1. |
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2. |
- Abdo, A. A., et al.
(författare)
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Multi-wavelength observations of the flaring gamma-ray blazar 3C 66A in 2008 October
- 2011
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 726:1, s. 43-
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Tidskriftsartikel (refereegranskat)abstract
- The BL Lacertae object 3C 66A was detected in a flaring state by the Fermi Large Area Telescope (LAT) and VERITAS in 2008 October. In addition to these gamma-ray observations, F-GAMMA, GASP-WEBT, PAIRITEL, MDM, ATOM, Swift, and Chandra provided radio to X-ray coverage. The available light curves show variability and, in particular, correlated flares are observed in the optical and Fermi-LAT gamma-ray band. The resulting spectral energy distribution can be well fitted using standard leptonic models with and without an external radiation field for inverse Compton scattering. It is found, however, that only the model with an external radiation field can accommodate the intra-night variability observed at optical wavelengths.
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3. |
- Abeysekara, A. U., et al.
(författare)
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VERITAS and Fermi-LAT Observations of TeV Gamma-Ray Sources Discovered by HAWC in the 2HWC Catalog
- 2018
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing. - 0004-637X .- 1538-4357. ; 866:1
-
Tidskriftsartikel (refereegranskat)abstract
- The High Altitude Water Cherenkov (HAWC) collaboration recently published their 2HWC catalog, listing 39 very high energy (VHE; >100 GeV) gamma-ray sources based on 507 days of observation. Among these, 19 sources are not associated with previously known teraelectronvolt (TeV) gamma-ray sources. We have studied 14 of these sources without known counterparts with VERITAS and Fermi-LAT. VERITAS detected weak gamma-ray emission in the 1 TeV-30 TeV band in the region of DA 495, a pulsar wind nebula coinciding with 2HWC J1953+294, confirming the discovery of the source by HAWC. We did not find any counterpart for the selected 14 new HAWC sources from our analysis of Fermi-LAT data for energies higher than 10 GeV. During the search, we detected gigaelectronvolt (GeV) gamma-ray emission coincident with a known TeV pulsar wind nebula, SNR G54.1+0.3 (VER J1930+188), and a 2HWC source, 2HWC J1930+188. The fluxes for isolated, steady sources in the 2HWC catalog are generally in good agreement with those measured by imaging atmospheric Cherenkov telescopes. However, the VERITAS fluxes for SNR G54.1+0.3, DA 495, and TeV J2032+4130 are lower than those measured by HAWC, and several new HAWC sources are not detected by VERITAS. This is likely due to a change in spectral shape, source extension, or the influence of diffuse emission in the source region.
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4. |
- Abramowski, A., et al.
(författare)
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Simultaneous multi-wavelength campaign on PKS 2005-489 in a high state
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 533, s. A110-
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Tidskriftsartikel (refereegranskat)abstract
- The high-frequency peaked BL Lac object PKS 2005-489 was the target of a multi-wavelength campaign with simultaneous observations in the TeV gamma-ray (H.E.S.S.), GeV gamma-ray (Fermi/LAT), X-ray (RXTE, Swift), UV (Swift) and optical (ATOM, Swift) bands. This campaign was carried out during a high flux state in the synchrotron regime. The flux in the optical and X-ray bands reached the level of the historical maxima. The hard GeV spectrum observed with Fermi/LAT connects well to the very high energy (VHE, E > 100 GeV) spectrum measured with H.E.S.S. with a peak energy between similar to 5 and 500 GeV. Compared to observations with contemporaneous coverage in the VHE and X-ray bands in 2004, the X-ray flux was similar to 50 times higher during the 2009 campaign while the TeV gamma-ray flux shows marginal variation over the years. The spectral energy distribution during this multi-wavelength campaign was fit by a one zone synchrotron self-Compton model with a well determined cutoff in X-rays. The parameters of a one zone SSC model are inconsistent with variability time scales. The variability behaviour over years with the large changes in synchrotron emission and small changes in the inverse Compton emission does not warrant an interpretation within a one-zone SSC model despite an apparently satisfying fit to the broadband data in 2009.
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5. |
- Abdalla, H., et al.
(författare)
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Gamma-ray blazar spectra with HESS II mono analysis : The case of PKS2155-304 and PG1553+113
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 600
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment's sensitivity to lower energies. The lowest energy threshold is obtained using monoscopic analysis of data taken with CT5, providing access to gamma-ray energies below 100 GeV for small zenith angle observations. Such an extension of the instrument's energy range is particularly beneficial for studies of active galactic nuclei with soft spectra, as expected for those at a redshift >= 0.5. The high-frequency peaked BL Lac objects PKS 2155-304 (z = 0.116) and PG 1553 + 113 (0.43 < z < 0.58) are among the brightest objects in the gamma-ray sky, both showing clear signatures of gamma-ray absorption at E > 100 GeV interpreted as being due to interactions with the extragalactic background light (EBL). Aims. The aims of this work are twofold: to demonstrate the monoscopic analysis of CT5 data with a low energy threshold, and to obtain accurate measurements of the spectral energy distributions (SED) of PKS 2155-304 and PG 1553 + 113 near their SED peaks at energies approximate to 100 GeV. Methods. Multiple observational campaigns of PKS 2155 304 and PG 1553 + 113 were conducted during 2013 and 2014 using the full H.E.S.S. II instrument (CT1-5). A monoscopic analysis of the data taken with the new CT5 telescope was developed along with an investigation into the systematic uncertainties on the spectral parameters which are derived from this analysis. Results. Using the data from CT5, the energy spectra of PKS 2155 304 and PG 1553 + 113 were reconstructed down to conservative threshold energies of 80 GeV for PKS 2155 304, which transits near zenith, and 110 GeV for the more northern PG 1553 + 113. The measured spectra, well fitted in both cases by a log-parabola spectral model ( with a 5.0 similar to statistical preference for non-zero curvature for PKS 2155 304 and 4.5 sigma for PG 1553+113), were found consistent with spectra derived from contemporaneous Fermi-LAT data, indicating a sharp break in the observed spectra of both sources at E approximate to 100 GeV. When corrected for EBL absorption, the intrinsic H.E.S.S. II mono and Fermi-LAT spectrum of PKS 2155 304 was found to show significant curvature. For PG 1553+113, however, no significant detection of curvature in the intrinsic spectrum could be found within statistical and systematic uncertainties.
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6. |
- Abdo, A. A., et al.
(författare)
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FERMI LARGE AREA TELESCOPE FIRST SOURCE CATALOG
- 2010
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 188:2, s. 405-436
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Tidskriftsartikel (refereegranskat)abstract
- We present a catalog of high-energy gamma-ray sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi), during the first 11 months of the science phase of the mission, which began on 2008 August 4. The First Fermi-LAT catalog (1FGL) contains 1451 sources detected and characterized in the 100 MeV to 100 GeV range. Source detection was based on the average flux over the 11 month period, and the threshold likelihood Test Statistic is 25, corresponding to a significance of just over 4 sigma. The 1FGL catalog includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and power-law spectral fits as well as flux measurements in five energy bands for each source. In addition, monthly light curves are provided. Using a protocol defined before launch we have tested for several populations of gamma-ray sources among the sources in the catalog. For individual LAT-detected sources we provide firm identifications or plausible associations with sources in other astronomical catalogs. Identifications are based on correlated variability with counterparts at other wavelengths, or on spin or orbital periodicity. For the catalogs and association criteria that we have selected, 630 of the sources are unassociated. Care was taken to characterize the sensitivity of the results to the model of interstellar diffuse gamma-ray emission used to model the bright foreground, with the result that 161 sources at low Galactic latitudes and toward bright local interstellar clouds are flagged as having properties that are strongly dependent on the model or as potentially being due to incorrectly modeled structure in the Galactic diffuse emission.
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7. |
- Abdo, A. A., et al.
(författare)
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Fermi Observations of High-Energy Gamma-Ray Emission from GRB 080916C
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 323:5922, s. 1688-1693
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray bursts (GRBs) are highly energetic explosions signaling the death of massive stars in distant galaxies. The Gamma-ray Burst Monitor and Large Area Telescope onboard the Fermi Observatory together record GRBs over a broad energy range spanning about 7 decades of gamma-ray energy. In September 2008, Fermi observed the exceptionally luminous GRB 080916C, with the largest apparent energy release yet measured. The high-energy gamma rays are observed to start later and persist longer than the lower energy photons. A simple spectral form fits the entire GRB spectrum, providing strong constraints on emission models. The known distance of the burst enables placing lower limits on the bulk Lorentz factor of the outflow and on the quantum gravity mass.
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8. |
- Acciari, V. A., et al.
(författare)
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Discovery of very high energy gamma rays from PKS 1424+240 and multiwavelength constraints on ITS redshift
- 2010
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Ingår i: ASTROPHYSICAL JOURNAL LETTERS. - 2041-8205. ; 708:2, s. L100-L106
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Tidskriftsartikel (refereegranskat)abstract
- We report the first detection of very high energy(83) (VHE) gamma-ray emission above 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift. The photon spectrum above 140 GeV measured by VERITAS is well described by a power law with a photon index of 3.8 +/- 0.5(stat) +/- 0.3(syst) and a flux normalization at 200 GeV of (5.1 +/- 0.9(stat) +/- 0.5(syst)) x 10(-11) TeV-1 cm(-2) s(-1), where stat and syst denote the statistical and systematical uncertainties, respectively. The VHE flux is steady over the observation period between MJD 54881 and 55003 (from 2009 February 19 to June 21). Flux variability is also not observed in contemporaneous high-energy observations with the Fermi Large Area Telescope. Contemporaneous X-ray and optical data were also obtained from the Swift XRT and MDM observatory, respectively. The broadband spectral energy distribution is well described by a one-zone synchrotron self-Compton model favoring a redshift of less than 0.1. Using the photon index measured with Fermi in combination with recent extragalactic background light absorption models it can be concluded from the VERITAS data that the redshift of PKS 1424+240 is less than 0.66.
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9. |
- Atwood, W. B., et al.
(författare)
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THE LARGE AREA TELESCOPE ON THE FERMI GAMMA-RAY SPACE TELESCOPE MISSION
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 697:2, s. 1071-1102
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Tidskriftsartikel (refereegranskat)abstract
- The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view (FoV), high-energy gamma-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. The LAT was built by an international collaboration with contributions from space agencies, high-energy particle physics institutes, and universities in France, Italy, Japan, Sweden, and the United States. This paper describes the LAT, its preflight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4 x 4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 (x, y) tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an eight-layer hodoscopic configuration with a total depth of 8.6 radiation lengths, giving both longitudinal and transverse information about the energy deposition pattern. The calorimeter's depth and segmentation enable the high-energy reach of the LAT and contribute significantly to background rejection. The aspect ratio of the tracker (height/width) is 0.4, allowing a large FoV (2.4 sr) and ensuring that most pair-conversion showers initiated in the tracker will pass into the calorimeter for energy measurement. Data obtained with the LAT are intended to (1) permit rapid notification of high-energy gamma-ray bursts and transients and facilitate monitoring of variable sources, (2) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (3) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (4) localize point sources to 0.3-2 arcmin, (5) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (6) measure the diffuse isotropic gamma-ray background up to TeV energies, and (7) explore the discovery space for dark matter.
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10. |
- Abdo, A. A., et al.
(författare)
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The spectral energy distribution of fermi bright blazars
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 716:1, s. 30-70
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Tidskriftsartikel (refereegranskat)abstract
- We have conducted a detailed investigation of the broadband spectral properties of the gamma-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/gamma-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these gamma-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log nu-log nu F-nu representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low-and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, alpha(ro), and optical to X-ray, alpha(ox), spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency (nu(S)(peak)) is positioned between 10(12.5) and 10(14.5) Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10(13) and 10(17) Hz in featureless BL Lacertae objects. We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter gamma-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum gamma-ray sources, the correlation between nu(S)(peak) and gamma-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.
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11. |
- Acciari, V. A., et al.
(författare)
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THE DISCOVERY OF gamma-RAY EMISSION FROM THE BLAZAR RGB J0710+591
- 2010
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Ingår i: ASTROPHYSICAL JOURNAL LETTERS. - 2041-8205. ; 715:1, s. L49-L55
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Tidskriftsartikel (refereegranskat)abstract
- The high-frequency-peaked BL Lacertae object RGB J0710+591 was observed in the very high-energy (VHE; E > 100 GeV) wave band by the VERITAS array of atmospheric Cherenkov telescopes. The observations, taken between 2008 December and 2009 March and totaling 22.1 hr, yield the discovery of VHE gamma rays from the source. RGB J0710+591 is detected at a statistical significance of 5.5 standard deviations (5.5 sigma) above the background, corresponding to an integral flux of (3.9 +/- 0.8) x 10(-12) cm(-2) s(-1) (3% of the Crab Nebula's flux) above 300 GeV. The observed spectrum can be fit by a power law from 0.31 to 4.6 TeV with a photon spectral index of 2.69 +/- 0.26(stat) +/- 0.20(sys). These data are complemented by contemporaneous multiwavelength data from the Fermi Large Area Telescope, the Swift X-ray Telescope, the Swift Ultra-Violet and Optical Telescope, and the Michigan-Dartmouth-MIT observatory. Modeling the broadband spectral energy distribution (SED) with an equilibrium synchrotron self-Compton model yields a good statistical fit to the data. The addition of an external-Compton component to the model does not improve the fit nor brings the system closer to equipartition. The combined Fermi and VERITAS data constrain the properties of the high-energy emission component of the source over 4 orders of magnitude and give measurements of the rising and falling sections of the SED.
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12. |
- Nolan, P. L., et al.
(författare)
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Fermi large area telescope second source catalog
- 2012
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 199:2, s. 31-
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Tidskriftsartikel (refereegranskat)abstract
- We present the second catalog of high-energy gamma-ray sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi), derived from data taken during the first 24 months of the science phase of the mission, which began on 2008 August 4. Source detection is based on the average flux over the 24 month period. The second Fermi-LAT catalog (2FGL) includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and spectral fits in terms of power-law, exponentially cutoff power-law, or log-normal forms. Also included are flux measurements in five energy bands and light curves on monthly intervals for each source. Twelve sources in the catalog are modeled as spatially extended. We provide a detailed comparison of the results from this catalog with those from the first Fermi-LAT catalog (1FGL). Although the diffuse Galactic and isotropic models used in the 2FGL analysis are improved compared to the 1FGL catalog, we attach caution flags to 162 of the sources to indicate possible confusion with residual imperfections in the diffuse model. The 2FGL catalog contains 1873 sources detected and characterized in the 100 MeV to 100 GeV range of which we consider 127 as being firmly identified and 1171 as being reliably associated with counterparts of known or likely gamma-ray-producing source classes.
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13. |
- Abdo, A. A., et al.
(författare)
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FERMI Large Area Telescope and multi-wavelength observations of the flaring activity of PKS 1510-089 between 2008 september and 2009 june
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 721:2, s. 1425-1447
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Tidskriftsartikel (refereegranskat)abstract
- We report on the multi-wavelength observations of PKS 1510-089 (a flat spectrum radio quasar (FSRQ) at z = 0.361) during its high activity period between 2008 September and 2009 June. During this 11 month period, the source was characterized by a complex variability at optical, UV, and gamma-ray bands, on timescales down to 6-12 hr. The brightest gamma-ray isotropic luminosity, recorded on 2009 March 26, was similar or equal to 2 x 1048 erg s-1. The spectrum in the Fermi Large Area Telescope energy range shows a mild curvature described well by a log-parabolic law, and can be understood as due to the Klein-Nishina effect. The. -ray flux has a complex correlation with the other wavelengths. There is no correlation at all with the X-ray band, a weak correlation with the UV, and a significant correlation with the optical flux. The. -ray flux seems to lead the optical one by about 13 days. From the UV photometry, we estimated a black hole mass of similar or equal to 5.4 x 10(8)M(circle dot) and an accretion rate of similar or equal to 0.5M(circle dot) yr(-1). Although the power in the thermal and non-thermal outputs is smaller compared to the very luminous and distant FSRQs, PKS 1510-089 exhibits a quite large Compton dominance and a prominent big blue bump (BBB) as observed in the most powerful gamma-ray quasars. The BBB was still prominent during the historical maximum optical state in 2009 May, but the optical/ UV spectral index was softer than in the quiescent state. This seems to indicate that the BBB was not completely dominated by the synchrotron emission during the highest optical state. We model the broadband spectrum assuming a leptonic scenario in which the inverse Compton emission is dominated by the scattering of soft photons produced externally to the jet. The resulting model-dependent jet energetic content is compatible with a scenario in which the jet is powered by the accretion disk, with a total efficiency within the Kerr black hole limit.
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14. |
- Abdo, A. A., et al.
(författare)
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FERMI/LARGE AREA TELESCOPE BRIGHT GAMMA-RAY SOURCE LIST
- 2009
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 183:1, s. 46-66
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Tidskriftsartikel (refereegranskat)abstract
- Following its launch in 2008 June, the Fermi Gamma-ray Space Telescope (Fermi) began a sky survey in August. The Large Area Telescope (LAT) on Fermi in three months produced a deeper and better resolved map of the gamma-ray sky than any previous space mission. We present here initial results for energies above 100 MeV for the 205 most significant (statistical significance greater than similar to 10 sigma) gamma-ray sources in these data. These are the best characterized and best localized point-like (i.e., spatially unresolved) gamma-ray sources in the early mission data.
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15. |
- Abdo, A. A., et al.
(författare)
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FERMI LARGE AREA TELESCOPE OBSERVATIONS OF THE VELA PULSAR
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 696:2, s. 1084-1093
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Tidskriftsartikel (refereegranskat)abstract
- The Vela pulsar is the brightest persistent source in the GeV sky and thus is the traditional first target for new gamma-ray observatories. We report here on initial Fermi Large Area Telescope observations during verification phase pointed exposure and early sky survey scanning. We have used the Vela signal to verify Fermi timing and angular resolution. The high-quality pulse profile, with some 32,400 pulsed photons at E >= 0.03 GeV, shows new features, including pulse structure as fine as 0.3 ms and a distinct third peak, which shifts in phase with energy. We examine the high-energy behavior of the pulsed emission; initial spectra suggest a phase-averaged power-law index of Gamma = 1.51(-0.04)(+0.05) with an exponential cutoff at E-c = 2.9 +/- 0.1 GeV. Spectral fits with generalized cutoffs of the form e(-(E/Ec)b) require b <= 1, which is inconsistent with magnetic pair attenuation, and thus favor outer-magnetosphere emission models. Finally, we report on upper limits to any unpulsed component, as might be associated with a surrounding pulsar wind nebula.
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16. |
- Abdo, A. A., et al.
(författare)
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Fermi Large Area Telescope Gamma-Ray Detection of the Radio Galaxy M87
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 707:1, s. 55-60
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Tidskriftsartikel (refereegranskat)abstract
- We report the Fermi Large Area Telescope (LAT) discovery of high-energy (MeV/GeV) γ-ray emission positionally consistent with the center of the radio galaxy M87, at a source significance of over 10σ in 10 months of all-sky survey data. Following the detections of Cen A and Per A, this makes M87 the third radio galaxy seen with the LAT. The faint point-like γ-ray source has a >100 MeV flux of 2.45 (±0.63) × 10–8 photons cm–2 s–1 (photon index = 2.26 ± 0.13) with no significant variability detected within the LAT observation. This flux is comparable with the previous EGRET upper limit (<2.18 × 10–8 photons cm–2 s–1, 2σ), thus there is no evidence for a significant MeV/GeV flare on decade timescales. Contemporaneous Chandra and Very Long Baseline Array data indicate low activity in the unresolved X-ray and radio core relative to previous observations, suggesting M87 is in a quiescent overall level over the first year of Fermi-LAT observations. The LAT γ-ray spectrum is modeled as synchrotron self-Compton (SSC) emission from the electron population producing the radio-to-X-ray emission in the core. The resultant SSC spectrum extrapolates smoothly from the LAT band to the historical-minimum TeV emission. Alternative models for the core and possible contributions from the kiloparsec-scale jet in M87 are considered, and cannot be excluded.
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17. |
- Abdo, A. A., et al.
(författare)
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Measurement of the Cosmic Ray e(+)+e(-) Spectrum from 20 GeV to 1 TeV with the Fermi Large Area Telescope
- 2009
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:Article number: 181101
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Tidskriftsartikel (refereegranskat)abstract
- Designed as a high-sensitivity gamma-ray observatory, the Fermi Large Area Telescope is also an electron detector with a large acceptance exceeding 2 m(2) sr at 300 GeV. Building on the gamma-ray analysis, we have developed an efficient electron detection strategy which provides sufficient background rejection for measurement of the steeply falling electron spectrum up to 1 TeV. Our high precision data show that the electron spectrum falls with energy as E-3.0 and does not exhibit prominent spectral features. Interpretations in terms of a conventional diffusive model as well as a potential local extra component are briefly discussed.
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18. |
- Abdollahi, S., et al.
(författare)
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Fermi Large Area Telescope Fourth Source Catalog
- 2020
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 247:1
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Tidskriftsartikel (refereegranskat)abstract
- We present the fourth Fermi Large Area Telescope catalog (4FGL) of gamma-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse gamma-ray emission, and two sets of light curves (one-year and two-month intervals). The 4FGL catalog includes 5064 sources above 4 sigma significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall, 358 sources are considered as identified based on angular extent, periodicity, or correlated variability observed at other wavelengths. For 1336 sources, we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.
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19. |
- Ackermann, M., et al.
(författare)
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Gamma-Ray Blazars within the First 2 Billion Years
- 2017
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Ingår i: Astrophysical Journal Letters. - : Institute of Physics Publishing (IOPP). - 2041-8205 .- 2041-8213. ; 837:1
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Tidskriftsartikel (refereegranskat)abstract
- The detection of high-redshift (z > 3) blazars enables the study of the evolution of the most luminous relativistic jets over cosmic time. More importantly, high-redshift blazars tend to host massive black holes and can be used to constrain the space density of heavy black holes in the early universe. Here, we report the first detection with the Fermi-Large Area Telescope of five γ-ray-emitting blazars beyond z = 3.1, more distant than any blazars previously detected in γ-rays. Among these five objects, NVSS J151002+570243 is now the most distant known γ-ray-emitting blazar at z = 4.31. These objects have steeply falling γ-ray spectral energy distributions (SEDs), and those that have been observed in X-rays have a very hard X-ray spectrum, both typical of powerful blazars. Their Compton dominance (ratio of the inverse Compton to synchrotron peak luminosities) is also very large (>20). All of these properties place these objects among the most extreme members of the blazar population. Their optical spectra and the modeling of their optical-UV SEDs confirm that these objects harbor massive black holes (MBH ∼ 10 8-10 Mo ). We find that, at z ≈ 4, the space density of >109 Mo black holes hosted in radio-loud and radio-quiet active galactic nuclei are similar, implying that radio-loudness may play a key role in rapid black hole growth in the early universe.
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20. |
- Abdo, A. A., et al.
(författare)
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BRIGHT ACTIVE GALACTIC NUCLEI SOURCE LIST FROM THE FIRST THREE MONTHS OF THE FERMI LARGE AREA TELESCOPE ALL-SKY SURVEY
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 700:1, s. 597-622
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Tidskriftsartikel (refereegranskat)abstract
- The first three months of sky-survey operation with the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope reveal 132 bright sources at |b| > 10 degrees with test statistic greater than 100 ( corresponding to about 10 sigma). Two methods, based on the CGRaBS, CRATES, and BZCat catalogs, indicate high-confidence associations of 106 of these sources with known active galactic nuclei (AGNs). This sample is referred to as the LAT Bright AGN Sample (LBAS). It contains two radio galaxies, namely, Centaurus A and NGC 1275, and 104 blazars consisting of 58 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and 4 blazars with unknown classification. Four new blazars were discovered on the basis of the LAT detections. Remarkably, the LBAS includes 10 high-energy-peaked BL Lacs (HBLs), sources which were previously difficult to detect in the GeV range. Another 10 lower-confidence associations are found. Only 33 of the sources, plus two at |b| < 10 degrees, were previously detected with Energetic Gamma-Ray Experiment Telescope( EGRET), probably due to variability. The analysis of the gamma-ray properties of the LBAS sources reveals that the average GeV spectra of BL Lac objects are significantly harder than the spectra of FSRQs. No significant correlation between radio and peak gamma-ray fluxes is observed. Blazar log N-log S distributions and luminosity functions are constructed to investigate the evolution of the different blazar classes, with positive evolution indicated for FSRQs but none for BL Lacs. The contribution of LAT blazars to the total extragalactic gamma-ray intensity is estimated.
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21. |
- Abdo, A. A., et al.
(författare)
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Fermi Gamma-Ray Imaging of a Radio Galaxy
- 2010
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 328:5979, s. 725-729
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Tidskriftsartikel (refereegranskat)abstract
- The Fermi Gamma-ray Space Telescope has detected the gamma-ray glow emanating from the giant radio lobes of the radio galaxy Centaurus A. The resolved gamma-ray image shows the lobes clearly separated from the central active source. In contrast to all other active galaxies detected so far in high-energy gamma-rays, the lobe flux constitutes a considerable portion (greater than one-half) of the total source emission. The gamma-ray emission from the lobes is interpreted as inverse Compton-scattered relic radiation from the cosmic microwave background, with additional contribution at higher energies from the infrared-to-optical extragalactic background light. These measurements provide gamma-ray constraints on the magnetic field and particle energy content in radio galaxy lobes, as well as a promising method to probe the cosmic relic photon fields.
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22. |
- Abdo, A. A., et al.
(författare)
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FERMI LARGE AREA TELESCOPE VIEW OF THE CORE OF THE RADIO GALAXY CENTAURUS A
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 719:2, s. 1433-1444
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Tidskriftsartikel (refereegranskat)abstract
- We present gamma-ray observations with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope of the nearby radio galaxy Centaurus A (Cen A). The previous EGRET detection is confirmed, and the localization is improved using data from the first 10 months of Fermi science operation. In previous work, we presented the detection of the lobes by the LAT; in this work, we concentrate on the gamma-ray core of Cen A. Flux levels as seen by the LAT are not significantly different from that found by EGRET, nor is the extremely soft LAT spectrum (Gamma = 2.67 +/- 0.10(stat) +/- 0.08(sys) where the photon flux is Phi alpha E-Gamma). The LAT core spectrum, extrapolated to higher energies, is marginally consistent with the non-simultaneous HESS spectrum of the source. The LAT observations are complemented by simultaneous observations from Suzaku, the Swift Burst Alert Telescope and X-ray Telescope, and radio observations with the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry program, along with a variety of non-simultaneous archival data from a variety of instruments and wavelengths to produce a spectral energy distribution (SED). We fit this broadband data set with a single-zone synchrotron/synchrotron self-Compton model, which describes the radio through GeV emission well, but fails to account for the non-simultaneous higher energy TeV emission observed by HESS from 2004 to 2008. The fit requires a low Doppler factor, in contrast to BL Lac objects which generally require larger values to fit their broadband SEDs. This indicates that the gamma-ray emission originates from a slower region than that from BL Lac objects, consistent with previous modeling results from Cen A. This slower region could be a slower moving layer around a fast spine, or a slower region farther out from the black hole in a decelerating flow. The fit parameters are also consistent with Cen A being able to accelerate ultra-high energy cosmic-rays, as hinted at by results from the Auger observatory.
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23. |
- Abdo, A. A., et al.
(författare)
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Gamma-ray emission concurrent with the nova in the symbiotic binary V407 cygni
- 2010
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 329:5993, s. 817-821
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Tidskriftsartikel (refereegranskat)abstract
- Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star. Current physical models posit that shocked expanding gas from the nova shell can produce x-ray emission, but emission at higher energies has not been widely expected. Here, we report the Fermi Large Area Telescope detection of variable γ-ray emission (0.1 to 10 billion electron volts) from the recently detected optical nova of the symbiotic star V407 Cygni. We propose that the material of the nova shell interacts with the dense ambient medium of the red giant primary and that particles can be accelerated effectively to produce π0 decay γ-rays from proton-proton interactions. Emission involving inverse Compton scattering of the red giant radiation is also considered and is not ruled out.
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24. |
- Abdo, A. A., et al.
(författare)
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Gamma-Ray Flares from the Crab Nebula
- 2011
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 331:6018, s. 739-742
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Tidskriftsartikel (refereegranskat)abstract
- A young and energetic pulsar powers the well-known Crab Nebula. Here, we describe two separate gamma-ray (photon energy greater than 100 mega-electron volts) flares from this source detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The first flare occurred in February 2009 and lasted approximately 16 days. The second flare was detected in September 2010 and lasted approximately 4 days. During these outbursts, the gamma-ray flux from the nebula increased by factors of four and six, respectively. The brevity of the flares implies that the gamma rays were emitted via synchrotron radiation from peta-electron-volt (10(15) electron volts) electrons in a region smaller than 1.4 x 10(-2) parsecs. These are the highest-energy particles that can be associated with a discrete astronomical source, and they pose challenges to particle acceleration theory.
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25. |
- Ackermann, M., et al.
(författare)
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Fermi LAT observations of cosmic-ray electrons from 7 GeV to 1 TeV
- 2010
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Ingår i: PHYSICAL REVIEW D. - 1550-7998. ; 82:9, s. 092004-
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of our analysis of cosmic-ray electrons using about 8 x 10(6) electron candidates detected in the first 12 months on-orbit by the Fermi Large Area Telescope. This work extends our previously published cosmic-ray electron spectrum down to 7 GeV, giving a spectral range of approximately 2.5 decades up to 1 TeV. We describe in detail the analysis and its validation using beam-test and on-orbit data. In addition, we describe the spectrum measured via a subset of events selected for the best energy resolution as a cross-check on the measurement using the full event sample. Our electron spectrum can be described with a power law proportional to E-3.08+/-0.05 with no prominent spectral features within systematic uncertainties. Within the limits of our uncertainties, we can accommodate a slight spectral hardening at around 100 GeV and a slight softening above 500 GeV.
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26. |
- Abdo, A. A., et al.
(författare)
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A Population of Gamma-Ray Millisecond Pulsars Seen with the Fermi Large Area Telescope
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 325:5942, s. 848-852
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Tidskriftsartikel (refereegranskat)abstract
- Pulsars are born with subsecond spin periods and slow by electromagnetic braking for several tens of millions of years, when detectable radiation ceases. A second life can occur for neutron stars in binary systems. They can acquire mass and angular momentum from their companions, to be spun up to millisecond periods and begin radiating again. We searched Fermi Large Area Telescope data for pulsations from all known millisecond pulsars (MSPs) outside of globular clusters, using rotation parameters from radio telescopes. Strong gamma-ray pulsations were detected for eight MSPs. The gamma-ray pulse profiles and spectral properties resemble those of young gamma-ray pulsars. The basic emission mechanism seems to be the same for MSPs and young pulsars, with the emission originating in regions far from the neutron star surface.
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27. |
- Abdo, A. A., et al.
(författare)
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Detection of 16 Gamma-Ray Pulsars Through Blind Frequency Searches Using the Fermi LAT
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 325:5942, s. 840-844
-
Tidskriftsartikel (refereegranskat)abstract
- Pulsars are rapidly rotating, highly magnetized neutron stars emitting radiation across the electromagnetic spectrum. Although there are more than 1800 known radio pulsars, until recently only seven were observed to pulse in gamma rays, and these were all discovered at other wavelengths. The Fermi Large Area Telescope (LAT) makes it possible to pinpoint neutron stars through their gamma-ray pulsations. We report the detection of 16 gamma-ray pulsars in blind frequency searches using the LAT. Most of these pulsars are coincident with previously unidentified gamma-ray sources, and many are associated with supernova remnants. Direct detection of gamma-ray pulsars enables studies of emission mechanisms, population statistics, and the energetics of pulsar wind nebulae and supernova remnants.
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28. |
- Abdo, A. A., et al.
(författare)
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Detection of High-Energy Gamma-Ray Emission from the Globular Cluster 47 Tucanae with Fermi
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 325:5942, s. 845-848
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Tidskriftsartikel (refereegranskat)abstract
- We report the detection of gamma-ray emissions above 200 megaelectron volts at a significance level of 17 sigma from the globular cluster 47 Tucanae, using data obtained with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. Globular clusters are expected to emit gamma rays because of the large populations of millisecond pulsars that they contain. The spectral shape of 47 Tucanae is consistent with gamma-ray emission from a population of millisecond pulsars. The observed gamma-ray luminosity implies an upper limit of 60 millisecond pulsars present in 47 Tucanae.
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29. |
- Abdo, A. A., et al.
(författare)
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EARLY FERMI GAMMA-RAY SPACE TELESCOPE OBSERVATIONS OF THE QUASAR 3C 454.3
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 699:1, s. 817-823
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Tidskriftsartikel (refereegranskat)abstract
- This is the first report of Fermi Gamma-Ray Space Telescope observations of the quasar 3C 454.3, which has been undergoing pronounced long-term outbursts since 2000. The data from the Large Area Telescope, covering 2008 July 7-October 6, indicate strong, highly variable.-ray emission with an average flux of similar to 3 x 10 (6) photons cm(-2) s(-1), for energies > 100 MeV. The gamma-ray flux is variable, with strong, distinct, symmetrically shaped flares for which the flux increases by a factor of several on a timescale of about 3 days. This variability indicates a compact emission region, and the requirement that the source is optically thin to pair production implies relativistic beaming with Doppler factor delta > 8, consistent with the values inferred from Very Long Baseline Interferometry observations of superluminal expansion (delta similar to 25). The observed gamma-ray spectrum is not consistent with a simple power law, but instead steepens strongly above similar to 2 GeV, and is well described by a broken power law with photon indices of similar to 2.3 and similar to 3.5 below and above the break, respectively. This is the first direct observation of a break in the spectrum of a high-luminosity blazar above 100 MeV, and it is likely direct evidence for an intrinsic break in the energy distribution of the radiating particles. Alternatively, the spectral softening above 2 GeV could be due to gamma-ray absorption via photon-photon pair production on the soft X-ray photon field of the host active galactic nucleus, but such an interpretation would require the dissipation region to be located very close (less than or similar to 100 gravitational radii) to the black hole, which would be inconsistent with the X-ray spectrum of the source.
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30. |
- Abdo, A. A., et al.
(författare)
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FERMI DISCOVERY OF GAMMA-RAY EMISSION FROM NGC 1275
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 699:1, s. 31-39
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of high-energy (E > 100 MeV) gamma-ray emission from NGC 1275, a giant elliptical galaxy lying at the center of the Perseus cluster of galaxies, based on observations made with the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope. The positional center of the gamma-ray source is only approximate to 3' away from the NGC 1275 nucleus, well within the 95% LAT error circle of approximate to 5'. The spatial distribution of gamma-ay photons is consistent with a point source. The average flux and power-law photon index measured with the LAT from 2008 August 4 to 2008 December 5 are F-gamma = (2.10 +/- 0.23) x 10(-7) ph (>100 MeV) cm(-2) s(-1) and Gamma = 2.17 +/- 0.05, respectively. The measurements are statistically consistent with constant flux during the four-month LAT observing period. Previous EGRET observations gave an upper limit of F-gamma < 3.72 x 10(-8) ph (>100 MeV) cm(-2) s(-1) to the gamma-ray flux from NGC 1275. This indicates that the source is variable on timescales of years to decades, and therefore restricts the fraction of emission that can be produced in extended regions of the galaxy cluster. Contemporaneous and historical radio observations are also reported. The broadband spectrum of NGC 1275 is modeled with a simple one-zone synchrotron/synchrotron self-Compton model and a model with a decelerating jet flow.
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31. |
- Abdo, A. A., et al.
(författare)
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Fermi Large Area Telescope constraints on the gamma-ray opacity of the universe
- 2010
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 723:2, s. 1082-1096
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Tidskriftsartikel (refereegranskat)abstract
- The extragalactic background light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above similar to 10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the gamma-ray flux of extragalactic sources such as blazars and gamma-ray bursts (GRBs). The Large Area Telescope on board Fermi detects a sample of gamma-ray blazars with redshift up to z similar to 3, and GRBs with redshift up to z similar to 4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of gamma-ray flux attenuation by the EBL. We place upper limits on the gamma-ray opacity of the universe at various energies and redshifts and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. can be ruled out with high confidence.
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32. |
- Abdo, A. A., et al.
(författare)
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FERMI/LARGE AREA TELESCOPE DISCOVERY OF GAMMA-RAY EMISSION FROM THE FLAT-SPECTRUM RADIO QUASAR PKS 1454-354
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 697:1, s. 934-941
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope of high-energy gamma-ray (GeV) emission from the flat-spectrum radio quasar PKS 1454-354 (z = 1.424). On 2008 September 4, the source rose to a peak flux of (3.5 +/- 0.7) x 10(-6) ph cm(-2) s(-1) (E > 100 MeV) on a timescale of hours and then slowly dropped over the following 2 days. No significant spectral changes occurred during the flare. Fermi/LAT observations also showed that PKS 1454-354 is the most probable counterpart of the unidentified EGRET source 3EG J1500-3509. Multiwavelength measurements performed during the following days (7 September with Swift; 6-7 September with the ground-based optical telescope Automated Telescope for Optical Monitoring; 13 September with the Australia Telescope Compact Array) resulted in radio, optical, UV, and X-ray fluxes greater than archival data, confirming the activity of PKS 1454-354.
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33. |
- Abdo, A. A., et al.
(författare)
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FERMI LARGE AREA TELESCOPE OBSERVATIONS OF MISALIGNED ACTIVE GALACTIC NUCLEI
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 720:1, s. 912-922
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Tidskriftsartikel (refereegranskat)abstract
- Analysis is presented for 15 months of data taken with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope for 11 non-blazar active galactic nuclei (AGNs), including seven FRI radio galaxies and four FRII radio sources consisting of two FRII radio galaxies and two steep spectrum radio quasars. The broad line FRI radio galaxy 3C 120 is reported here as a gamma-ray source for the first time. The analysis is based on directional associations of LAT sources with radio sources in the 3CR, 3CRR, and MS4 (collectively referred to as 3C-MS) catalogs. Seven of the eleven LAT sources associated with 3C-MS radio sources have spectral indices larger than 2.3 and, except for the FRI radio galaxy NGC 1275 that shows possible spectral curvature, are well described by a power law. No evidence for time variability is found for any sources other than NGC 1275. The gamma-ray luminosities of FRI radio galaxies are significantly smaller than those of the BL Lac objects detected by the LAT, whereas the gamma-ray luminosities of the FRII sources are quite similar to those of FSRQs, which could reflect different beaming factors for the gamma-ray emission. A core dominance (CD) study of the 3CRR sample indicates that sources closer to the jet axis are preferentially detected with the Fermi LAT, insofar as the gamma-ray-detected misaligned AGNs have larger CD at a given average radio flux. The results are discussed in view of the AGN unification scenario.
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34. |
- Abdo, A. A., et al.
(författare)
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FERMI LAT DISCOVERY OF EXTENDED GAMMA-RAY EMISSION IN THE DIRECTION OF SUPERNOVA REMNANT W51C
- 2009
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Ingår i: Astrophysical Journal Letters. - 2041-8205. ; 706:1, s. L1-L6
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Tidskriftsartikel (refereegranskat)abstract
- The discovery of bright gamma-ray emission coincident with supernova remnant (SNR) W51C is reported using the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. W51C is a middle-aged remnant (similar to 10(4) yr) with intense radio synchrotron emission in its shell and known to be interacting with a molecular cloud. The gamma-ray emission is spatially extended, broadly consistent with the radio and X-ray extent of SNR W51C. The energy spectrum in the 0.2-50 GeV band exhibits steepening toward high energies. The luminosity is greater than 1 x 10(36) erg s(-1) given the distance constraint of D > 5.5 kpc, which makes this object one of the most luminous gamma-ray sources in our Galaxy. The observed gamma-rays can be explained reasonably by a combination of efficient acceleration of nuclear cosmic rays at supernova shocks and shock-cloud interactions. The decay of neutral pi mesons produced in hadronic collisions provides a plausible explanation for the gamma-ray emission. The product of the average gas density and the total energy content of the accelerated protons amounts to (n) over bar W-H(p) similar or equal to 5 x 10(51) (D/6 kpc)(2) erg cm(-3). Electron density constraints from the radio and X-ray bands render it difficult to explain the LAT signal as due to inverse Compton scattering. The Fermi LAT source coincident with SNR W51C sheds new light on the origin of Galactic cosmic rays.
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35. |
- Abdo, A. A., et al.
(författare)
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Fermi/LAT observations of LS 5039
- 2009
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Ingår i: Astrophysical Journal Letters. - 0571-7248 .- 2041-8205. ; 706:1, s. L56-L61
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Tidskriftsartikel (refereegranskat)abstract
- The first results from observations of the high-mass X-ray binary LS 5039 using the Fermi Gamma-ray Space Telescope data between 2008 August and 2009 June are presented. Our results indicate variability that is consistent with the binary period, with the emission being modulated with a period of 3.903 ± 0.005 days; the first detection of this modulation at GeV energies. The light curve is characterized by a broad peak around superior conjunction in agreement with inverse Compton scattering models. The spectrum is represented by a power law with an exponential cutoff, yielding an overall flux (100 MeV-300 GeV) of 4.9 ± 0.5(stat) ± 1.8(syst) ×10–7 photon cm–2 s–1, with a cutoff at 2.1 ± 0.3(stat) ± 1.1(syst) GeV and photon index Γ = 1.9 ± 0.1(stat) ± 0.3(syst). The spectrum is observed to vary with orbital phase, specifically between inferior and superior conjunction. We suggest that the presence of a cutoff in the spectrum may be indicative of magnetospheric emission similar to the emission seen in many pulsars by Fermi.
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36. |
- Abdo, A. A., et al.
(författare)
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FERMI OBSERVATIONS OF GRB 090902B : A DISTINCT SPECTRAL COMPONENT IN THE PROMPT AND DELAYED EMISSION
- 2009
-
Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 0004-637X .- 1538-4357. ; 706:1, s. L138-L144
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Tidskriftsartikel (refereegranskat)abstract
- We report on the observation of the bright, long gamma-ray burst (GRB), GRB 090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on-board the Fermi observatory. This was one of the brightest GRBs to have been observed by the LAT, which detected several hundred photons during the prompt phase. With a redshift of z = 1.822, this burst is among the most luminous detected by Fermi. Time-resolved spectral analysis reveals a significant power-law component in the LAT data that is distinct from the usual Band model emission that is seen in the sub-MeV energy range. This power-law component appears to extrapolate from the GeV range to the lowest energies and is more intense than the Band component, both below similar to 50 keV and above 100 MeV. The Band component undergoes substantial spectral evolution over the entire course of the burst, while the photon index of the power-law component remains constant for most of the prompt phase, then hardens significantly toward the end. After the prompt phase, power-law emission persists in the LAT data as late as 1 ks post-trigger, with its flux declining as t(-1.5). The LAT detected a photon with the highest energy so far measured from a GRB, 33.4(-3.5)(+ 2.7) GeV. This event arrived 82 s after the GBM trigger and similar to 50 s after the prompt phase emission had ended in the GBM band. We discuss the implications of these results for models of GRB emission and for constraints on models of the extragalactic background light.
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37. |
- Abdo, A. A., et al.
(författare)
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SPECTRAL PROPERTIES OF BRIGHT FERMI-DETECTED BLAZARS IN THE GAMMA-RAY BAND
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 710:2, s. 1271-1285
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Tidskriftsartikel (refereegranskat)abstract
- The gamma-ray energy spectra of bright blazars of the LAT Bright AGN Sample LBAS) are investigated using Fermi-LAT data. Spectral properties hardness, curvature, and variability) established using a data set accumulated over 6 months of operation are presented and discussed for different blazar classes and subclasses: flat spectrum radio quasars (FSRQs), low-synchrotron peaked BLLacs (LSP-BLLacs), intermediate-synchrotron peaked BLLacs (ISP-BLLacs), and high-synchrotron peaked BLLacs (HSP-BLLacs). The distribution of photon index G, obtained from a power-law fit above 100 MeV) is found to correlate strongly with blazar subclass. The change in spectral index from that averaged over the 6 months observing period is < 0.2-0.3 when the flux varies by about an order of magnitude, with a tendency toward harder spectra when the flux is brighter for FSRQs and LSP-BLLacs. A strong departure from a single power-law spectrum appears to be a common feature for FSRQs. This feature is also present for some high-luminosity LSP-BLLacs, and a small number of ISP-BLLacs. It is absent in all LBAS HSP-BLLacs. For 3C 454.3 and AO 0235+164, the two brightest FSRQ source and LSP-BLLac source, respectively, a broken power law (BPL) gives the most acceptable of power law, BPL, and curved forms. The consequences of these findings are discussed.
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38. |
- Abdo, A. A., et al.
(författare)
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The Fermi Gamma-Ray Space Telescope Discovers the Pulsar in the Young Galactic Supernova Remnant CTA 1
- 2008
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 322:5905, s. 1218-1221
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Tidskriftsartikel (refereegranskat)abstract
- Energetic young pulsars and expanding blast waves [ supernova remnants (SNRs)] are the most visible remains after massive stars, ending their lives, explode in core-collapse supernovae. The Fermi Gamma- Ray Space Telescope has unveiled a radio quiet pulsar located near the center of the compact synchrotron nebula inside the supernova remnant CTA 1. The pulsar, discovered through its gamma- ray pulsations, has a period of 316.86 milliseconds and a period derivative of 3.614 x 10(-13) seconds per second. Its characteristic age of 10(4) years is comparable to that estimated for the SNR. We speculate that most unidentified Galactic gamma- ray sources associated with star- forming regions and SNRs are such young pulsars.
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39. |
- Abdo, A. A., et al.
(författare)
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The first catalog of active galactic nuclei detected by the Fermi large area telescope
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 715:1, s. 429-457
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Tidskriftsartikel (refereegranskat)abstract
- We present the first catalog of active galactic nuclei (AGNs) detected by the Large Area Telescope (LAT), corresponding to 11 months of data collected in scientific operation mode. The First LAT AGN Catalog (1LAC) includes 671 gamma-ray sources located at high Galactic latitudes (|b| > 10 degrees) that are detected with a test statistic greater than 25 and associated statistically with AGNs. Some LAT sources are associated with multiple AGNs, and consequently, the catalog includes 709 AGNs, comprising 300 BL Lacertae objects, 296 flat-spectrum radio quasars, 41 AGNs of other types, and 72 AGNs of unknown type. We also classify the blazars based on their spectral energy distributions as archival radio, optical, and X-ray data permit. In addition to the formal 1LAC sample, we provide AGN associations for 51 low-latitude LAT sources and AGN "affiliations" (unquantified counterpart candidates) for 104 high-latitude LAT sources without AGN associations. The overlap of the 1LAC with existing gamma-ray AGN catalogs (LBAS, EGRET, AGILE, Swift, INTEGRAL, TeVCat) is briefly discussed. Various properties-such as gamma-ray fluxes and photon power-law spectral indices, redshifts, gamma-ray luminosities, variability, and archival radio luminosities-and their correlations are presented and discussed for the different blazar classes. We compare the 1LAC results with predictions regarding the gamma-ray AGN populations, and we comment on the power of the sample to address the question of the blazar sequence.
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40. |
- Ackermann, M., et al.
(författare)
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Fermi observations of GRB 090510 : A short-hard gamma-ray burst with an additional, hard power-law component from 10 keV to GeV energies
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 716:2, s. 1178-1190
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Tidskriftsartikel (refereegranskat)abstract
- We present detailed observations of the bright short-hard gamma-ray burst GRB 090510 made with the Gammaray Burst Monitor (GBM) and Large Area Telescope (LAT) on board the Fermi observatory. GRB 090510 is the first burst detected by the LAT that shows strong evidence for a deviation from a Band spectral fitting function during the prompt emission phase. The time-integrated spectrum is fit by the sum of a Band function with E-peak = 3.9 +/- 0.3 MeV, which is the highest yet measured, and a hard power-law component with photon index -1.62 +/- 0.03 that dominates the emission below approximate to 20 keV and above approximate to 100 MeV. The onset of the high-energy spectral component appears to be delayed by similar to 0.1 s with respect to the onset of a component well fit with a single Band function. A faint GBM pulse and a LAT photon are detected 0.5 s before the main pulse. During the prompt phase, the LAT detected a photon with energy 30.5(-2.6)(+5.8) GeV, the highest ever measured from a short GRB. Observation of this photon sets a minimum bulk outflow Lorentz factor, Gamma greater than or similar to 1200, using simple.. opacity arguments for this GRB at redshift z = 0.903 and a variability timescale on the order of tens of ms for the approximate to 100 keV-few MeV flux. Stricter high confidence estimates imply Gamma greater than or similar to 1000 and still require that the outflows powering short GRBs are at least as highly relativistic as those of long-duration GRBs. Implications of the temporal behavior and power-law shape of the additional component on synchrotron/synchrotron self-Compton, external-shock synchrotron, and hadronic models are considered.
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41. |
- Ackermann, M., et al.
(författare)
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MULTI-WAVELENGTH OBSERVATIONS OF BLAZAR AO 0235+164 IN THE 2008-2009 FLARING STATE
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 751:2
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Tidskriftsartikel (refereegranskat)abstract
- The blazarAO 0235+164 (z=0.94) has been one of the most active objects observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008. In addition to the continuous coverage by Fermi, contemporaneous observations were carried out from the radio to gamma-ray bands between 2008 September and 2009 February. In this paper, we summarize the rich multi-wavelength data collected during the campaign (including F-GAMMA, GASP-WEBT, Kanata, OVRO, RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation between the light curves measured in the different energy bands, and interpret the resulting spectral energy distributions in the context of well-known blazar emission models. We find that the gamma-ray activity is well correlated with a series of near-IR/optical flares, accompanied by an increase in the optical polarization degree. On the other hand, the X-ray light curve shows a distinct 20 day high state of unusually soft spectrum, which does not match the extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret this feature as the bulk Compton emission by cold electrons contained in the jet, which requires an accretion disk corona with an effective covering factor of 19% at a distance of 100 R-g. We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus.
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42. |
- Abdo, A. A., et al.
(författare)
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Detection of the Small Magellanic Cloud in gamma-rays with Fermi/LAT
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 523, s. A46-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The flux of gamma rays with energies greater than 100 MeV is dominated by diffuse emission coming from cosmic-rays (CRs) illuminating the interstellar medium (ISM) of our Galaxy through the processes of Bremsstrahlung, pion production and decay, and inverse-Compton scattering. The study of this diffuse emission provides insight into the origin and transport of cosmic rays. Aims. We searched for gamma-ray emission from the Small Magellanic Cloud (SMC) in order to derive constraints on the cosmic-ray population and transport in an external system with properties different from the Milky Way. Methods. We analysed the first 17 months of continuous all-sky observations by the Large Area Telescope (LAT) of the Fermi mission to determine the spatial distribution, flux and spectrum of the gamma-ray emission from the SMC. We also used past radio synchrotron observations of the SMC to study the population of CR electrons specifically. Results. We obtained the first detection of the SMC in high-energy gamma rays, with an integrated >100 MeV flux of (3.7 +/- 0.7) x 10(-8) ph cm(-2) s(-1), with additional systematic uncertainty of <= 16%. The emission is steady and from an extended source similar to 3 degrees in size. It is not clearly correlated with the distribution of massive stars or neutral gas, nor with known pulsars or supernova remnants, but a certain correlation with supergiant shells is observed. Conclusions. The observed flux implies an upper limit on the average CR nuclei density in the SMC of similar to 15% of the value measured locally in the Milky Way. The population of high-energy pulsars of the SMC may account for a substantial fraction of the gamma-ray flux, which would make the inferred CR nuclei density even lower. The average density of CR electrons derived from radio synchrotron observations is consistent with the same reduction factor but the uncertainties are large. From our current knowledge of the SMC, such a low CR density does not seem to be due to a lower rate of CR injection and rather indicates a smaller CR confinement volume characteristic size.
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43. |
- Abdo, A. A., et al.
(författare)
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Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes
- 2009
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Ingår i: Physical Review D. Particles and fields. - 0556-2821 .- 1089-4918. ; 103:25
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Tidskriftsartikel (refereegranskat)abstract
- The diffuse galactic γ-ray emission is produced by cosmic rays (CRs) interacting with the interstellar gas and radiation field. Measurements by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory indicated excess γ-ray emission ≳1GeV relative to diffuse galactic γ-ray emission models consistent with directly measured CR spectra (the so-called “EGRET GeV excess”). The Large Area Telescope (LAT) instrument on the Fermi Gamma-Ray Space Telescope has measured the diffuse γ-ray emission with improved sensitivity and resolution compared to EGRET. We report on LAT measurements for energies 100 MeV to 10 GeV and galactic latitudes 10°≤|b|≤20°. The LAT spectrum for this region of the sky is well reproduced by a diffuse galactic γ-ray emission model that is consistent with local CR spectra and inconsistent with the EGRET GeV excess.
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44. |
- Abdo, A. A., et al.
(författare)
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Fermi large area telescope observations of PSR J1836+5925
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 712:2, s. 1209-1218
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Tidskriftsartikel (refereegranskat)abstract
- The discovery of the gamma-ray pulsar PSR J1836+5925, powering the formerly unidentified EGRET source 3EG J1835+5918, was one of the early accomplishments of the Fermi Large Area Telescope (LAT). Sitting 25 degrees off the Galactic plane, PSR J1836+5925 is a 173 ms pulsar with a characteristic age of 1.8 million years, a spindown luminosity of 1.1 x 10(34) erg s(-1), and a large off-peak (OP) emission component, making it quite unusual among the known gamma-ray pulsar population. We present an analysis of one year of LAT data, including an updated timing solution, detailed spectral results, and a long-term light curve showing no indication of variability. No evidence for a surrounding pulsar wind nebula is seen and the spectral characteristics of the OP emission indicate it is likely magnetospheric. Analysis of recent XMM-Newton observations of the X-ray counterpart yields a detailed characterization of its spectrum, which, like Geminga, is consistent with that of a neutron star showing evidence for both magnetospheric and thermal emission.
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45. |
- Abdo, A. A., et al.
(författare)
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FERMI-LAT OBSERVATIONS OF THE GEMINGA PULSAR
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 720:1, s. 272-283
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Tidskriftsartikel (refereegranskat)abstract
- We report on the Fermi-LAT observations of the Geminga pulsar, the second brightest non-variable GeV source in the gamma-ray sky and the first example of a radio-quiet gamma-ray pulsar. The observations cover one year, from the launch of the Fermi satellite through 2009 June 15. A data sample of over 60,000 photons enabled us to build a timing solution based solely on gamma-rays. Timing analysis shows two prominent peaks, separated by Delta phi = 0.497 +/- 0.004 in phase, which narrow with increasing energy. Pulsed gamma-rays are observed beyond 18 GeV, precluding emission below 2.7 stellar radii because of magnetic absorption. The phase-averaged spectrum was fitted with a power law with exponential cutoff of spectral index Gamma = (1.30 +/- 0.01 +/- 0.04), cutoff energy E-0 = (2.46 +/- 0.04 +/- 0.17) GeV, and an integral photon flux above 0.1 GeV of (4.14 +/- 0.02 +/- 0.32) x 10(-6) cm(-2) s(-1). The first uncertainties are statistical and the second ones are systematic. The phase-resolved spectroscopy shows a clear evolution of the spectral parameters, with the spectral index reaching a minimum value just before the leading peak and the cutoff energy having maxima around the peaks. The phase-resolved spectroscopy reveals that pulsar emission is present at all rotational phases. The spectral shape, broad pulse profile, and maximum photon energy favor the outer magnetospheric emission scenarios.
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46. |
- Abdo, A. A., et al.
(författare)
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OBSERVATION OF SUPERNOVA REMNANT IC 443 WITH THE FERMI LARGE AREA TELESCOPE
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 712:1, s. 459-468
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Tidskriftsartikel (refereegranskat)abstract
- We report observation of the supernova remnant (SNR) IC 443 (G189.1+3.0) with the Fermi Gamma-ray Space Telescope Large Area Telescope (LAT) in the energy band between 200 MeV and 50 GeV. IC 443 is a shell-type SNR with mixed morphology located off the outer Galactic plane where high-energy emission has been detected in the X-ray, GeV and TeV gamma-ray bands. Past observations suggest IC 443 has been interacting with surrounding interstellar matter. Proximity between dense shocked molecular clouds and GeV-TeV gamma-ray emission regions detected by EGRET, MAGIC, and VERITAS suggests an interpretation that cosmic-ray (CR) particles are accelerated by the SNR. With the high gamma-ray statistics and broad energy coverage provided by the LAT, we accurately characterize the gamma-ray emission produced by the CRs accelerated at IC 443. The emission region is extended in the energy band with theta(68) = 0 degrees.27 +/- 0 degrees.01(stat) +/- 0 degrees.03(sys) for an assumed two-dimensional Gaussian profile and overlaps almost completely with the extended source region of VERITAS. Its centroid is displaced significantly from the known pulsar wind nebula (PWN) which suggests the PWN is not the major contributor in the present energy band. The observed spectrum changes its power-law slope continuously and continues smoothly to the MAGIC and VERITAS data points. The combined gamma-ray spectrum (200 MeV < E < 2 TeV) is reproduced well by decays of neutral pions produced by a broken power-law proton spectrum with a break around 70 GeV.
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47. |
- Abdo, A. A., et al.
(författare)
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THE FERMI-LAT HIGH-LATITUDE SURVEY : SOURCE COUNT DISTRIBUTIONS AND THE ORIGIN OF THE EXTRAGALACTIC DIFFUSE BACKGROUND
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 720:1, s. 435-453
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Tidskriftsartikel (refereegranskat)abstract
- This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the Fermi-LAT survey. In this work, we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, Fermi sources are on average steeper than previously found (e.g., in the bright source list) with an average photon index of 2.40 +/- 0.02 over the entire 0.1-100 GeV energy band. We confirm that flat spectrum radio quasars have steeper spectra than BL Lacertae objects with an average index of 2.48 +/- 0.02 versus 2.18 +/- 0.02. Using several methods, we build the deepest source count distribution at GeV energies, deriving that the intrinsic source (i.e., blazar) surface density at F-100 >= 10(-9) ph cm(2) s(-1) is 0.12(-0.02)(+0.03) deg(-2). The integration of the source count distribution yields that point sources contribute 16(+/- 1.8)% (+/- 7% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT, we can rule out the hypothesis that pointlike sources (i.e., blazars) produce a larger fraction of the diffuse emission.
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48. |
- Abdo, A. A., et al.
(författare)
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The first fermi large area telescope catalog of gamma-ray pulsars
- 2010
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 187:2, s. 460-494
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Tidskriftsartikel (refereegranskat)abstract
- The dramatic increase in the number of known gamma-ray pulsars since the launch of the Fermi Gamma-ray Space Telescope (formerly GLAST) offers the first opportunity to study a sizable population of these high-energy objects. This catalog summarizes 46 high-confidence pulsed detections using the first six months of data taken by the Large Area Telescope (LAT), Fermi's main instrument. Sixteen previously unknown pulsars were discovered by searching for pulsed signals at the positions of bright gamma-ray sources seen with the LAT, or at the positions of objects suspected to be neutron stars based on observations at other wavelengths. The dimmest observed flux among these gamma-ray-selected pulsars is 6.0 x 10(-8) ph cm(-2) s(-1) (for E > 100 MeV). Pulsed gamma-ray emission was discovered from 24 known pulsars by using ephemerides (timing solutions) derived from monitoring radio pulsars. Eight of these new gamma-ray pulsars are millisecond pulsars. The dimmest observed flux among the radio-selected pulsars is 1.4 x 10(-8) ph cm(-2) s(-1) (for E > 100 MeV). The remaining six gamma-ray pulsars were known since the Compton Gamma Ray Observatory mission, or before. The limiting flux for pulse detection is non-uniform over the sky owing to different background levels, especially near the Galactic plane. The pulsed energy spectra can be described by a power law with an exponential cutoff, with cutoff energies in the range similar to 1-5 GeV. The rotational energy-loss rate ((E) over dot) of these neutron stars spans five decades, from similar to 3 x 10(33) erg s(-1) to 5 x 10(38) erg s(-1), and the apparent efficiencies for conversion to gammaray emission range from similar to 0.1% to similar to unity, although distance uncertainties complicate efficiency estimates. The pulse shapes show substantial diversity, but roughly 75% of the gamma-ray pulse profiles have two peaks, separated by greater than or similar to 0.2 of rotational phase. For most of the pulsars, gamma-ray emission appears to come mainly from the outer magnetosphere, while polar-cap emission remains plausible for a remaining few. Spatial associations imply that many of these pulsars power pulsar wind nebulae. Finally, these discoveries suggest that gamma-ray-selected young pulsars are born at a rate comparable to that of their radio-selected cousins and that the birthrate of all young gamma-ray-detected pulsars is a substantial fraction of the expected Galactic supernova rate.
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49. |
- Abdo, A. A., et al.
(författare)
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THE VELA PULSAR : RESULTS FROM THE FIRST YEAR OF FERMI LAT OBSERVATIONS
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 713:1, s. 154-165
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Tidskriftsartikel (refereegranskat)abstract
- We report on analysis of timing and spectroscopy of the Vela pulsar using 11 months of observations with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. The intrinsic brightness of Vela at GeV energies combined with the angular resolution and sensitivity of the LAT allows us to make the most detailed study to date of the energy-dependent light curves and phase-resolved spectra, using a LAT-derived timing model. The light curve consists of two peaks (P1 and P2) connected by bridge emission containing a third peak (P3). We have confirmed the strong decrease of the P1/P2 ratio with increasing energy seen with EGRET and previous Fermi LAT data, and observe that P1 disappears above 20 GeV. The increase with energy of the mean phase of the P3 component can be followed with much greater detail, showing that P3 and P2 are present up to the highest energies of pulsation. We find significant pulsed emission at phases outside the main profile, indicating that magnetospheric emission exists over 80% of the pulsar period. With increased high-energy counts the phase-averaged spectrum is seen to depart from a power law with simple exponential cutoff, and is better fit with a more gradual cutoff. The spectra in fixed-count phase bins are well fit with power laws with exponential cutoffs, revealing a strong and complex phase dependence of the cutoff energy, especially in the peaks. By combining these results with predictions of the outer magnetosphere models that map emission characteristics to phase, it will be possible to probe the particle acceleration and the structure of the pulsar magnetosphere with unprecedented detail.
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50. |
- Abdollahi, S., et al.
(författare)
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Incremental Fermi Large Area Telescope Fourth Source Catalog
- 2022
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 260:2
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Tidskriftsartikel (refereegranskat)abstract
- We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area Telescope (LAT) catalog of γ-ray sources. Based on the first 12 years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss the degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources.
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