| 1. |
- 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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| 2. |
- Abdo, A. A., et al.
(författare)
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THE FIRST FERMI MULTIFREQUENCY CAMPAIGN ON BL LACERTAE : CHARACTERIZING THE LOW-ACTIVITY STATE OF THE EPONYMOUS BLAZAR
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 730:2, s. 101-
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Tidskriftsartikel (refereegranskat)abstract
- We report on observations of BL Lacertae during the first 18 months of Fermi LAT science operations and present results from a 48 day multifrequency coordinated campaign from 2008 August 19 to 2008 October 7. The radio to gamma-ray behavior of BL Lac is unveiled during a low-activity state thanks to the coordinated observations of radio-band (Metsahovi and VLBA), near-IR/optical (Tuorla, Steward, OAGH, and MDM), and X-ray (RXTE and Swift) observatories. No variability was resolved in gamma rays during the campaign, and the brightness level was 15 times lower than the level of the 1997 EGRET outburst. Moderate and uncorrelated variability has been detected in UV and X-rays. The X-ray spectrum is found to be concave, indicating the transition region between the low- and high-energy components of the spectral energy distribution (SED). VLBA observation detected a synchrotron spectrum self-absorption turnover in the innermost part of the radio jet appearing to be elongated and inhomogeneous, and constrained the average magnetic field there to be less than 3 G. Over the following months, BL Lac appeared variable in gamma rays, showing flares (in 2009 April and 2010 January). There is no evidence for the correlation of gamma rays with the optical flux monitored from the ground in 18 months. The SED may be described by a single-zone or a two-zone synchrotron self-Compton (SSC) model, but a hybrid SSC plus external radiation Compton model seems to be preferred based on the observed variability and the fact that it provides a fit closest to equipartition.
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| 3. |
- Ackermann, M., et al.
(författare)
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THE THIRD CATALOG OF ACTIVE GALACTIC NUCLEI DETECTED BY THE FERMI LARGE AREA TELESCOPE
- 2015
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 810:1
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Tidskriftsartikel (refereegranskat)abstract
- The third catalog of active galactic nuclei (AGNs) detected by the Fermi-LAT (3LAC) is presented. It is based on the third Fermi-LAT catalog (3FGL) of sources detected between 100 MeV and 300 GeV with a Test Statistic greater than 25, between 2008 August 4 and 2012 July 31. The 3LAC includes 1591 AGNs located at high Galactic latitudes (vertical bar b vertical bar > 10 degrees), a 71% increase over the second catalog based on 2 years of data. There are 28 duplicate associations, thus 1563 of the 2192 high-latitude gamma-ray sources of the 3FGL catalog are AGNs. Most of them (98%) are blazars. About half of the newly detected blazars are of unknown type, i.e., they lack spectroscopic information of sufficient quality to determine the strength of their emission lines. Based on their gamma-ray spectral properties, these sources are evenly split between flat-spectrum radio quasars (FSRQs) and BL Lacs. The most abundant detected BL Lacs are of the high-synchrotron-peaked (HSP) type. About 50% of the BL Lacs have no measured redshifts. A few new rare outliers (HSP-FSRQs and high-luminosity HSP BL Lacs) are reported. The general properties of the 3LAC sample confirm previous findings from earlier catalogs. The fraction of 3LAC blazars in the total population of blazars listed in BZCAT remains non-negligible even at the faint ends of the BZCAT-blazar radio, optical, and X-ray flux distributions, which hints that even the faintest known blazars could eventually shine in gamma-rays at LAT-detection levels. The energy-flux distributions of the different blazar populations are in good agreement with extrapolation from earlier catalogs.
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| 4. |
- 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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| 5. |
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| 6. |
- Sokolovsky, K. V., et al.
(författare)
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Two active states of the narrow-line gamma-ray-loud AGN GB 1310+487
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 565
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Tidskriftsartikel (refereegranskat)abstract
- Context. Previously unremarkable, the extragalactic radio source GB 1310+487 showed gamma-ray flare on 2009 November 18, reaching a daily flux of similar to 10(-6) photons cm(-2) s(-1) at energies E > 100 MeV and became one of the brightest GeV sources for about two weeks. Its optical spectrum shows strong forbidden-line emission while lacking broad permitted lines, which is not typical for a blazar. Instead, the spectrum resembles those of narrow emission-line galaxies. Aims. We investigate changes in the object's radio-to-GeV spectral energy distribution (SED) during and after the prominent gamma-ray flare with the aim of determining the nature of the object and of constraining the origin of the variable high-energy emission. Methods. The data collected by the Fermi and AGILE satellites at gamma-ray energies; Swift at X-ray and ultraviolet (UV); the Kanata, NOT, and Keck telescopes at optical; OAGH and WISE at infrared (IR); and IRAM 30m, OVRO 40m, Effelsberg 100 m, RATAN-600, and VLBA at radio are analyzed together to trace the SED evolution on timescales of months. Results. The gamma-ray/ radio-loud narrow-line active galactic nucleus (AGN) is located at redshift z = 0.638. It shines through an unrelated foreground galaxy at z = 0.500. The AGN light is probably amplified by gravitational lensing. The AGN SED shows a two-humped structure typical of blazars and gamma-ray-loud narrow-line Seyfert 1 galaxies, with the high-energy (inverse-Compton) emission dominating by more than an order of magnitude over the low-energy (synchrotron) emission during gamma-ray flares. The difference between the two SED humps is smaller during the low-activity state. Fermi observations reveal a strong correlation between the gamma-ray flux and spectral index, with the hardest spectrum observed during the brightest gamma-ray state. The gamma-ray flares occurred before and during a slow rising trend in the radio, but no direct association between gamma-ray and radio flares could be established. Conclusions. If the gamma-ray flux is a mixture of synchrotron self-Compton and external Compton emission, the observed GeV spectral variability may result from varying relative contributions of these two emission components. This explanation fits the observed changes in the overall IR to gamma-ray SED.
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| 7. |
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| 8. |
- D'Ammando, F., et al.
(författare)
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Long-term monitoring of PKS 0537-441 with Fermi-LAT and multiwavelength observations
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 431:3, s. 2481-2492
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Tidskriftsartikel (refereegranskat)abstract
- We report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through gamma-rays by Submillimeter Array, Rapid Eye Mounting, Automatic Telescope for Optical Monitoring (ATOM), Swift and Fermi mostly during 2008 August-2010 April. Strong variability has been observed in gamma-rays, with two major flaring episodes (2009 July and 2010 March) and a harder-when-brighter behaviour, quite common for flat spectrum radio quasars and low-synchrotron-peaked BL Lacertae objects (BL Lacs), in 2010 March. In the same way, the spectral energy distribution (SED) of the source cannot be modelled by a simple synchrotron self-Compton model, as opposed to many BL Lacs, but the addition of an external Compton component of seed photons from a dust torus is needed. The 230 GHz light curve showed an increase simultaneous with the gamma-ray one, indicating co-spatiality of the mm and gamma-ray emission region likely at large distance from the central engine. The low, average, and high activity SED of the source could be fit changing only the electron distribution parameters, but two breaks in the electron distribution are necessary. The ensuing extra spectral break, located at near-infrared (NIR)-optical frequencies, together with that in gamma-rays seem to indicate a common origin, most likely due to an intrinsic feature in the underlying electron distribution. An overall correlation between the gamma-ray band with the R band and K band has been observed with no significant time lag. On the other hand, when inspecting the light curves on short time-scales some differences are evident. In particular, flaring activity has been detected in NIR and optical bands with no evident gamma-ray counterparts in 2009 September and November. Moderate variability has been observed in X-rays with no correlation between flux and photon index. An increase of the detected X-ray flux with no counterpart at the other wavelengths has been observed in 2008 October, suggesting once more a complex correlation between the emission at different energy bands.
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| 9. |
- Marti-Vidal, Ivan, 1980, et al.
(författare)
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On the calibration of full-polarization 86GHz global VLBI observations
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 542, s. A107-
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Tidskriftsartikel (refereegranskat)abstract
- We report the development of a semi-automatic pipeline for the calibration of 86GHz full-polarization observations performed with the Global Millimeter-VLBI array (GMVA) and describe the calibration strategy followed in the data reduction. Our calibration pipeline involves non-standard procedures, since VLBI polarimetry at frequencies above 43GHz has not yet been well established. We also present, for the first time, a full-polarization global-VLBI image at 86GHz (source 3C 345), as an example of the final product of our calibration pipeline, and discuss the effect of instrumental limitations on the fidelity of the polarization images. Our calibration strategy is not exclusive to the GMVA, and could be applied to other VLBI arrays at millimeter wavelengths. The use of this pipeline will allow GMVA observers to obtain fully calibrated datasets shortly after the data correlation.
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| 10. |
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