1. |
- Niemi, MEK, et al.
(författare)
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- 2021
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swepub:Mat__t
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2. |
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3. |
- Kanai, M, et al.
(författare)
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- 2023
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swepub:Mat__t
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4. |
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- 2019
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Tidskriftsartikel (refereegranskat)
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5. |
- 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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6. |
- 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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7. |
- 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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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. |
- 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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10. |
- 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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