| 1. |
- Abdo, A. A., et al.
(author)
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Fermi Observations of High-Energy Gamma-Ray Emission from GRB 080916C
- 2009
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 323:5922, s. 1688-1693
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Journal article (peer-reviewed)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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| 2. |
- Abdo, A. A., et al.
(author)
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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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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 700:1, s. 597-622
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Journal article (peer-reviewed)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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| 3. |
- Abdo, A. A., et al.
(author)
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EARLY FERMI GAMMA-RAY SPACE TELESCOPE OBSERVATIONS OF THE QUASAR 3C 454.3
- 2009
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 699:1, s. 817-823
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Journal article (peer-reviewed)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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| 4. |
- Abdo, A. A., et al.
(author)
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FERMI OBSERVATIONS OF GRB 090902B : A DISTINCT SPECTRAL COMPONENT IN THE PROMPT AND DELAYED EMISSION
- 2009
-
In: Astrophysical Journal Letters. - 2041-8205 .- 0004-637X .- 1538-4357. ; 706:1, s. L138-L144
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Journal article (peer-reviewed)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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| 5. |
- Abdo, A. A., et al.
(author)
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The on-orbit calibration of the Fermi Large Area Telescope
- 2009
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In: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 32:3-4, s. 193-219
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Journal article (peer-reviewed)abstract
- The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope began its on-orbit operations on June 23, 2008. Calibrations, defined in a generic sense, correspond to synchronization of trigger signals, optimization of delays for latching data, determination of detector thresholds, gains and responses, evaluation of the perimeter of the South Atlantic Anomaly (SAA), measurements of live time, of absolute time, and internal and spacecraft boresight alignments. Here we describe on-orbit calibration results obtained using known astrophysical sources, galactic cosmic rays, and charge injection into the front-end electronics of each detector. Instrument response functions will be described in a separate publication. This paper demonstrates the stability of calibrations and describes minor changes observed since launch. These results have been used to calibrate the LAT datasets to be publicly released in August 2009.
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| 6. |
- Band, D. L., et al.
(author)
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PROSPECTS FOR GRB SCIENCE WITH THE FERMI LARGE AREA TELESCOPE
- 2009
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 701:2, s. 1673-1694
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Research review (peer-reviewed)abstract
- The Large Area Telescope (LAT) instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst (GRB) phenomena in the > 100 MeV band. The synergy with Fermi's Gamma-ray Burst Monitor detectors will link these observations to those in the well explored 10-1000 keV range; the addition of the > 100 MeV band observations will resolve theoretical uncertainties about burst emission in both the prompt and afterglow phases. Trigger algorithms will be applied to the LAT data both onboard the spacecraft and on the ground. The sensitivity of these triggers will differ because of the available computing resources onboard and on the ground. Here we present the LAT's burst detection methodologies and the instrument's GRB capabilities.
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| 7. |
- Foley, S., et al.
(author)
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Global Characteristics Of Gamma-Ray Bursts Observed With Integral
- 2009
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In: Baltic Astronomy. - 1392-0049. ; 18:3-4, s. 279-283
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Journal article (peer-reviewed)abstract
- The gamma-ray instruments on board INTEGRAL have detected and localized 62 gamma-ray bursts (GRBs) to date. The peak flux distribution of these bursts shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. Spectral lags, i.e., the time delay in the arrival of low-energy gamma rays with respect to high-energy gamma rays, are measured for 31 of the GRBs. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags < 0.75 s (between 25-50 keV and 50-300 keV) and one with long lags > 0.75 s. Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags. They are mainly observed in the direction of the supergalactic plane and hence reflect the local large-scale structure of the Universe. This low-luminosity population appears to be distinct from typical cosmological GRBs.
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| 8. |
- Foley, S., et al.
(author)
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Spectral lags of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs
- 2009
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In: Gamma-ray bursts. - : American Institute of Physics (AIP). - 9780735406704 ; , s. 362-367
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Conference paper (peer-reviewed)abstract
- The γ-ray instruments on board INTEGRAL detected and localised 47 GRBs from its launch in October 2002 up to July 2007. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ∼0.15ph cm 2 s- 1 is ∼1400yr -1 in the fully coded field of view of IBIS. Spectral lags i.e. the time delay in the arrival of low-energy γ-rays with respect to high-energy γ-rays, are measured for 31 of the GRBs. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags<0.75 s (between 25-50 keV and 50-300 keV) and one with long lags > 0.75 s. Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies, and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q = -0.225 ±0.090 and hence reflect the local large-scale structure of the Universe. The rate of long-lag GRBs with inferred low luminosity is ∼25% of Type Ib/c SNe. Some of these bursts could be produced by the collapse of a massive star without a SN. Alternatively, they could result from a different progenitor, such as the merger of two white dwarfs or a white dwarf with a neutron star or black hole, possibly in the cluster environment without a host galaxy.
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| 9. |
- Foley, S., et al.
(author)
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The spectral lag distribution of swift gamma-ray bursts
- 2009
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In: Gamma-Ray Busts. - : American Institute of Physics (AIP). - 9780735406704 ; , s. 403-405
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Conference paper (peer-reviewed)abstract
- Swift has detected 350 GRBs from its launch in November 2004 up to August 2008. Approximately one third of these bursts have a measured redshift. We present the spectral lag distributions of long and short-duration Swift GRBs. The spectral lags are determined using a cross-correlation analysis between lightcurves in the 15-25 and 50-100 keV BAT energy channels. The proposed relationship between spectral lag and isotropic peak luminosity is investigated for the GRBs with known redshift. The sensitivity of BAT does not allow the lags of very faint bursts to be measured and so a correlation between faint, long-lag Swift GRBs and the supergalactic plane cannot be confirmed using this sample.
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| 10. |
- McGlynn, Sinéad, et al.
(author)
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High energy emission and polarisation limits for the INTEGRAL burst GRB061122
- 2009
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 499:2, s. 465-472
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Journal article (peer-reviewed)abstract
- Context. GRB061122 is one of the brightest GRBs detected within INTEGRAL's field of view to date, with a peak flux (20-200 keV) of 32 photons cm(-2) s(-1) and fluence of 2 x 10(-5) erg cm(-2). The Spectrometer aboard INTEGRAL, SPI, can measure linear polarisation in bright GRBs through the process of Compton scattering in the Germanium detectors. Polarisation measurements of the prompt emission are relatively rare. The spectral and polarisation results can be combined to provide vital information about the circumburst region. Aims. The two gamma-ray detectors on INTEGRAL were used to investigate the spectral characteristics of GRB061122. A search for linear polarisation in the prompt emission was carried out on GRB061122 using the SPI multiple event data in the energy range 100 keV-1 MeV. The X-ray properties were examined using data from the X-Ray Telescope (XRT) on Swift. Methods. The gamma-ray spectral and temporal properties of GRB061122 were determined using IBIS and SPI. The afterglow properties were obtained using XRT. The multiple event data of GRB061122 from SPI were analysed and compared with the predicted instrument response obtained from Monte-Carlo simulations using the GEANT 4 INTEGRAL mass model. The chi(2) distributions between the real and simulated data as a function of the percentage polarisation and polarisation angle were calculated and limits on the level and angle of polarisation were obtained from the best-fit value of chi(2). Results. The prompt spectrum was best fit by a combination of a blackbody and a power-law model (the quasithermal model), with evidence for high energy emission continuing above 8 MeV. A pseudo-redshift value of pz = 0.95 +/- 0.18 was determined using the spectral fit parameters. The isotropic energy at this pseudo-redshift is 8.5 x 10(52) erg. The jet opening angle was estimated to be smaller than 2.8 degrees or larger than 11.9 degrees from the X-ray lightcurve. An upper limit of 60% polarisation was determined for the prompt emission of GRB061122, using the multiple event data from the spectrometer on INTEGRAL. Conclusions. The high energy emission observed in the spectrum may be due to the reverse shock interacting with the GRB ejecta when it is decelerated by the circumburst medium. This behaviour has been observed in a small fraction of GRBs to date, but is expected to be more commonly observed by the Fermi Gamma-ray Space Telescope. The conditions for polarisation are met if the jet opening angle is less than 2.8 degrees, but further constraints on the level of polarisation are not possible.
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