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F. Ryde,
M. Axelsson,
B. B. Zhang,
S. McGlynn,
A. Pe'er,
C. Lundman,
S. Larsson, M. Battelino,
B. Zhang,
E. Bissaldi, [......],
J. Chiang,
F. de Palma,
S. Guiriec,
J. Larsson,
F. Longo,
S. McBreen,
N. Omodei,
V. Petrosian,
R. Preece,
and A. J. van der Horst
[show abstract]
[hide abstract]
ABSTRACT: The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300 GeV. Here we show that the prompt burst spectrum is consistent with emission from the jet photosphere combined with nonthermal emission described by a single power law with photon index –1.9. The photosphere gives rise to a strong quasi-blackbody spectrum which is somewhat broader than a single Planck function and has a characteristic temperature of ~290 keV. We model the photospheric emission with a multicolor blackbody, and its shape indicates that the photospheric radius increases at higher latitudes. We derive the averaged photospheric radius R ph = (1.1 ± 0.3) × 1012 Y 1/4 cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of 2 and has a maximal value of Γ = 750 Y 1/4. Here, Y is the ratio between the total fireball energy and the energy emitted in the gamma rays. We find that during the first quarter of the prompt phase the photospheric emission dominates, which explains the delayed onset of the observed flux in the LAT compared to the GBM. We interpret the broadband emission as synchrotron emission at R ~ 4 × 1015 cm. Our analysis emphasizes the importance of having high temporal resolution when performing spectral analysis on gamma-ray bursts, since there is strong spectral evolution.
The Astrophysical Journal Letters 01/2010; 709(2):L172. · 5.53 Impact Factor
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D. L. Band,
M. Axelsson,
L Baldini,
G. Barbiellini,
M. G. Baring,
D. Bastieri, M. Battelino,
R. Bellazzini,
E. Bissaldi,
G. Bogaert, [......],
P. M. Saz Parkinson,
J. D. Scargle,
C. Sgrò,
T. Shimokawabe,
P D Smith,
G. Spandre,
P. Spinelli,
M. Stamatikos,
B. L. Winer,
R. Yamazaki
[show abstract]
[hide abstract]
ABSTRACT: The LAT instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst phenomena in the > 100 MeV band. The synergy with Fermi's GBM 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. Comment: Accepted by ApJ
06/2009;
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A A Abdo,
M Ackermann,
M Ajello,
W B Atwood,
M Axelsson,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri, M Battelino, [......],
V Vasileiou,
N Vilchez,
V Vitale,
A P Waite,
E Wallace,
P Wang,
B L Winer,
K S Wood,
T Ylinen,
M Ziegler
[show abstract]
[hide abstract]
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.
Physical Review Letters 06/2009; 102(18):181101. · 7.37 Impact Factor
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A A Abdo,
M Ackermann,
M Arimoto,
K Asano,
W B Atwood,
M Axelsson,
L Baldini,
J Ballet,
D L Band,
G Barbiellini, [......],
A von Kienlin,
A P Waite,
D A Williams,
C Wilson-Hodge,
B L Winer,
K S Wood,
X F Wu,
R Yamazaki,
T Ylinen,
M Ziegler
[show abstract]
[hide abstract]
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 gammaray 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.
Science 03/2009; 323(5922):1688-93. · 31.20 Impact Factor
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A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri, M. Battelino,
B. M. Baughman, [......],
M. Kramer,
F. Kuehn,
M. Kuss,
J. Lande,
L. Latronico,
S. -H. Lee,
M. Lemoine-Goumard,
F. Longo,
F. Loparco,
others
Astrophysical Journal. 01/2009; 700(2):1059-1066.
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A. A. Abdo,
M. Ackermann,
W. B. Atwood,
M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri, M. Battelino,
B. M. Baughman, [......],
F. Longo,
F. Loparco,
B. Lott,
M. N. Lovellette,
P. Lubrano,
G. M. Madejski,
A. Makeev,
M. Marelli,
M. N. Mazziotta,
others
Astrophysical Journal. 01/2009; 699(2):1171-1177.
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A. A. Abdo,
M. Ackermann,
M. Ajello,
M. Axelsson,
L Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri, M. Battelino,
B. M. Baughman, [......],
A P Waite,
B. L. Winer,
K. S. Wood,
T. Ylinen,
J. A. Zensus,
M Ziegler,
G. Ghisellini,
L. Maraschi,
F. Tavecchio,
E. Angelakis
[show abstract]
[hide abstract]
ABSTRACT: We report the discovery by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope of high-energy. gamma-ray emission from the peculiar quasar PMN J0948+0022 (z = 0.5846). The optical spectrum of this object exhibits rather narrow H beta (FWHM(H beta) similar to 1500 km s(-1)), weak forbidden lines, and is therefore classified as a narrow-line type I quasar. This class of objects is thought to have relatively small black hole mass and to accrete at a high Eddington ratio. The radio loudness and variability of the compact radio core indicate the presence of a relativistic jet. Quasi-simultaneous radio/optical/X-ray and gamma-ray observations are presented. Both radio and gamma-ray emissions (observed over five months) are strongly variable. The simultaneous optical and X-ray data from Swift show a blue continuum attributed to the accretion disk and a hard X-ray spectrum attributed to the jet. The resulting broadband spectral energy distribution (SED) and, in particular, the gamma-ray spectrum measured by Fermi are similar to those of more powerful Flat-Spectrum Radio Quasars (FSRQs). A comparison of the radio and gamma-ray characteristics of PMN J0948+0022 with the other blazars detected by LAT shows that this source has a relatively low radio and gamma-ray power with respect to other FSRQs. The physical parameters obtained from modeling the SED also fall at the low power end of the FSRQ parameter region discussed in Celotti & Ghisellini. We suggest that the similarity of the SED of PMN J0948+0022 to that of more massive and more powerful quasars can be understood in a scenario in which the SED properties depend on the Eddington ratio rather than on the absolute power.
Astrophysical Journal, v.699, 976-984 (2009). 01/2009;
-
A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood,
M. Axelsson,
L. Baldini,
J. Ballet,
D. L. Band,
G. Barbiellini,
D. Bastieri, [......],
W. N. Johnson,
T. Kamae,
H. Katagiri,
J. Kataoka,
N. Kawai,
M. Kerr,
J. Knoedlseder,
D. Kocevski,
M. L. Kocian,
others
Astrophysical Journal Supplement Series. 01/2009; 183(1):46-66.
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A. A. Abdo,
M. Ackermann,
M. Arimoto,
K. Asano,
W. B. Atwood,
M. Axelsson,
L. Baldini,
J. Ballet,
D. L. Band,
G. Barbiellini, [......],
M. -H. Grondin,
J. E. Grove,
L. Guillemot,
S. Guiriec,
G. Haller,
Y. Hanabata,
A. K. Harding,
M. Hayashida,
E. Hays,
others
Science. 01/2009; 323(5922):1688-1693.
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A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood,
M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri, M. Battelino, [......],
M. L. Kocian,
F. Kuehn,
M. Kuss,
L. Latronico,
S. H. Lee,
M. Lemoine-Goumard,
F. Longo,
F. Loparco,
B. Lott,
others
[show abstract]
[hide abstract]
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.
Astrophysical Journal. 01/2009; 699(1):817-823.
-
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
U. Barres de Almeida,
A. R. Bazer-Bachi,
Y. Becherini,
B. Behera,
K. Bernl¨ohr,
C. Boisson,
A. Bochow, [......],
G. Tosti,
A. Tramacere,
Y Uchiyama,
T. L. Usher,
N. Vilchez,
M. Villata,
V Vitale,
A P Waite,
K. S. Wood,
T. Ylinen
[show abstract]
[hide abstract]
ABSTRACT: We report on the first simultaneous observations that cover the optical, X-ray, and high energy gamma-ray bands of the BL Lac object PKS 2155-304. The gamma-ray bands were observed for 11 days, between 25 August and 6 September 2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S. atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral energy distribution with the new generation of gamma-ray telescopes. The ATOM telescope and the RXTE and Swift observatories provided optical and X-ray coverage of the low-energy component over the same time period. The object was close to the lowest archival X-ray and Very High Energy state, whereas the optical flux was much higher. The light curves show relatively little (~30%$) variability overall when compared to past flaring episodes, but we find a clear optical/VHE correlation and evidence for a correlation of the X-rays with the high energy spectral index. Contrary to previous observations in the flaring state, we do not find any correlation between the X-ray and VHE components. Although synchrotron self-Compton models are often invoked to explain the SEDs of BL Lac objects, the most common versions of these models are at odds with the correlated variability we find in the various bands for PKS 2155-304.
The Astrophysical Journal / The Astrophysical Journal Letters; Astrophysical Journal Letters to the Editor.
-
A. A. Abdo,
M. Ackermann,
M. Ajello,
W B Atwood,
M. Axelsson,
L Baldini,
J. Ballet,
D. L. Band,
G. Barbiellini,
D. Bastieri, [......],
N. Vilchez,
V Vitale,
A P Waite,
E. Wallace,
P Wang,
K. Watters,
B. L. Winer,
K. S. Wood,
T. Ylinen,
M Ziegler
Astrophysical Journal, Supplement Series, v.183, 46-66 (2009).
-
A. A. Abdo,
M. Ackermann,
M. Ajello,
M. Axelsson,
L Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri, M. Battelino,
B. M. Baughman, [......],
Y Uchiyama,
T. L. Usher,
N. Vilchez,
V Vitale,
A P Waite,
B. L. Winer,
K. S. Wood,
Y. Ylinen,
J. A. Zensus,
M Ziegler
[show abstract]
[hide abstract]
ABSTRACT: We report the discovery by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope of high-energy γ-ray emission from the peculiar quasar PMN J0948+0022 (z = 0.5846). The optical spectrum of this object exhibits rather narrow Hβ (FWHM(Hβ) ~1500 km s^–1), weak forbidden lines, and is therefore classified as a narrow-line type I quasar. This class of objects is thought to have relatively small black hole mass and to accrete at a high Eddington ratio. The radio loudness and variability of the compact radio core indicate the presence of a relativistic jet. Quasi-simultaneous radio/optical/X-ray and γ-ray observations are presented. Both radio and γ-ray emissions (observed over five months) are strongly variable. The simultaneous optical and X-ray data from Swift show a blue continuum attributed to the accretion disk and a hard X-ray spectrum attributed to the jet. The resulting broadband spectral energy distribution (SED) and, in particular, the γ-ray spectrum measured by Fermi are similar to those of more powerful Flat-Spectrum Radio Quasars (FSRQs). A comparison of the radio and γ-ray characteristics of PMN J0948+0022 with the other blazars detected by LAT shows that this source has a relatively low radio and γ-ray power with respect to other FSRQs. The physical parameters obtained from modeling the SED also fall at the low power end of the FSRQ parameter region discussed in Celotti & Ghisellini. We suggest that the similarity of the SED of PMN J0948+0022 to that of more massive and more powerful quasars can be understood in a scenario in which the SED properties depend on the Eddington ratio rather than on the absolute power.
-
A. A. Abdo,
M. Ackermann,
M. Ajello,
W B Atwood,
M. Axelsson,
L Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri, M. Battelino, [......],
V. Vasileiou,
N. Vilchez,
V Vitale,
A P Waite,
E. Wallace,
P Wang,
B. L. Winer,
K. S. Wood,
T. Ylinen,
M Ziegler
Physical Review Letters, v.102 (2009).
