-
A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol, [......],
P. Lubrano,
G. M. Madejski,
A. Makeev,
M. N. Mazziotta,
W. McConville,
J. E. McEnery,
C. Meurer,
P. F. Michelson,
W. Mitthumsiri,
others
[show abstract]
[hide abstract]
ABSTRACT: We report the detection of high-energy gamma-ray emission from two starburst galaxies using data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. Steady point-like emission above 200 MeV has been detected at significance levels of 6.8 sigma and 4.8 sigma, respectively, from sources positionally coincident with locations of the starburst galaxies M82 and NGC 253. The total fluxes of the sources are consistent with gamma-ray emission originating from the interaction of cosmic rays with local interstellar gas and radiation fields and constitute evidence for a link between massive star formation and gamma-ray emission in star-forming galaxies.
Astrophysical Journal Letters. 01/2010; 709(2):L152-L157.
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A. A. Abdo,
M. Ackermann,
I. Agudo,
M. Ajello,
H. D. Aller,
M. F. Aller,
E. Angelakis,
A. A. Arkharov, M. Axelsson,
U. Bach, [......],
T. Glanzman,
G. Godfrey,
I. A. Grenier,
C. Gronwall,
J. E. Grove,
L. Guillemot,
S. Guiriec,
M. A. Gurwell,
D. Hadasch,
others
[show abstract]
[hide abstract]
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.
Astrophysical Journal. 01/2010; 716(1):30-70.
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A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
M. G. Baring,
D. Bastieri, [......],
M. Kramer,
F. Kuehn,
M. Kuss,
J. Lande,
L. Latronico,
S. H. Lee,
M. Lemoine-Goumard,
F. Longo,
F. Loparco,
others
[show abstract]
[hide abstract]
ABSTRACT: We report on gamma-ray observations of the Crab Pulsar and Nebula using 8 months of survey data with the Fermi Large Area Telescope (LAT). The high quality light curve obtained using the ephemeris provided by the Nancay and Jodrell Bank radio telescopes shows two main peaks stable in phase with energy. The first gamma-ray peak leads the radio main pulse by (281 +/- 12 +/- 21) mu s, giving new constraints on the production site of non-thermal emission in pulsar magnetospheres. The first uncertainty is due to gamma-ray statistics, and the second arises from the rotation parameters. The improved sensitivity and the unprecedented statistics afforded by the LAT enable precise measurement of the Crab Pulsar spectral parameters: cut-off energy at E(c) = (5.8 +/- 0.5 +/- 1.2) GeV, spectral index of Gamma = (1.97 +/- 0.02 +/- 0.06) and integral photon flux above 100 MeV of (2.09 +/- 0.03 +/- 0.18) x 10(-6) cm(-2) s(-1). The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Pulsed gamma-ray photons are observed up to similar to 20 GeV which precludes emission near the stellar surface, below altitudes of around 4-5 stellar radii in phase intervals encompassing the two main peaks. A detailed phase-resolved spectral analysis is also performed: the hardest emission from the Crab Pulsar comes from the bridge region between the two gamma-ray peaks while the softest comes from the falling edge of the second peak. The spectrum of the nebula in the energy range 100 MeV-300 GeV is well described by the sum of two power laws of indices Gamma(sync) = (3.99 +/- 0.12 +/- 0.08) and Gamma(IC) = (1.64 +/- 0.05 +/- 0.07), corresponding to the falling edge of the synchrotron and the rising edge of the inverse Compton (IC) components, respectively. This latter, which links up naturally with the spectral data points of Cherenkov experiments, is well reproduced via IC scattering from standard magnetohydrodynamic nebula models, and does not require any additional radiation mechanism.
Astrophysical Journal. 01/2010; 708(2):1254-1267.
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A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol, [......],
L. Latronico,
M. Lemoine-Goumard,
F. Longo,
F. Loparco,
B. Lott,
M. N. Lovellette,
P. Lubrano,
G. M. Madejski,
A. Makeev,
others
[show abstract]
[hide abstract]
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.
Astrophysical Journal. 01/2010; 710(2):1271-1285.
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M. De Pasquale,
P. Schady,
N. P. M. Kuin,
M. J. Page,
P. A. Curran,
S. Zane,
S. R. Oates,
S. T. Holland,
A. A. Breeveld,
E. A. Hoversten, [......],
T. Kamae,
H. Katagiri,
J. Kataoka,
N. Kawai,
M. Kerr,
R. M. Kippen,
J. Knodlseder,
D. Kocevski,
M. Kuss,
others
[show abstract]
[hide abstract]
ABSTRACT: We present the observations of GRB090510 performed by the Fermi Gamma-Ray Space Telescope and the Swift observatory. This is a bright, short burst that shows an extended emission detected in the GeV range. Furthermore, its optical emission initially rises, a feature so far observed only in long bursts, while the X-ray flux shows an initial shallow decrease, followed by a steeper decay. This exceptional behavior enables us to investigate the physical properties of the gamma-ray burst outflow, poorly known in short bursts. We discuss internal and external shock models for the broadband energy emission of this object.
Astrophysical Journal Letters. 01/2010; 709(2):L146-L151.
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A. A. Abdo,
M. Ackermann,
I. Agudo,
M. Ajello,
A. Allafort,
H. D. Aller,
M. F. Aller,
E. Antolini,
A. A. Arkharov, M. Axelsson, [......],
D. Horan,
R. E. Hughes,
G. Johannesson,
A. S. Johnson,
W. N. Johnson,
T. Kamae,
H. Katagiri,
J. Kataoka,
N. Kawai,
others
Astrophysical Journal. 01/2010; 721(2):1425-1447.
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A. A. Abdo,
M. Ackermann,
M. Ajello,
A. Allafort,
E. Antolini,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini, [......],
S. E. Healey,
A. B. Hill,
D. Horan,
R. E. Hughes,
G. Iafrate,
R. Itoh,
G. Johannesson,
A. S. Johnson,
R. P. Johnson,
others
Astrophysical Journal. 01/2010; 715(1):429-457.
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A. A. Abdo,
M. Ackermann,
M. Ajello, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
B. M. Baughman,
K. Bechtol, [......],
M. Kadler,
T. Kamae,
H. Katagiri,
J. Kataoka,
N. Kawai,
M. Kerr,
J. Knodlseder,
M. L. Kocian,
M. Kuss,
others
[show abstract]
[hide abstract]
ABSTRACT: It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight(1). The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (gamma)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and gamma-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 10(5) gravitational radii.
Nature. 01/2010; 463(7283):919-923.
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A. A. Abdo,
M. Ackermann,
M. Ajello,
K. Asano,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri, [......],
W. N. Johnson,
T. Kamae,
H. Katagiri,
J. Kataoka,
N. Kawai,
M. Kerr,
R. M. Kippen,
J. Knoedlseder,
D. Kocevski,
others
Astrophysical Journal. 01/2010; 712(1):558-564.
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M. Ackermann,
K. Asano,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
M. G. Baring,
D. Bastieri,
K. Bechtol, [......],
P. Lubrano,
A. Makeev,
M. N. Mazziotta,
J. E. McEnery,
S. McGlynn,
C. Meegan,
P. Meszaros,
P. F. Michelson,
W. Mitthumsiri,
others
[show abstract]
[hide abstract]
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.
Astrophysical Journal. 01/2010; 716(2):1178-1190.
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A. A. Abdo,
M. Ackermann,
M. Ajello,
A. Allafort,
E. Antolini,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini, [......],
S. Guiriec,
M. Gustafsson,
D. Hadasch,
Y. Hanabata,
A. K. Harding,
M. Hayashida,
E. Hays,
S. E. Healey,
A. B. Hill,
others
Astrophysical Journal Supplement Series. 01/2010; 188(2):405-436.
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P. Weltevrede,
A. A. Abdo,
M. Ackermann,
M. Ajello, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
B. M. Baughman, [......],
N. Vilchez,
V. Vitale,
A. P. Waite,
P. Wang,
N. Wang,
K. Watters,
B. L. Winer,
K. S. Wood,
T. Ylinen,
and M. Ziegler
[show abstract]
[hide abstract]
ABSTRACT: We report the detection of pulsed γ-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850, and J1718-3825 using the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their γ-ray light curves are (at least given the current count statistics) single peaked. For two pulsars, there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the γ-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the γ-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the γ-ray light curves with high-energy beam models.
The Astrophysical Journal 12/2009; 708(2):1426. · 6.02 Impact Factor
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A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
M. G. Baring,
D. Bastieri, [......],
V. Vitale,
A. P. Waite,
E. Wallace,
P. Wang,
K. Watters,
P. Weltevrede,
B. L. Winer,
K. S. Wood,
T. Ylinen,
and M. Ziegler
[show abstract]
[hide abstract]
ABSTRACT: We report on γ-ray observations of the Crab Pulsar and Nebula using 8 months of survey data with the Fermi Large Area Telescope (LAT). The high quality light curve obtained using the ephemeris provided by the Nançay and Jodrell Bank radio telescopes shows two main peaks stable in phase with energy. The first γ-ray peak leads the radio main pulse by (281 ± 12 ± 21) μs, giving new constraints on the production site of non-thermal emission in pulsar magnetospheres. The first uncertainty is due to γ-ray statistics, and the second arises from the rotation parameters. The improved sensitivity and the unprecedented statistics afforded by the LAT enable precise measurement of the Crab Pulsar spectral parameters: cut-off energy at Ec = (5.8 ± 0.5 ± 1.2) GeV, spectral index of Γ = (1.97 ± 0.02 ± 0.06) and integral photon flux above 100 MeV of (2.09 ± 0.03 ± 0.18) × 10–6 cm–2 s–1. The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Pulsed γ-ray photons are observed up to ~ 20 GeV which precludes emission near the stellar surface, below altitudes of around 4-5 stellar radii in phase intervals encompassing the two main peaks. A detailed phase-resolved spectral analysis is also performed: the hardest emission from the Crab Pulsar comes from the bridge region between the two γ-ray peaks while the softest comes from the falling edge of the second peak. The spectrum of the nebula in the energy range 100 MeV-300 GeV is well described by the sum of two power laws of indices Γsync = (3.99 ± 0.12 ± 0.08) and ΓIC = (1.64 ± 0.05 ± 0.07), corresponding to the falling edge of the synchrotron and the rising edge of the inverse Compton (IC) components, respectively. This latter, which links up naturally with the spectral data points of Cherenkov experiments, is well reproduced via IC scattering from standard magnetohydrodynamic nebula models, and does not require any additional radiation mechanism.
The Astrophysical Journal 12/2009; 708(2):1254. · 6.02 Impact Factor
-
A. A. Abdo,
M. Ackermann,
M. Ajello, M. Axelsson,
L Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
B. M. Baughman,
K. Bechtol, [......],
M. N. Lovellette,
P Lubrano,
G. M. Madejski,
A. Makeev,
W. Max-Moerbeck,
M. N. Mazziotta,
W. McConville,
J. E. McEnery,
C. Meurer,
P. F. Michelson
[show abstract]
[hide abstract]
ABSTRACT: (Abridged) We have conducted a detailed investigation of the broad-band 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 three 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 broad-band 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 SEDs 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 broad-band colors (i.e. the radio to optical and optical to X-ray spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency $\nu_p^S$ is positioned between 10$^{12.5}$ and 10$^{14.5}$ Hz in broad-lined 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 SEDs, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. (...) Comment: 85 pages, 38 figures, submitted to ApJ
12/2009;
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A A Abdo,
M Ackermann,
M Ajello,
B Anderson,
W B Atwood, M Axelsson,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri, [......],
T L Usher,
V Vasileiou,
N Vilchez,
V Vitale,
A P Waite,
P Wang,
B L Winer,
K S Wood,
T Ylinen,
M Ziegler
[show abstract]
[hide abstract]
ABSTRACT: The diffuse galactic gamma-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 gamma-ray emission greater, > or approximately equal to 1 GeV relative to diffuse galactic gamma-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 gamma-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 degrees < or = |b| < or = 20 degrees. The LAT spectrum for this region of the sky is well reproduced by a diffuse galactic gamma-ray emission model that is consistent with local CR spectra and inconsistent with the EGRET GeV excess.
Physical Review Letters 12/2009; 103(25):251101. · 7.37 Impact Factor
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A. A. Abdo,
M. Ackermann,
M. Ajello,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
M. G. Baring,
D. Bastieri, [......],
V. Vitale,
A. P. Waite,
P. Wang,
N. Wang,
K. Watters,
P. Weltevrede,
B. L. Winer,
K. S. Wood,
T. Ylinen,
and M. Ziegler
[show abstract]
[hide abstract]
ABSTRACT: We report the detection of γ-ray pulsations (≥0.1 GeV) from PSR J2229+6114 and PSR J1048–5832, the latter having been detected as a low-significance pulsar by EGRET. Data in the γ-ray band were acquired by the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope, while the radio rotational ephemerides used to fold the γ-ray light curves were obtained using the Green Bank Telescope, the Lovell telescope at Jodrell Bank, and the Parkes Telescope. The two young radio pulsars, located within the error circles of the previously unidentified EGRET sources 3EG J1048–5840 and 3EG J2227+6122, present spin-down characteristics similar to the Vela pulsar. PSR J1048–5832 shows two sharp peaks at phases 0.15 ± 0.01 and 0.57 ± 0.01 relative to the radio pulse confirming the EGRET light curve, while PSR J2229+6114 presents a very broad peak at phase 0.49 ± 0.01. The γ-ray spectra above 0.1 GeV of both pulsars are fit with power laws having exponential cutoffs near 3 GeV, leading to integral photon fluxes of (2.19 ± 0.22 ± 0.32) × 10–7 cm–2 s–1 for PSR J1048–5832 and (3.77 ± 0.22 ± 0.44) × 10–7 cm–2 s–1 for PSR J2229+6114. The first uncertainty is statistical and the second is systematic. PSR J1048–5832 is one of the two LAT sources which were entangled together as 3EG J1048–5840. These detections add to the growing number of young γ-ray pulsars that make up the dominant population of GeV γ-ray sources in the Galactic plane.
The Astrophysical Journal 11/2009; 706(2):1331. · 6.02 Impact Factor
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A. A. Abdo,
M. Ackermann,
M. Ajello,
K. Asano,
W. B. Atwood, M. Axelsson,
L. Baldini,
J. Ballet,
G. Barbiellini,
M. G. Baring, [......],
V. Vitale,
A. von Kienlin,
A. P. Waite,
P. Wang,
C. Wilson-Hodge,
B. L. Winer,
K. S. Wood,
R. Yamazaki,
T. Ylinen,
and M. Ziegler
[show abstract]
[hide abstract]
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 ~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+2.7 –3.5 GeV. This event arrived 82 s after the GBM trigger and ~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.
The Astrophysical Journal 11/2009; 706(1):L138. · 6.02 Impact Factor
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A A Abdo,
M Ackermann,
M Ajello, M Axelsson,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
B M Baughman,
K Bechtol, [......],
V Vasileiou,
N Vilchez,
V Vitale,
A P Waite,
P Wang,
J Wilms,
B L Winer,
K S Wood,
T Ylinen,
M Ziegler
[show abstract]
[hide abstract]
ABSTRACT: Microquasars are accreting black holes or neutron stars in binary systems with associated relativistic jets. Despite their frequent outburst activity, they have never been unambiguously detected emitting high-energy gamma rays. The Fermi Large Area Telescope (LAT) has detected a variable high-energy source coinciding with the position of the x-ray binary and microquasar Cygnus X-3. Its identification with Cygnus X-3 is secured by the detection of its orbital period in gamma rays, as well as the correlation of the LAT flux with radio emission from the relativistic jets of Cygnus X-3. The gamma-ray emission probably originates from within the binary system, opening new areas in which to study the formation of relativistic jets.
Science 11/2009; 326(5959):1512-6. · 31.20 Impact Factor
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A A Abdo,
M Ackermann,
M Ajello,
K Asano,
W B Atwood, M Axelsson,
L Baldini,
J Ballet,
G Barbiellini,
M G Baring, [......],
A von Kienlin,
A P Waite,
P Wang,
C Wilson-Hodge,
B L Winer,
K S Wood,
X F Wu,
R Yamazaki,
T Ylinen,
M Ziegler
[show abstract]
[hide abstract]
ABSTRACT: A cornerstone of Einstein's special relativity is Lorentz invariance-the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l(Planck) approximately 1.62 x 10(-33) cm or E(Planck) = M(Planck)c(2) approximately 1.22 x 10(19) GeV), at which quantum effects are expected to strongly affect the nature of space-time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves. Here we report the detection of emission up to approximately 31 GeV from the distant and short GRB 090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space-time on a very small scale linearly alters the speed of light.
Nature 10/2009; 462(7271):331-4. · 36.28 Impact Factor
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A A Abdo,
M Ackermann,
M Ajello,
W B Atwood, M Axelsson,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
B M Baughman, [......],
T L Usher,
V Vasileiou,
N Vilchez,
V Vitale,
P Wang,
N Webb,
B L Winer,
K S Wood,
T Ylinen,
M Ziegler
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ABSTRACT: We report the detection of gamma-ray emissions above 200 megaelectron volts at a significance level of 17sigma 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.
Science 09/2009; 325(5942):845-8. · 31.20 Impact Factor
