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J. Holder,
R. W. Atkins,
H. M. Badran,
G Blaylock,
S. M. Bradbury,
J. H. Buckley,
K. L. Byrum,
D. A. Carter-Lewis,
O. Celik,
Y. C. K. Chow, [......],
S. P. Swordy,
A. Syson,
J. A. Toner,
L. Valcarcel,
V. V. Vassiliev,
S. P. Wakely,
T. C. Weekes,
R. J. White,
D A Williams,
R Wagner
[show abstract]
[hide abstract]
ABSTRACT: The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV gamma-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications. (c) 2006 Elsevier B.V. All rights reserved.
Astroparticle Physics 05/2013; 25:391. · 3.22 Impact Factor
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V. A. Acciari,
E. Aliu,
M. Beilicke,
W. Benbow,
M. Bottcher,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev,
Y. Butt,
O. Celik, [......],
R Wagner,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A Weinstein,
R. J. White,
D A Williams,
S. A. Wissel,
M. Wood,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We report the detection of very high energy gamma- ray emission from the intermediate- frequency- peaked BL Lacertae object W Comae (z = 0.102) by VERITAS. The source was observed between 2008 January and April. A strong outburst of gamma-ray emission was measured in the middle of March, lasting for only 4 days. The energy spectrum measured during the two highest flare nights is fit by a power law and is found to be very steep, with a differential photon spectral index of Gamma = 3.81 +/- 0.35(stat) +/- 0.34(syst). The integral photon flux above 200 GeV during those two nights corresponds to roughly 9% of the flux from the Crab Nebula. Quasi-simultaneous Swift observations at X-ray energies were triggered by the VERITAS observations. The spectral energy distribution of the flare data can be described by synchrotron self-Compton (SSC) or external Compton (EC) leptonic jet models.
The Astrophysical Journal Letters 05/2013; 684:L73. · 5.53 Impact Factor
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V. A. Acciari,
E. Aliu,
T. Arlen,
M. Bautista,
M. Beilicke,
W. Benbow,
M. Bottcher,
S. M. Bradbury,
V. Bugaev,
Y. Butt, [......],
R. G. Wagner,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A Weinstein,
R. J. White,
D A Williams,
S. Wissel,
M. Wood,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We present results from a long-term monitoring campaign on the TeV binary LSI +61° 303 with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between 2006 September and 2008 February. The binary was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8σ (2006/2007) and 7.3σ (2007/2008) significance level for emission above 500 GeV. The source was detected during active periods with flux values ranging from 5% to 20% of the Crab Nebula, varying over the course of a single orbital cycle. Additionally, the observations conducted in the 2007-2008 observing season show marginal evidence (at the 3.6σ significance level) for TeV emission outside the apastron passage of the compact object around the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show large variability with flux values typically varying between 0.5 and 3.0 ×10-11 erg cm-2 s-1 over a single orbital cycle. The contemporaneous X-ray and TeV data are examined and it is shown that the TeV sampling is not dense enough to detect a correlation between the two bands.
The Astrophysical Journal 05/2013; 700:1034. · 6.02 Impact Factor
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M Ackermann,
M Ajello,
A Allafort,
K Asano,
W ~B Atwood,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
K Bechtol, [......],
V Vitale,
A Kienlin,
A ~P Waite,
E Wallace,
P Weltevrede,
B ~L Winer,
K ~S Wood,
M Wood,
Z Yang,
S Zimmer
apj. 03/2013; 765:54.
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M Ackermann,
M Ajello,
A Allafort,
L Baldini,
J Ballet,
G Barbiellini,
M G Baring,
D Bastieri,
K Bechtol,
R Bellazzini, [......],
G Vianello,
V Vitale,
A P Waite,
M Werner,
B L Winer,
K S Wood,
M Wood,
R Yamazaki,
Z Yang,
S Zimmer
[show abstract]
[hide abstract]
ABSTRACT: Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. When accelerated protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma rays. This offers a compelling way to detect the acceleration sites of protons. The identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse Compton scattering. We detected the characteristic pion-decay feature in the gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area Telescope. This detection provides direct evidence that cosmic-ray protons are accelerated in SNRs.
Science 02/2013; 339(6121):807-11. · 31.20 Impact Factor
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M Ackermann,
M Ajello,
A Allafort,
P Schady,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
R Bellazzini,
R D Blandford, [......],
E Troja,
T L Usher,
J Vandenbroucke,
V Vasileiou,
G Vianello,
V Vitale,
A P Waite,
B L Winer,
K S Wood,
M Wood
[show abstract]
[hide abstract]
ABSTRACT: The light emitted by stars and accreting compact objects through the history of the universe is encoded in the intensity of the extragalactic background light (EBL). Knowledge of the EBL is important to understand the nature of star formation and galaxy evolution, but direct measurements of the EBL are limited by galactic and other foreground emissions. Here, we report an absorption feature seen in the combined spectra of a sample of gamma-ray blazars out to a redshift of z ∼ 1.6. This feature is caused by attenuation of gamma rays by the EBL at optical to ultraviolet frequencies and allowed us to measure the EBL flux density in this frequency band.
Science 11/2012; · 31.20 Impact Factor
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H J Pletsch,
L Guillemot,
H Fehrmann,
B Allen,
M Kramer,
C Aulbert,
M Ackermann,
M Ajello,
A de Angelis,
W B Atwood, [......],
J Vandenbroucke,
V Vasileiou,
G Vianello,
V Vitale,
A P Waite,
B L Winer,
K S Wood,
M Wood,
Z Yang,
S Zimmer
[show abstract]
[hide abstract]
ABSTRACT: Millisecond pulsars, old neutron stars spun-up by accreting matter from a companion star, can reach high rotation rates of hundreds of revolutions per second. Until now, all such "recycled" rotation-powered pulsars have been detected by their spin-modulated radio emission. In a computing-intensive blind search of gamma-ray data from the Fermi Large Area Telescope (with partial constraints from optical data), we detected a 2.5-millisecond pulsar, PSR J1311-3430. This unambiguously explains a formerly unidentified gamma-ray source that had been a decade-long enigma, confirming previous conjectures. The pulsar is in a circular orbit with an orbital period of only 93 minutes, the shortest of any spin-powered pulsar binary ever found.
Science 10/2012; · 31.20 Impact Factor
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A ~A Abdo,
M Ackermann,
M Ajello,
W ~B Atwoo,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini, [......],
V Vasileiou,
G Vianello,
V Vitale,
A ~P Waite,
P Wang,
B ~L Winer,
D ~L Wood,
K ~S Wood,
Z Yang,
S Zimmer
apj. 10/2012; 758:140.
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M Ackermann,
M Ajello,
A Allafort,
L Baldini,
J Ballet,
D Bastieri,
K Bechtol,
R Bellazzini,
B Berenji,
E ~D Bloom, [......],
D ~F Torres,
G Tosti,
E Troja,
Y Uchiyama,
J Vandenbroucke,
G Vianello,
V Vitale,
A ~P Waite,
M Wood,
Z Yang
apj. 08/2012; 755:164.
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M. Ackermann,
M. Ajello,
A. Allafort,
E. Antolini,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
R. Bellazzini,
B. Berenji, [......],
V. Vitale,
A. P. Waite,
E. Wallace,
P. Wang,
B. L. Winer,
M. T. Wolff,
D. L. Wood,
K. S. Wood,
Z. Yang,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: The Fermi Large Area Telescope (LAT) First Source Catalog (1FGL) provided spatial, spectral, and temporal properties for a large number of γ-ray sources using a uniform analysis method. After correlating with the most-complete catalogs of source types known to emit γ rays, 630 of these sources are "unassociated" (i.e., have no obvious counterparts at other wavelengths). Here, we employ two statistical analyses of the primary γ-ray characteristics for these unassociated sources in an effort to correlate their γ-ray properties with the active galactic nucleus (AGN) and pulsar populations in 1FGL. Based on the correlation results, we classify 221 AGN-like and 134 pulsar-like sources in the 1FGL unassociated sources. The results of these source "classifications" appear to match the expected source distributions, especially at high Galactic latitudes. While useful for planning future multiwavelength follow-up observations, these analyses use limited inputs, and their predictions should not be considered equivalent to "probable source classes" for these sources. We discuss multiwavelength results and catalog cross-correlations to date, and provide new source associations for 229 Fermi-LAT sources that had no association listed in the 1FGL catalog. By validating the source classifications against these new associations, we find that the new association matches the predicted source class in ~80% of the sources.
The Astrophysical Journal 06/2012; 753(1):83. · 6.02 Impact Factor
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Fermi LAT Collaboration,
M. Ackermann,
M. Ajello,
A. Allafort,
L. Baldini,
J. Ballet,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
B. Berenji, [......],
D. F. Torres,
G. Tosti,
E. Troja,
Y. Uchiyama,
J. Vandenbroucke,
G. Vianello,
V. Vitale,
A. P. Waite,
M. Wood,
Z. Yang
[show abstract]
[hide abstract]
ABSTRACT: Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray
telescopes suggest that galaxies rapidly forming massive stars are more
luminous at gamma-ray energies compared to their quiescent relatives. Building
upon those results, we examine a sample of 69 dwarf, spiral, and luminous and
ultraluminous infrared galaxies at photon energies 0.1-100 GeV using 3 years of
data collected by the Large Area Telescope (LAT) on the \textit{Fermi Gamma-ray
Space Telescope} (\textit{Fermi}). Measured fluxes from significantly detected
sources and flux upper limits for the remaining galaxies are used to explore
the physics of cosmic rays in galaxies. We find further evidence for
quasi-linear scaling relations between gamma-ray luminosity and both radio
continuum luminosity and total infrared luminosity which apply both to
quiescent galaxies of the Local Group and low-redshift starburst galaxies
(conservative $P$-values $\lesssim0.05$ accounting for statistical and
systematic uncertainties). The normalizations of these scaling relations
correspond to luminosity ratios of $\log(L_{0.1-100 \rm{GeV}}/L_{1.4 \rm{GHz}})
= 1.7 \pm 0.1_{\rm (statistical)} \pm 0.2_{\rm (dispersion)}$ and
$\log(L_{0.1-100 \rm{GeV}}/L_{8-1000 \mu\rm{m}}) = -4.3 \pm 0.1_{\rm
(statistical)} \pm 0.2_{\rm (dispersion)}$ for a galaxy with a star formation
rate of 1 $M_{\odot}$ yr$^{-1}$, assuming a Chabrier initial mass function.
Using the relationship between infrared luminosity and gamma-ray luminosity,
the collective intensity of unresolved star-forming galaxies at redshifts
$0<z<2.5$ above 0.1 GeV is estimated to be 0.4-2.4 $\times 10^{-6}$ ph
cm$^{-2}$ s$^{-1}$ sr$^{-1}$ (4-23% of the intensity of the isotropic diffuse
component measured with the LAT). We anticipate that $\sim10$ galaxies could be
detected by their cosmic-ray induced gamma-ray emission during a 10-year
\textit{Fermi} mission.
06/2012;
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M. Ackermann,
M. Ajello,
W. B. Atwood,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
B. Berenji, [......],
G. Vianello,
V. Vitale,
A. P. Waite,
P. Wang,
B. L. Winer,
K. S. Wood,
M. Wood,
Z. Yang,
M. Ziegler,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: The γ-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse γ-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X CO factor, the ratio between integrated CO-line intensity and H2 column density, the fluxes and spectra of the γ-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as γ-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.
The Astrophysical Journal 04/2012; 750(1):3. · 6.02 Impact Factor
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M Ackermann,
M Ajello,
A Allafort,
W ~B Atwood,
L Baldini,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini,
P ~N Bhat, [......],
J Vandenbroucke,
V Vasileiou,
G Vianello,
V Vitale,
A Kienlin,
A ~P Waite,
C Wilson-Hodge,
D ~L Wood,
K ~S Wood,
Z Yang
apj. 04/2012; 748:151.
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P. L. Nolan,
A. A. Abdo,
M. Ackermann,
M. Ajello,
A. Allafort,
E. Antolini,
W. B. Atwood,
M. Axelsson,
L. Baldini,
J. Ballet, [......],
A. P. Waite,
E. Wallace,
P. Wang,
M. Werner,
B. L. Winer,
D. L. Wood,
K. S. Wood,
M. Wood,
Z. Yang,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We present the second catalog of high-energy γ-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 γ-ray-producing source classes.
The Astrophysical Journal Supplement Series 03/2012; 199(2):31. · 13.46 Impact Factor
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R. Buehler,
J. D. Scargle,
R. D. Blandford,
L. Baldini,
M. G. Baring,
A. Belfiore,
E. Charles,
J. Chiang,
F. D'Ammando,
C. D. Dermer, [......],
J. E. Grove,
A. K. Harding, E. Hays,
M. Kerr,
F. Massaro,
M. N. Mazziotta,
R. W. Romani,
P. M. Saz Parkinson,
A. F. Tennant,
and M. C. Weisskopf
[show abstract]
[hide abstract]
ABSTRACT: The Large Area Telescope on board the Fermi satellite observed a gamma-ray flare in the Crab Nebula lasting for approximately nine days in April of 2011. The source, which at optical wavelengths has a size of 11 lt-yr across, doubled its gamma-ray flux within eight hours. The peak photon flux was (186 ± 6) × 10–7 cm–2 s–1 above 100 MeV, which corresponds to a 30-fold increase compared to the average value. During the flare, a new component emerged in the spectral energy distribution, which peaked at an energy of (375 ± 26) MeV at flare maximum. The observations imply that the emission region was likely relativistically beamed toward us and that variations in its motion are responsible for the observed spectral variability.
The Astrophysical Journal 03/2012; 749(1):26. · 6.02 Impact Factor
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The Astronomer's Telegram. 03/2012; 3978:1.
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M. Ackermann,
M. Ajello,
A. Allafort,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
B. Berenji, [......],
J. Vandenbroucke,
V. Vasileiou,
G. Vianello,
V. Vitale,
A. P. Waite,
B. L. Winer,
K. S. Wood,
M. Wood,
Z. Yang,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We report on a systematic investigation of the γ-ray properties of 120 hard X-ray-selected Seyfert galaxies classified as "radio-quiet" objects, utilizing the three-year accumulation of Fermi Large Area Telescope (LAT) data. Our sample of Seyfert galaxies is selected using the Swift Burst Alert Telescope 58 month catalog, restricting the analysis to the bright sources with average hard X-ray fluxes F 14 – 195 keV ≥ 2.5 × 10–11 erg cm–2 s–1 at high Galactic latitudes (|b| > 10°). In order to remove "radio-loud" objects from the sample, we use the "hard X-ray radio loudness parameter," R rX, defined as the ratio of the total 1.4 GHz radio to 14-195 keV hard X-ray energy fluxes. Among 120 X-ray bright Seyfert galaxies with R rX <10–4, we did not find a statistically significant γ-ray excess (TS > 25) positionally coincident with any target Seyferts, with possible exceptions of ESO 323-G077 and NGC 6814. The mean value of the 95% confidence level γ-ray upper limit for the integrated photon flux above 100 MeV from the analyzed Seyferts is 4 × 10–9 photons cm–2 s–1 , and the upper limits derived for several objects reach 1 × 10–9 photons cm–2 s–1 . Our results indicate that no prominent γ-ray emission component related to active galactic nucleus activity is present in the spectra of Seyferts around GeV energies. The Fermi-LAT upper limits derived for our sample probe the ratio of γ-ray to X-ray luminosities L γ/L X < 0.1, and even <0.01 in some cases. The obtained results impose novel constraints on the models for high-energy radiation of "radio-quiet" Seyfert galaxies.
The Astrophysical Journal 02/2012; 747(2):104. · 6.02 Impact Factor
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Fermi LAT Collaboration,
M Ackermann,
M Ajello,
J Ballet,
G Barbiellini,
D Bastieri,
A Belfiore,
R Bellazzini,
B Berenji,
R D Blandford, [......],
M Wood,
Z Yang,
S Zimmer,
M J Coe,
F Di Mille,
P G Edwards,
M D Filipović,
J L Payne,
J Stevens,
M A P Torres
[show abstract]
[hide abstract]
ABSTRACT: Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6-day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.
Science 01/2012; 335(6065):189-93. · 31.20 Impact Factor
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A A Abdo,
M Ackermann,
M Ajello,
W B Atwoo,
L Baldini,
J Ballet,
G Barbiellini,
D Bastieri,
K Bechtol,
R Bellazzini, [......],
V Vasileiou,
G Vianello,
V Vitale,
A P Waite,
P Wang,
B L Winer,
D L Wood,
K S Wood,
Z Yang,
S Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We report on the detection of high-energy γ-ray emission from the Moon during the first 24 months of observations by the Fermi Large Area Telescope (LAT). This emission comes from particle cascades produced by cosmic-ray (CR) nuclei and electrons interacting with the lunar surface. The differential spectrum of the Moon is soft and can be described as a log-parabolic function with an effective cutoff at 2-3 GeV, while the average integral flux measured with the LAT from the beginning of observations in 2008 August to the end of 2010 August is F (> ##IMG## [http://ej.iop.org/images/0004-637X/758/2/140/apj445159ieqn1.gif] $100 rm MeV) =(1.04pm 0.01,rm [statistical error]pm 0.1,rm [systematic error])times 10^-6$ cm –2 s –1 . This flux is about a factor 2-3 higher than that observed between 1991 and 1994 by the EGRET experiment on board the Compton Gamma Ray Observatory , F (>100 MeV) ##IMG## [http://ej.iop.org/icons/Entities/ap.gif] ≈ 5 × 10 –7 cm –2 s –1 , when solar activity was relatively high. The higher γ-ray flux measured by Fermi is consistent with the deep solar minimum conditions during the first 24 months of the mission, which reduced effects of heliospheric modulation, and thus increased the heliospheric flux of Galactic CRs. A detailed comparison of the light curve with McMurdo Neutron Monitor rates suggests a correlation of the trends. The Moon and the Sun are so far the only known bright emitters of γ-rays with fast celestial motion. Their paths across the sky are projected onto the Galactic center and high Galactic latitudes as well as onto other areas crowded with high-energy γ-ray sources. Analysis of the lunar and solar emission may thus be important for studies of weak and transient sources near the ecliptic.
The Astrophysical Journal. 01/2012; 758(2):140.
-
M. Ajello,
A. Allafort,
L. Baldini,
J. Ballet,
G. Barbiellini,
D. Bastieri,
K. Bechtol,
R. Bellazzini,
B. Berenji,
R. D. Blandford, [......],
A. P. Waite,
P. Wang,
B. L. Winer,
K. S. Wood,
H. Yamamoto,
R. Yamazaki,
Z. Yang,
H. Yasuda,
M. Ziegler,
and S. Zimmer
[show abstract]
[hide abstract]
ABSTRACT: We present a detailed analysis of the GeV gamma-ray emission toward the supernova remnant (SNR) G8.7–0.1 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. An investigation of the relationship between G8.7–0.1 and the TeV unidentified source HESS J1804–216 provides us with an important clue on diffusion process of cosmic rays if particle acceleration operates in the SNR. The GeV gamma-ray emission is extended with most of the emission in positional coincidence with the SNR G8.7–0.1 and a lesser part located outside the western boundary of G8.7–0.1. The region of the gamma-ray emission overlaps spatially connected molecular clouds, implying a physical connection for the gamma-ray structure. The total gamma-ray spectrum measured with LAT from 200 MeV-100 GeV can be described by a broken power-law function with a break of 2.4 ± 0.6 (stat) ± 1.2 (sys) GeV, and photon indices of 2.10 ± 0.06 (stat) ± 0.10 (sys) below the break and 2.70 ± 0.12 (stat) ± 0.14 (sys) above the break. Given the spatial association among the gamma rays, the radio emission of G8.7–0.1, and the molecular clouds, the decay of π0s produced by particles accelerated in the SNR and hitting the molecular clouds naturally explains the GeV gamma-ray spectrum. We also find that the GeV morphology is not well represented by the TeV emission from HESS J1804–216 and that the spectrum in the GeV band is not consistent with the extrapolation of the TeV gamma-ray spectrum. The spectral index of the TeV emission is consistent with the particle spectral index predicted by a theory that assumes energy-dependent diffusion of particles accelerated in an SNR. We discuss the possibility that the TeV spectrum originates from the interaction of particles accelerated in G8.7–0.1 with molecular clouds, and we constrain the diffusion coefficient of the particles.
The Astrophysical Journal 12/2011; 744(1):80. · 6.02 Impact Factor
