| 1. |
- Abdo, A. A., et al.
(författare)
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Fermi Observations of γ-Ray Emission from the Moon
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 758:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on the detection of high-energy gamma-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(> 100 MeV) = (1.04 +/- 0.01 [statistical error] +/- 0.1 [systematic error]) x 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 boardthe Compton Gamma Ray Observatory, F(> 100 MeV) approximate to 5 x 10(-7) cm-2 s-1, when solar activity was relatively high. The higher gamma-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 gamma-rays with fast celestial motion. Their paths across thesky are projected onto the Galactic center and high Galactic latitudes as well as onto other areas crowded with high-energy gamma-ray sources. Analysis ofthe lunar and solar emission may thus be important for studies of weak and transient sources near the ecliptic.
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| 2. |
- Abdo, A. A., et al.
(författare)
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FERMI LARGE AREA TELESCOPE OBSERVATIONS OF MISALIGNED ACTIVE GALACTIC NUCLEI
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 720:1, s. 912-922
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Tidskriftsartikel (refereegranskat)abstract
- Analysis is presented for 15 months of data taken with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope for 11 non-blazar active galactic nuclei (AGNs), including seven FRI radio galaxies and four FRII radio sources consisting of two FRII radio galaxies and two steep spectrum radio quasars. The broad line FRI radio galaxy 3C 120 is reported here as a gamma-ray source for the first time. The analysis is based on directional associations of LAT sources with radio sources in the 3CR, 3CRR, and MS4 (collectively referred to as 3C-MS) catalogs. Seven of the eleven LAT sources associated with 3C-MS radio sources have spectral indices larger than 2.3 and, except for the FRI radio galaxy NGC 1275 that shows possible spectral curvature, are well described by a power law. No evidence for time variability is found for any sources other than NGC 1275. The gamma-ray luminosities of FRI radio galaxies are significantly smaller than those of the BL Lac objects detected by the LAT, whereas the gamma-ray luminosities of the FRII sources are quite similar to those of FSRQs, which could reflect different beaming factors for the gamma-ray emission. A core dominance (CD) study of the 3CRR sample indicates that sources closer to the jet axis are preferentially detected with the Fermi LAT, insofar as the gamma-ray-detected misaligned AGNs have larger CD at a given average radio flux. The results are discussed in view of the AGN unification scenario.
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| 3. |
- Abdo, A. A., et al.
(författare)
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SUZAKU OBSERVATIONS OF LUMINOUS QUASARS : REVEALING THE NATURE OF HIGH-ENERGY BLAZAR EMISSION IN LOW-LEVEL ACTIVITY STATES
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 716:1, s. 835-849
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Tidskriftsartikel (refereegranskat)abstract
- We present the results from the Suzaku X-ray observations of five flat-spectrum radio quasars (FSRQs), namely PKS 0208-512, Q 0827+243, PKS 1127-145, PKS 1510-089, and 3C 454.3. All these sources were additionally monitored simultaneously or quasi-simultaneously by the Fermi satellite in gamma rays and the Swift UVOT in the UV and optical bands, respectively. We constructed their broadband spectra covering the frequency range from 10(14) Hz up to 10(25) Hz, and those reveal the nature of high-energy emission of luminous blazars in their low-activity states. The analyzed X-ray spectra are well fitted by a power-law model with photoelectric absorption. In the case of PKS 0208-512, PKS 1127-145, and 3C 454.3, the X-ray continuum showed indication of hardening at low energies. Moreover, when compared with the previous X-ray observations, we see a significantly increasing contribution of low-energy photons to the total X-ray fluxes when the sources are getting fainter. The same behavior can be noted in the Suzaku data alone. A likely explanation involves a variable, flat-spectrum component produced via inverse-Compton emission, plus an additional, possibly steady soft X-ray component prominent when the source gets fainter. This soft X-ray excess is represented either by a steep power-law (photon indices Gamma similar to 3-5) or a blackbody-type emission with temperatures kT similar to 0.1-0.2 keV. We model the broadband spectra of the five observed FSRQs using synchrotron self-Compton and/or external-Compton radiation models. Our modeling suggests that the difference between the low-and high-activity states in luminous blazars is due to the different total kinetic power of the jet, most likely related to varying bulk Lorentz factor of the outflow within the blazar emission zone.
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| 4. |
- Abramowski, A., et al.
(författare)
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Detection of very-high-energy gamma-ray emission from the vicinity of PSR B1706-44 and G 343.1-2.3 with HESS
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 528, s. A143-
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Tidskriftsartikel (refereegranskat)abstract
- The gamma-ray pulsar PSR B1706-44 and the adjacent supernova remnant (SNR) candidate G 343.1-2.3 were observed by H. E. S. S. during a dedicated observation campaign in 2007. As a result of this observation campaign, a new source of very-high-energy (VHE; E > 100 GeV) gamma-ray emission, H.E.S.S. J1708-443, was detected with a statistical significance of 7 sigma, although no significant point-like emission was detected at the position of the energetic pulsar itself. In this paper, the morphological and spectral analyses of the newly-discovered TeV source are presented. The centroid of H. E. S. S. J1708-443 is considerably offset from the pulsar and located near the apparent center of the SNR, at alpha(J2000) = 17(h)08(m)11(s) +/- 17(s) and delta(J2000) = -44 degrees 20' +/- 4'. The source is found to be significantly more extended than the H. E. S. S. point spread function (similar to 0.1 degrees), with an intrinsic Gaussian width of 0.29 degrees +/- 0.04 degrees. Its integral flux between 1 and 10 TeV is similar to 3.8 x 10(-1)2 ph cm(-2) s(-1), equivalent to 17% of the Crab Nebula flux in the same energy range. The measured energy spectrum is well-fit by a power law with a relatively hard photon index Gamma = 2.0 +/- 0.1(stat) +/-0.2(sys). Additional multi-wavelength data, including 330 MHz VLA observations, were used to investigate the VHE gamma-ray source's possible associations with the pulsar wind nebula of PSR B1706-44 and/or with the complex radio structure of the partial shell-type SNR G 343.1-2.3.
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| 5. |
- Abramowski, A., et al.
(författare)
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HESS constraints on dark matter annihilations towards the sculptor and carina dwarf galaxies
- 2011
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 34:8, s. 608-616
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Tidskriftsartikel (refereegranskat)abstract
- The Sculptor and Carina dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov telescope array between January 2008 and December 2009. The data sets consist of a total of 11.8 h and 14.811 of high quality data, respectively. No gamma-ray signal was detected at the nominal positions of these galaxies above 220 GeV and 320 GeV, respectively. Upper limits on the gamma-ray fluxes at 95% CL assuming two forms for the spectral energy distribution (a power law shape and one derived from dark matter annihilation) are obtained at the level of 10(-13)-10(-12) cm(-2) s(-1) in the TeV range. Constraints on the velocity weighted dark matter particle annihilation cross section for both Sculptor and Carina dwarf galaxies range from 10(-21) cm(3) s(-1) down to similar to 10(-2)2 cm(3) s(-1) on the dark matter halo model used. Possible enhancements of the gamma-ray flux are studied: the Sommerfeld effect, which is found to exclude some dark matter particle masses, the internal Bremsstrahlung and clumps in the dark-matter halo distributions. (C) 2010 Elsevier B.V. All rights reserved.
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| 6. |
- Abramowski, A., et al.
(författare)
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HESS J1943+213 : a candidate extreme BL Lacertae object
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 529, s. A49-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The H. E. S. S. Cherenkov telescope array has been surveying the Galactic plane for new VHE (>100 GeV) gamma-ray sources. Aims. We report on a newly detected point-like source, HESS J1943+213. This source coincides with an unidentified hard X-ray source IGR J19443+2117, which was proposed to have radio and infrared counterparts. Methods. We combine new H. E. S. S., Fermi/LAT and Nancay Radio Telescope observations with pre-existing non-simultaneous multi-wavelength observations of IGR J19443+2117 and discuss the likely source associations as well as the interpretation as an active galactic nucleus, a gamma-ray binary or a pulsar wind nebula. Results. HESS J1943+213 is detected at the significance level of 7.9 sigma (post-trials) at RA(J2000) = 19(h)43(m)55(s) +/- 1(stat)(s) +/- 1(sys)(s), Dec(J2000) = +21 degrees 18'8 '' +/- 17(stat)'' +/- 20(sys)''. The source has a soft spectrum with photon index Gamma = 3.1 +/- 0.3(stat) +/- 0.2(sys) and a flux above 470 GeV of (1.3 +/- 0.2(stat) +/- 0.3(sys)) x 10(-12) cm(-2) s(-1). There is no Fermi/LAT counterpart down to a flux limit of 6 x 10(-9) cm(-2) s(-1) in the 0.1-100 GeV energy range (95% confidence upper limit calculated for an assumed power-law model with a photon index Gamma = 2.0). The data from radio to VHE gamma-rays do not show any significant variability. Conclusions. The lack of a massive stellar counterpart disfavors the binary hypothesis, while the soft VHE spectrum would be very unusual in case of a pulsar wind nebula. In addition, the distance estimates for Galactic counterparts places them outside of the Milky Way. All available observations favor an interpretation as an extreme, high-frequency peaked BL Lac object with a redshift z > 0.14. This would be the first time a blazar is detected serendipitously from ground-based VHE observations, and the first VHE AGN detected in the Galactic Plane.
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| 7. |
- Abramowski, A., et al.
(författare)
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Search for Lorentz Invariance breaking with a likelihood fit of the PKS 2155-304 flare data taken on MJD 53944
- 2011
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 34:9, s. 738-747
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Tidskriftsartikel (refereegranskat)abstract
- Several models of Quantum Gravity predict Lorentz Symmetry breaking at energy scales approaching the Planck scale (similar to 10(19) GeV). With present photon data from the observations of distant astrophysical sources, it is possible to constrain the Lorentz Symmetry breaking linear term in the standard photon dispersion relations. Gamma Ray Bursts (GRB) and flaring Active Galactic Nuclei (AGN) are complementary to each other for this purpose, since they are observed at different distances in different energy ranges and with different levels of variability. Following a previous publication of the High Energy Stereoscopic System (H.E.S.S.) collaboration [1], a more sensitive event-by-event method consisting of a likelihood fit is applied to PKS 2155-304 flare data of MJD 53944 (July 28, 2006) as used in the previous publication. The previous limit on the linear term is improved by a factor of similar to 3 up to M(QG)(1), > 2.1 X 10(1B) GeV and is currently the best result obtained with blazars. The sensitivity to the quadratic term is lower and provides a limit of M(QG)(q) > 6.4 x 10(10) GeV, which is the best value obtained so far with an AGN and similar to the best limits obtained with GRB. (C) 2011 Elsevier B.V. All rights reserved.
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| 8. |
- Abramowski, A., et al.
(författare)
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VHE gamma-ray emission of PKS 2155-304 : spectral and temporal variability
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 520, s. A83-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Observations of very high-energy.-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects lead to a better understanding of the mechanisms in play. Aims. To investigate the spectral and temporal variability of VHE (>100 GeV) gamma-rays of the well-known high-frequency-peaked BL Lac object PKS 2155-304 with the HESS imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 were analyzed. Spectra were derived on time scales ranging from 3 years to 4 min. Light curve variability was studied through doubling timescales and structure functions and compared with red noise process simulations. Results. The source was found to be in a low state from 2005 to 2007, except for a set of exceptional flares that occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 +/- 0.09(stat) +/- 0.86(syst)) x 10(-11) cm(-2) s(-1) above 200 GeV, or approximately 15% of the Crab Nebula, and a power-law photon index of Gamma = 3.53 +/- 0.06(stat) +/- 0.10(syst). During the flares of July 2006, doubling timescales of similar to 2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behavior at low and high fluxes, which is a new phenomenon in VHE gamma-ray emitting blazars. The variability amplitude characterized by the fractional rms F-var is strongly energy-dependent and is proportional to E-0.19 +/- 0.01. The light curve rms correlates with the flux. This is the signature of a multiplicative process that can be accounted for as a red noise with a Fourier index of similar to 2. Conclusions. This unique data set shows evidence of a low-level.-ray emission state from PKS 2155-304 that possibly has a different origin than the outbursts. The discovery of the light curve ognormal behavior might be an indicator of the origin of aperiodic variability in blazars.
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| 9. |
- Acero, F., et al.
(författare)
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HESS upper limits on very high energy gamma-ray emission from the microquasar GRS 1915+105
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 508:3, s. 1135-1140
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Tidskriftsartikel (refereegranskat)abstract
- Context. High energy particles reside in the relativistic jets of microquasars, making them possible sources of very high energy radiation (VHE, > 100 GeV). Detecting this emission would provide a new handle on jet physics. Aims. Observations of the microquasar GRS 1915+105 with the HESS telescope array were undertaken in 2004-2008 to search for VHE emission. Methods. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. Results. There is no evidence for a VHE gamma-ray signal either from the direction of the microquasar or its vicinity. An upper limit of 6.1 x 10(-13) ph cm(-2) s(-1) (99.9% confidence level) is set on the photon flux above 410 GeV, equivalent to a VHE luminosity of similar to 10(34) erg s(-1) at 11 kpc. Conclusions. The VHE to X-ray luminosity ratio in GRS 1915+105 is at least four orders of magnitude lower than the ratio observed in gamma-ray binaries. The VHE radiative efficiency of the compact jet is less than 0.01% based on its estimated total power of 10(38) erg s(-1). Particle acceleration in GRS 1915+105 is not efficient at high energies and/or the magnetic field is too strong. It is also possible that VHE gamma-rays are produced by GRS 1915+105, but the emission is highly time-dependent.
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| 10. |
- Acero, F., et al.
(författare)
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Localizing the VHE gamma-ray source at the Galactic Centre
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 402:3, s. 1877-1882
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Tidskriftsartikel (refereegranskat)abstract
- The inner 10 pc of our Galaxy contains many counterpart candidates of the very high energy (VHE; > 100 GeV) gamma-ray point source HESS J1745-290. Within the point spread function of the H.E.S.S. measurement, at least three objects are capable of accelerating particles to VHE and beyond and of providing the observed gamma-ray flux. Previous attempts to address this source confusion were hampered by the fact that the projected distances between these objects were of the order of the error circle radius of the emission centroid (34 arcsec, dominated by the pointing uncertainty of the H.E.S.S. instrument). Here we present H.E.S.S. data of the Galactic Centre region, recorded with an improved control of the instrument pointing compared to H.E.S.S. standard pointing procedures. Stars observed during gamma-ray observations by optical guiding cameras mounted on each H.E.S.S. telescope are used for off-line pointing calibration, thereby decreasing the systematic pointing uncertainties from 20 to 6 arcsec per axis. The position of HESS J1745-290 is obtained by fitting a multi-Gaussian profile to the background-subtracted gamma-ray count map. A spatial comparison of the best-fitting position of HESS J1745-290 with the position and morphology of candidate counterparts is performed. The position is, within a total error circle radius of 13 arcsec, coincident with the position of the supermassive black hole Sgr A* and the recently discovered pulsar wind nebula candidate G359.95-0.04. It is significantly displaced from the centroid of the supernova remnant Sgr A East, excluding this object with high probability as the dominant source of the VHE gamma-ray emission.
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