| 1. |
- Aharonian, F., et al.
(författare)
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SIMULTANEOUS OBSERVATIONS OF PKS 2155-304 WITH HESS, FERMI, RXTE, AND ATOM : SPECTRAL ENERGY DISTRIBUTIONS AND VARIABILITY IN A LOW STATE
- 2009
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 0004-637X .- 1538-4357. ; 696:2, s. L150-L155
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Tidskriftsartikel (refereegranskat)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 2008 August 25 and 2008 September 6 (MJD 54704-54715), jointly with the Fermi Gamma-ray Space Telescope and the HESS atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral energy distribution (SED) 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 (VHE; > 100 GeV) state, whereas the optical flux was much higher. The light curves show relatively little (similar to 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.
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| 2. |
- Acciari, V. A., et al.
(författare)
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Radio Imaging of the Very-High-Energy gamma-Ray Emission Region in the Central Engine of a Radio Galaxy
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 325:5939, s. 444-448
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Tidskriftsartikel (refereegranskat)abstract
- The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10(12) electron volts and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.
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| 3. |
- Ageron, M., et al.
(författare)
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The ANTARES optical beacon system
- 2007
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 578:3, s. 498-509
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Tidskriftsartikel (refereegranskat)abstract
- ANTARES is a neutrino telescope being deployed in the Mediterranean Sea. It consists of a three-dimensional array of photomultiplier tubes that can detect the Cherenkov light induced by charged particles produced in the interactions of neutrinos with the surrounding medium. High angular resolution can be achieved, in particular, when a muon is produced, provided that the Cherenkov photons are detected with sufficient timing precision. Considerations of the intrinsic time uncertainties stemming from the transit time spread in the photomultiplier tubes and the mechanism of transmission of light in sea water lead to the conclusion that a relative time accuracy of the order of 0.5 ns is desirable. Accordingly, different time calibration systems have been developed for the ANTARES telescope. In this article, a system based on Optical Beacons, a set of external and well-controlled pulsed light sources located throughout the detector, is described. This calibration system takes into account the optical properties of sea water, which is used as the detection volume of the ANTARES telescope. The design, tests, construction and first results of the two types of beacons, LED and laser-based, are presented. (C) 2007 Elsevier B.V. All rights reserved.
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| 4. |
- Aguilar, J. A., et al.
(författare)
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First results of the instrumentation line for the deep-sea ANTARES neutrino telescope
- 2006
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Ingår i: Astroparticle physics. - : Elsevier. - 0927-6505 .- 1873-2852. ; 26:4-5, s. 314-324
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Tidskriftsartikel (refereegranskat)abstract
- In 2005, the ANTARES Collaboration deployed and operated at a depth of 2500 m a so-called Mini Instrumentation Line equipped with Optical Modules (MILOM) at the ANTARES site. The various data acquired during the continuous operation from April to December 2005 of the MILOM confirm the satisfactory performance of the Optical Modules, their front-end electronics and readout system. as well as the calibration devices of the detector. The in situ measurement of the Optical Module time response yields a resolution better than 0.5 ns. The performance of the acoustic positioning system, which enables the spatial reconstruction of the ANTARES detector with a precision of about 10 cm, is verified. These results demonstrate that with the full ANTARES neutrino telescope the design angular resolution of better than 0.3 degrees can be realistically achieved.
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| 5. |
- Aguilar, J. A., et al.
(författare)
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The data acquisition system for the ANTARES neutrino telescope
- 2007
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 570:1, s. 107-116
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Tidskriftsartikel (refereegranskat)abstract
- The ANTARES neutrino telescope is being constructed in the Mediterranean Sea. It consists of a large three-dimensional array of photo-multiplier tubes. The data acquisition system of the detector takes care of the digitisation of the photo-multiplier tube signals, data transport, data filtering, and data storage. The detector is operated using a control program interfaced with all elements. The design and the implementation of the data acquisition system are described. (c) 2006 Elsevier B.V. All rights reserved.
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| 6. |
- Ageron, M., et al.
(författare)
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Studies of a full-scale mechanical prototype line for the ANTARES neutrino telescope and tests of a prototype instrument for deep-sea acoustic measurements
- 2007
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 581:3, s. 695-708
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Tidskriftsartikel (refereegranskat)abstract
- full-scale mechanical prototype line was deployed to a depth of 2500 m to test the leak tightness of the electronics containers and the pressure-resistant properties of an electromechanical cable under evaluation for use in the ANTARES deep-sea neutrino telescope. During a month-long immersion study, line parameter data were taken using miniature autonomous data loggers and shore-based optical time domain reflectometry. Details of the mechanical prototype line, the electromechanical cable and data acquisition are presented. Data taken during the immersion study revealed deficiencies in the pressure resistance of the electromechanical cable terminations at the entry points to the electronics containers. The improvements to the termination, which have been integrated into subsequent detection lines, are discussed. The line also allowed deep-sea acoustic measurements with a prototype hydrophone system. The technical setup of this system is described, and the first results of the data analysis are presented. (c) 2007 Elsevier B.V. All rights reserved.
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| 7. |
- Aharonian, F., et al.
(författare)
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Constraints on the multi-TeV particle population in the Coma galaxy cluster with HESS observations
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 502:2, s. 437-443
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Galaxy clusters are key targets in the search for ultra high energy particle accelerators. The Coma cluster represents one of the best candidates for such a search owing to its high mass, proximity, and the established non-thermal radio emission centred on the cluster core. Methods. The HESS (High Energy Stereoscopic System) telescopes observed Coma for similar to 8 h in a search for gamma-ray emission at energies > 1 TeV. The large 3.5 degrees FWHM field of view of HESS is ideal for viewing a range of targets at various sizes including the Coma cluster core, the radio-relic (1253+275) and merger/infall (NGC 4839) regions to the southwest, and features greater than 1 degrees away. Results. No evidence for point-like nor extended TeV gamma-ray emission was found and upper limits to the TeV flux F(E) for E > 1, > 5, and > 10 TeV were set for the Coma core and other regions. Converting these limits to an energy flux E(2)F(E) the lowest or most constraining is the E > 5 TeV upper limit for the Coma core (0.2 degrees radius) at similar to 8% Crab flux units or similar to 10(-13) ph cm(-2) s(-1). Conclusions. The upper limits for the Coma core were compared with a prediction for the gamma-ray emission from proton-proton interactions, the level of which ultimately scales with the mass of the Coma cluster. A direct constraint using our most stringent limit for E > 5 TeV, on the total energy content in non-thermal protons with injection energy spectrum alpha E(-2.1) and spatial distribution following the thermal gas in the cluster, is found to be similar to 0.2 times the thermal energy, or similar to 10(62) erg. The E > 5 TeV gamma-ray threshold in this case corresponds to cosmic-ray proton energies greater than or similar to 50 TeV. Our upper limits rule out the most optimistic theoretical models for gamma ray emission from clusters and complement radio observations which constrain the cosmic ray content in clusters at significantly lower proton energies, subject to assumptions on the magnetic field strength.
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| 8. |
- Aharonian, F., et al.
(författare)
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Detection of very high energy radiation from HESS J1908+063 confirms the Milagro unidentified source MGRO J1908+06
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 499:3, s. 723-728
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Detection of gamma-ray source above 300 GeV is reported, confirming the unidentified source MGRO J1908+06, discovered by the Milagro collaboration at a median energy of 20 TeV. Methods. The source was observed during 27 h as part of the extension of the HESS Galactic plane survey to longitudes > 30 degrees. Results. HESS J1908+063 is detected at a significance level of 10.9 sigma with an integral flux above 1 TeV of (3.76 +/- 0.29 (stat) +/- 0.75(sys)) x 10(-12) ph cm(-2) s(-1), and a spectral photon index Gamma = 2.10 +/- 0.07 (stat) +/- 0.2(sys). The positions and fluxes of HESS J1908+063 and MGRO J1908+06 are in good agreement. Possible counterparts at other wavelengths and the origin of the gamma-ray emission are discussed. The nearby unidentified GeV source, GRO J1908+0556 (GeV) which also remains unidentified and the new Fermi pulsar 0FGL J1907.5+0617, may be connected to the TeV source.
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| 9. |
- Aharonian, F., et al.
(författare)
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Discovery of very high energy γ-ray emission from centaurus a with H.E.S.S.
- 2009
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Ingår i: Astrophysical Journal Letters. - : Institute of Physics (IOP). - 2041-8205. ; 695:1, s. L40-L44
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of faint very high energy (VHE; E > 100 GeV) gamma-ray emission from the radio galaxy Centaurus A in observations performed with the High Energy Stereoscopic System (H.E.S.S.) experiment, an imaging atmospheric Cherenkov telescope array consisting of four telescopes located in Namibia. Centaurus A has been observed for more than 120 hr. A signal with a statistical significance of 5.0 sigma is detected from the region including the radio core and the inner kpc jets. The integral flux above an energy threshold of similar to 250 GeV is measured to be 0.8% of the flux of the Crab Nebula (apparent luminosity: L(> 250 GeV) approximate to 2.6 x 1039 erg s(-1), adopting a distance of 3.8 Mpc). The spectrum can be described by a power law with a photon index of 2.7 +/- 0.5(stat) +/- 0.2(sys). No significant flux variability is detected in the data set. However, the low flux only allows detection of variability on the timescale of days to flux increments above a factor of similar to 15-20 (3 sigma and 4 sigma, respectively). The discovery of VHE gamma-ray emission from Centaurus A reveals particle acceleration in the source to > TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.
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| 10. |
- Aharonian, F., et al.
(författare)
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Energy Spectrum of Cosmic-Ray Electrons at TeV Energies
- 2008
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Ingår i: Physical Review Letters. - : APS Physics. - 0031-9007 .- 1079-7114. ; 101:26
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Tidskriftsartikel (refereegranskat)abstract
- The very large collection area of ground-based gamma-ray telescopes gives them a substantial advantage over balloon or satellite based instruments in the detection of very-high-energy (> 600 GeV) cosmic-ray electrons. Here we present the electron spectrum derived from data taken with the High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes. In this measurement, the first of this type, we are able to extend the measurement of the electron spectrum beyond the range accessible to direct measurements. We find evidence for a substantial steepening in the energy spectrum above 600 GeV compared to lower energies.
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| 11. |
- Aharonian, F., et al.
(författare)
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HESS upper limit on the very high energy gamma-ray emission from the globular cluster 47 Tucanae
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 499:1, s. 273-277
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Tidskriftsartikel (refereegranskat)abstract
- Observations of the globular cluster 47 Tucanae (NGC 104), which contains at least 23 ms pulsars, were performed with the HESS telescope system. The observations lead to an upper limit of F(E > 800 GeV) < 6.7 x 10(-13) cm(-2) s(-1) on the integral gamma-ray photon flux from 47 Tucanae. Considering millisecond pulsars as the unique potential source of gamma-rays in the globular cluster, constraints based on emission models are derived: on the magnetic field in the average pulsar nebula and on the conversion efficiency of spin-down power to gamma-ray photons or to relativistic leptons.
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| 12. |
- Aharonian, F., et al.
(författare)
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Limits on an Energy Dependence of the Speed of Light from a Flare of the Active Galaxy PKS 2155-304
- 2008
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Ingår i: Physical Review Letters. - : APS Physics. - 0031-9007 .- 1079-7114. ; 101:17
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Tidskriftsartikel (refereegranskat)abstract
- In the past few decades, several models have predicted an energy dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this Letter a search for such time lags during the High Energy Stereoscopic System observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.
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| 13. |
- Aharonian, F., et al.
(författare)
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Simultaneous HESS and Chandra observations of Sagitarius A(star) during an X-ray flare
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 492:1, s. L25-L28
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Tidskriftsartikel (refereegranskat)abstract
- The rapidly varying (similar to 10 min timescale) non-thermal X-ray emission observed from Sgr A(star) implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV gamma-ray source HESS J1745-290 is coincident with Sgr A(star) and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint HESS/Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of approximate to 9 increase in the X-ray flux of Sgr A(star), no evidence is found for an increase in the TeV gamma-ray flux from this region. We find that an increase in the gamma-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the gamma-ray emission is produced within similar to 1014 cm (similar to 100 RS) of the supermassive black hole.
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| 14. |
- Aharonian, F., et al.
(författare)
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Simultaneous multiwavelength observations of the second exceptional gamma-ray flare of PKS2155-304 in July 2006
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 502:3, s. 749-770
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Forskningsöversikt (refereegranskat)abstract
- Aims. The X-ray-TeV connection and the evolution of the emitting particle population is studied in high-energy peaked BL Lac objects, by obtaining spectral information in both bands on sub-hour timescales. Methods. Simultaneous observations with HESS, Chandra and the Bronberg optical observatory were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent a major gamma-ray outburst during its high-activity state of Summer 2006. This event took place about 44 h after the other major outburst of the night of July 27-28, which is known for its ultrafast variability. An unprecedented 6 to 8 h of simultaneous, uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2 min timescales, respectively. Results. The source exhibited one major flare along the night, at high energies. The gamma-ray flux reached a maximum of similar to 11 times the Crab flux (>400 GeV), with rise/decay timescales of similar to 1 h, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE gamma-ray bands is strongly correlated, with no evidence of lags. The spectra also evolve with similar patterns, and are always soft ( photon index G > 2), indicating no strong shift of the peaks in the spectral energy distribution towards higher energies. Only at the flare maximum is there evidence that the gamma-ray peak is inside the observed passband, at similar to 400-600 GeV. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (similar to 22x) are only accompanied by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in a high-energy peaked BL Lac object a large Compton dominance (L(C)/L(S) similar to 10) - rapidly evolving - and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge the common scenarios for the TeV-blazar emission.
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| 15. |
- Aharonian, F., et al.
(författare)
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Spectrum and variability of the Galactic center VHE gamma-ray source HESS J1745-290
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 503:3, s. 817-825
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Tidskriftsartikel (refereegranskat)abstract
- Aims. A detailed study of the spectrum and variability of the source HESS J1745-290 in the Galactic Center (GC) region using new data from the H. E. S. S. array of Cherenkov telescopes is presented. Flaring activity and quasi periodic oscillations (QPO) of HESS J1745-290 are investigated. Methods. The image analysis is performed with a combination of a semi-analytical shower model and the statistical moment-based Hillas technique. The spectrum and lightcurves of HESS J1745-290 are derived with a likelihood method based on a spectral shape hypothesis. Rayleigh tests and Fourier analysis of the H. E. S. S. GC signal are used to study the periodicity of the source. Results. With a three-fold increase in statistics compared to previous work, a deviation from a simple power law spectrum is detected for the first time. The measured energy spectrum over the three years 2004, 2005 and 2006 of data taking is compatible with both a power law spectrum with an exponential cut-off and a broken power law spectrum. The curvature of the energy spectrum is likely to be intrinsic to the photon source, as opposed to effects of interstellar absorption. The power law spectrum with an exponential cut-off is characterized by a photon index of 2.10 +/- 0.04(stat) +/- 0.10(syst) and a cut-off energy at 15.7 +/- 3.4(stat) +/- 2.5(syst) TeV. The broken power law spectrum exhibits spectral indices of 2.02 +/- 0.08(stat) +/- 0.10(syst) and 2.63 +/- 0.14(stat) +/- 0.10(syst) with a break energy at 2.57 +/- 0.19(stat) +/- 0.44(syst) TeV. No significant flux variation is found. Increases in the gamma-ray flux of HESS J1745-290 by at least a factor of two would be required for a 3 sigma detection of a flare with time scales of an hour. Investigation of possible QPO activity at periods claimed to be detected in X-rays does not show any periodicities in the H. E. S. S. signal.
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| 16. |
- Aharonian, F., et al.
(författare)
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Very high energy gamma-ray observations of the galaxy clusters Abell 496 and Abell 85 with HESS
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 495:1, s. 27-35
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The nearby galaxy clusters Abell 496 and Abell 85 are studied in the very high-energy (VHE, E > 100 GeV) band to investigate VHE cosmic rays (CRs) in this class of objects which are the largest gravitationally bound systems in the Universe. Methods. HESS, an array of four imaging atmospheric cherenkov telescopes (IACT), was used to observe the targets in the range of VHE gamma rays. Results. No significant gamma-ray signal is found at the respective position of the two clusters with several different source size assumptions for each target. In particular, emission regions corresponding to the high-density core, to the extension of the entire X-ray emission in these clusters, and to the very extended region where the accretion shock is expected are investigated. Upper limits are derived for the gamma-ray flux at energies E > 570 GeV for Abell 496 and E > 460 GeV for Abell 85. Conclusions. From the non-detection in VHE gamma rays, upper limits on the total energy of hadronic CRs in the clusters are calculated. If the cosmic-ray energy density follows the large-scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium (ICM) is 51% for Abell 496 and only 8% for Abell 85 due to its higher mass and higher gas density. These upper limits are compared with theoretical estimates. They predict about similar to 10% of the thermal energy of the ICM in non-thermal particles. The observations presented here can constrain these predictions especially for the case of the Abell 85 cluster.
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| 17. |
- Aharonian, F., et al.
(författare)
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Probing the ATIC peak in the cosmic-ray electron spectrum with HESS
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 508:2, s. 561-564
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Tidskriftsartikel (refereegranskat)abstract
- The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has - together with the PAMELA detection of a rise in the positron fraction up to approximate to 100 GeV - motivated many interpretations in terms of dark matter scenarios; alternative explanations assume a nearby electron source like a pulsar or supernova remnant. Here we present a measurement of the cosmic-ray electron spectrum with H. E. S. S. starting at 340 GeV. While the overall electron flux measured by H. E. S. S. is consistent with the ATIC data within statistical and systematic errors, the H. E. S. S. data exclude a pronounced peak in the electron spectrum as suggested for interpretation by ATIC. The H. E. S. S. data follow a power-law spectrum with spectral index of 3.0 +/- 0.1(stat.) +/- 0.3(syst.), which steepens at about 1 TeV.
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| 18. |
- Acero, F., et al.
(författare)
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Detection of Gamma Rays from a Starburst Galaxy
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 326:5956, s. 1080-1082
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Tidskriftsartikel (refereegranskat)abstract
- Starburst galaxies exhibit in their central regions a highly increased rate of supernovae, the remnants of which are thought to accelerate energetic cosmic rays up to energies of similar to 10(15) electron volts. We report the detection of gamma rays-tracers of such cosmic rays-from the starburst galaxy NGC 253 using the High Energy Stereoscopic System (H. E. S. S.) array of imaging atmospheric Cherenkov telescopes. The gamma-ray flux above 220 billion electron volts is F = (5.5 +/- 1.0(stat) +/- 2.8(sys)) x 10(-13) cm(-2) s(-1), implying a cosmic-ray density about three orders of magnitude larger than that in the center of the Milky Way. The fraction of cosmic-ray energy channeled into gamma rays in this starburst environment is five times as large as that in our Galaxy.
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| 19. |
- 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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| 20. |
- Aharonian, F., et al.
(författare)
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Very high energy gamma-ray observations of the binary PSR B1259-63/SS2883 around the 2007 Periastron
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 507:1, s. 389-396
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Tidskriftsartikel (refereegranskat)abstract
- Aims. This article presents very-high-energy (VHE; E > 100 GeV) data from the gamma-ray binary PSR B1259-63 as taken during the years 2005, 2006 and before as well as shortly after the 2007 periastron passage. These data extend the knowledge of the lightcurve of this object to all phases of the 3.4 year binary orbit. The lightcurve constrains physical mechanisms present in this TeV source. Methods. Observations of VHE gamma-rays with the HESS telescope array using the Imaging Atmospheric Cherenkov Technique were performed. The HESS instrument features an angular resolution of < 0.1 degrees and an energy resolution of < 20%. Gamma-ray events in an energy range of 0.5-70 TeV were recorded. From these data, energy spectra and lightcurve with a monthly time sampling were extracted. Results. VHE gamma-ray emission from PSR B1259-63 was detected with an overall significance of 9.5 standard deviations using 55 h of exposure, obtained from April to August 2007. The monthly flux of gamma-rays during the observation period was measured, yielding VHE lightcurve data for the early pre-periastron phase of the system for the first time. No spectral variability was found on timescales of months. The spectrum is described by a power law with a photon index of Gamma = 2.8 +/- 0.2(stat) +/- 0.2(sys) and flux normalisation Phi(0) = (1.1 +/- 0.1(stat) +/- 0.2(sys)) x 10(-12) TeV(-1) cm(-2) s(-1). PSR B1259-63 was also monitored in 2005 and 2006, far from periastron passage, comprising 8.9 h and 7.5 h of exposure, respectively. No significant excess of.-rays is seen in those observations. Conclusions. PSR B1259-63 has been re-confirmed as a variable TeV gamma-ray emitter. The firm detection of VHE photons emitted at a true anomaly theta approximate to -0.35 of the pulsar orbit, i.e. already similar to 50 days prior to the periastron passage, disfavors the stellar disc target scenario as a primary emission mechanism, based on current knowledge about the companion star's disc inclination, extension, and density profile.
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