| 11. |
- Aharonian, F., et al.
(författare)
-
Spectrum and variability of the Galactic center VHE gamma-ray source HESS J1745-290
- 2009
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 503:3, s. 817-825
-
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.
|
|
| 12. |
- Aharonian, F., et al.
(författare)
-
Very high energy gamma-ray observations of the binary PSR B1259-63/SS2883 around the 2007 Periastron
- 2009
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 507:1, s. 389-396
-
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.
|
|
| 13. |
- Aharonian, F., et al.
(författare)
-
Very high energy gamma-ray observations of the galaxy clusters Abell 496 and Abell 85 with HESS
- 2009
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 495:1, s. 27-35
-
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.
|
|
| 14. |
- Becherini, Yvonne, et al.
(författare)
-
HESS-II reconstruction strategy and performance in the low-energy (20-150 GeV) domain
- 2009
-
Ingår i: HIGH ENERGY GAMMA-RAY ASTRONOMY. - : American Institute of Physics (AIP). - 9780735406162 ; , s. 738-741
-
Konferensbidrag (refereegranskat)abstract
- In mid-2009 a notable upgrade of the H.E.S.S. telescope system will take place: a new telescope with a 600 m(2) mirror area and very-high-resolution camera (0.07 degrees) will be positioned at the centre of the present configuration, with the aim of lowering the threshold and enhance its sensitivity in the 100 GeV to several TeV energy range. HESS-II will permit the investigation of the lower energy gamma-ray spectra in various cosmic accelerators, giving information on the origin of the gamma-rays observed, and will detect AGNs with a redshift greater than 0.2 (being less affected by absorption by Extragalactic Background Light - EBL - in this energy range) and will search for new classes of very high energy gamma-ray emitters (pulsars, microquasars, GRB, and dark matter candidates).
|
|
| 15. |
|
|
