Observations of Selected AGN with H.E.S.S.

A sample of selected active galactic nuclei (AGN) was observed in 2003 and 2004 with the High Energy Stereoscopic System (H.E.S.S.), an array of imaging atmospheric-Cherenkov telescopes in Namibia. The redshifts of these candidate very-high-energy (VHE, >100 GeV) γ-ray emitters range from ≉=0.00183 to ≉=0.333. Significant detections were already reported for some of these objects, such as PKS 2155-304 and Mkn421. Marginal evidence (3.1σ) for a signal is found from large-zenith-angle observations of Mkn501, corresponding to an integral flux of I(>1.65 TeV) = (1.5±0.6_stat ±0.3_syst)x 10^-12cm^-2s^-1 or ~15% of the Crab Nebula flux. Integral flux upper limits for 19 other AGN, based on exposures of ~1 to ~8 hours live time, and with average energy thresholds between 160 GeV and 610 GeV, range from 0.4% to 5.1% of the Crab Nebula flux. All the upper limits are the most constraining ever reported for these objects

H.E.S.S. observations of galaxy clusters

Clusters of galaxies, the largest gravitationally bound objects in the universe, are expected to contain a significant population of hadronic and leptonic cosmic rays. Potential sources for these particles are merger and accretion shocks, starburst driven galactic winds and radio galaxies. Furthermore, since galaxy clusters confine cosmic ray protons up to energies of at least 1 PeV for a time longer than the Hubble time they act as storehouses and accumulate all the hadronic particles which are accelerated within them. Consequently clusters of galaxies are potential sources of VHE (> 100 GeV) gamma rays. Motivated by these considerations, promising galaxy clusters are observed with the H.E.S.S. experiment as part of an ongoing campaign. Here, upper limits for the VHE gamma ray emission for the Abell 496 and Coma cluster systems are reported.Comment: Contribution to the 30th ICRC, Merida Mexico, July 200

Discovery of Very High Energy gamma - ray emission from the extreme BL Lac object H2356-309 with H.E.S.S

The understanding of acceleration mechanisms in active galactic nuclei (AGN) jets and the measurement of the extragalactic-background-light (EBL) density are closely linked and require the detection of a large sample of very-high-energy (VHE) emitting extragalactic objects at varying redshifts. We report here on the discovery with the H.E.S.S. (High Energy Stereoscopic System) atmospheric-Cherenkov telescopes of the VHE Gamma-ray emission from H2356 - 309, an extreme BL Lac object located at a redshift of 0.165. The observations of this object, which was previously proposed as a southern-hemisphere VHE candidate source, were performed between June and December 2004. The total exposure is 38.9 hours live time, after data quality selection, which yields the detection of a signal at the level of 9.0$\sigma$ (standard deviations) .Comment: To appear on proceeding of 29th International Cosmic Ray Conference (ICRC 2005

A Spectacular VHE Gamma-Ray Outburst from PKS 2155-304 in 2006

Since 2002 the VHE (>100 GeV) gamma-ray flux of the high-frequency peaked BL Lac PKS 2155-304 has been monitored with the High Energy Stereoscopic System (HESS). An extreme gamma-ray outburst was detected in the early hours of July 28, 2006 (MJD 53944). The average flux above 200 GeV observed during this outburst is ~7 times the flux observed from the Crab Nebula above the same threshold. Peak fluxes are measured with one-minute time scale resolution at more than twice this average value. Variability is seen up to ~600 s in the Fourier power spectrum, and well-resolved bursts varying on time scales of ~200 seconds are observed. There are no strong indications for spectral variability within the data. Assuming the emission region has a size comparable to the Schwarzschild radius of a ~10^9 solar mass black hole, Doppler factors greater than 100 are required to accommodate the observed variability time scales.Comment: 4 pages, 3 figures; To appear in the Proceedings of the 30th ICRC (Merida, Mexico

Preliminary results from a search for TeV gamma-ray emission from SN1987A and the surrounding field with H.E.S.S

H.E.S.S. has observed the very young supernova remnant SN1987A during 2003 in a search for TeV γ-ray emission. These observations were taken during a build-up phase of H.E.S.S. with 2 operating telescopes, ∼6400 days after the initial explosion. Preliminary analysis so far reveals no convincing evidence for TeV emission and the 99% upper limit is compared with a predicted light curve. The H.E.S.S. field of view encompasses a number of other interesting objects including the X-ray shell 30 Dor C, the Crab-like plerion PSR B0540-69, the SNR N157B, and the X-ray binary LMC X-1. These objects may be associated with several features seen in the H.E.S.S. skymaps at marginal significances, and further observations in 2004/2005 with 4 telescopes will be valuable for confirmation. © 2005 American Institute of Physics