K. Bernlöhr

The H.E.S.S. Survey of the inner galaxy in very high energy gamma rays.

We report on a survey of the inner part of the Galactic Plane in very high energy gamma-rays, with the H.E.S.S. Cherenkov telescope system. The Galactic Plane between +-30deg in longitude and +-3deg in latitude relative to the Galactic Centre was observed in 500 pointings for a total of 230 hours, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 sigma after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere. Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae.We report on a survey of the inner part of the Galactic plane in very high energy gamma rays with the H.E.S.S. Cerenkov telescope system. The Galactic plane between +/-30° in longitude and +/-3° in latitude relative to the Galactic center was observed in 500 pointings for a total of 230 hr, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 σ after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere (Aharonian and coworkers). Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae.

High-energy particle acceleration in the shell of a supernova remnant

A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays). The origin of these particles remains uncertain. Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically, unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT in the X-ray spectrum and later claimed as a source of high-energy γ-rays of TeV energies (1u2009TeV = 1012u2009eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100u2009TeV, consistent with current ideas of particle acceleration in young SNR shocks.

Very high energy gamma rays from the direction of Sagittarius A

We report the detection of a point-like source of very high energy (VHE) gamma-rays coincident within 1 of Sgr A*, obtained with the H.E.S.S. array of Cherenkov telescopes. The gamma-rays exhibit a power-law energy spectrum with a spectral index of -2.2 +/- 0.09 +/- 0.15 and a flux above the 165 GeV threshold of (1.82 +/- 0.22) times 10^{-7} m^{-2} s^{-1}. The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration.H.E.S.S. – the High Energy Stereoscopic System– is a new system of large atmospheric Cherenkov telescopes for GeV/TeV astronomy. Each of the four telescopes of 107 m mirror area is equipped with a 960-pixel photomulitiplier-tube camera. This paper describes the methods used to convert the photomultiplier signals into the quantities needed for Cherenkov image analysis. Two independent calibration techniques have been applied in parallel to provide an estimation of uncertainties. Results on the long-term stability of the H.E.S.S. cameras are also presented.

H.E.S.S. observations of PKS 2155-304

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (~45 sigma) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2 sigma). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

Calibration of cameras of the H.E.S.S. detector

We report the detection of a point-like source of very high en ergy (VHE) γ-rays coincident within 1 ′ of Sgr A, obtained with the H.E.S.S. array of Cherenkov telescopes. The γ-rays exhibit a power-law energy spectrum with a spectral in dex of −2.2±0.09±0.15 and a flux above the 165 GeV threshold of (1 .82±0.22)·10−7m−2s−1. The measured flux and spectrum di ffer substantially from recent results reported in particular b y the CANGAROO collaboration.

Multi-wavelength observations of PKS 2155-304 with HESS

The High Energy Stereoscopic System (HESS) has observed the high-frequency peaked BL Lac object PKS 2155−304 in 2003 between October 19 and November 26 in Very High Energy (VHE) γ-rays (E ≥ 160 GeV for these observations). Observations were carried out simultaneously with the Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer satellite (RXTE), the Robotic Optical Transient Search Experiment (ROTSE) and the Nancay decimetric radiotelescope (NRT). Intra-night variability is seen in the VHE band, the source being detected with a high significance on each night it was observed. Variability is also found in the X-ray and optical bands on kilosecond timescales, along with flux-dependent spectral changes in the X-rays. A transient X-ray event with a 1500 s timescale is detected, making this the fastest X-ray flare seen in this object. No correlation can be established between the X-ray and the γ-ray fluxes, or any of the other wavebands, over the small range of observed variability. The average HESS spectrum shows a very soft power law shape with a photon index of 3.37 ± 0.07stat ± 0.10sys. The energy outputs in the 2–10 keV and in the VHE γ-ray range are found to be similar, with the X-rays and the optical fluxes at a level comparable to some of the lowest historical measurements, indicating that PKS 2155−304 was in a low or quiescent state during the observations. Both a leptonic and a hadronic model are used to derive source parameters from these observations. These parameters are found to be sensitive to the model of Extragalactic Background Light (EBL) that attenuates the VHE signal at this source’s redshift (z = 0.117).

Observations of Mkn 421 in 2004 with HESS at large zenith angles

Mkn 421 was observed during a high flux state for nine nights in April and May 2004 with the fully operational High Energy Stereoscopic System (H.E.S.S.) in Namibia. The observations were carried out at zenith angles of 60

A possible association of the new VHE gamma-ray source HESS J1825-137 with the pulsar wind nebula G18.0-0.7

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Serendipitous discovery of the unidentified extended TeV gamma-ray source HESS J1303-631

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Upper limits to the SN1006 multi-TeV gamma-ray flux from HESS observations

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