Starburst galaxies exhibit in their central regions a highly increased rate of supernovae, which are expected to generate a high density of cosmic rays accelerated in the supernova shocks. The detection of gamma rays from the starburst galaxy NGC 253 in deep H.E.S.S. observations indeed indicates a cosmic ray density about three orders of magnitude larger than in the center of the Milky Way. With a flux of 0.3% of the Crab Nebula flux, NGC 253 is the faintest very high energy gamma ray source detected so far. The paper appeared in Science Express, Sept. 24, 2009 and on the preprint server: arXiv:0909.4651. See also MPG press release.

Galaxy clusters act as storehouses for cosmic rays, escape times from a cluster typically being larger than the age of the Universe. Upper limits on the very high energy gamma rays flux determined with H.E.S.S. constrain the content of high energy particles in the Coma cluster; depending on the detailed assumptions, limits are as low as 20% of the thermal energy content of the cluster, ruling out the most optimistic models for gamma-ray emission from clusters. The paper is in press in A&A and can be found on the preprint server: arXiv:0907.0727.

The three leading teams in the field of very-high-energy gamma-ray astrophysics, H.E.S.S., MAGIC, and VERITAS, joined up with a team of radio astronomers to, for the first time, reveal the precise location of particle acceleration in the close-by giant radio galaxy Messier 87. Simultaneous observations at the lowest and highest ends of the electromagnetic spectrum indicate that this active galactic nucleus accelerates elementary particles to very high energies in the immediate vicinity of its central black hole. The paper has been published in Science Express. See also H.E.S.S. press release in English, German, French, MPG press release, NRAO press release, MAGIC press release, VERITAS press release.

Simultaneous HESS/CHANDRA/optical observations were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent its second major gamma-ray outburst in Summer 2006. The emission in the X-ray and gamma ray bands is found to be strongly correlated, both in flux and spectrum, with no evidence of lags; the detailed patterns of correlation between wavebands challenge the common scenarios for the TeV-blazar emission. The paper is in press in A&A and can be found on the preprint server: arXiv:0906.2002.

Analysis of a three-year (2004, 2005, 2006) data set of observations of HESS J1745-290 reveals emission following a power-law energy spectrum with a cutoff at high energies. No indications of flares or (quasi-)periodic oscillations - as observed in IR and X-rays - are detected; the VHE gamma-ray source is steady. The paper is accepted for publication in A&A and can be found on the preprint server: arXiv:0906.1247.

The previous H.E.S.S. measurement of the cosmic-ray electron spectrum (arXiv:0811.3894) is now extended down to 340 GeV, covering the region of the bump reported by ATIC, which is possibly attributed to dark matter annihilation. The spectrum measured by H.E.S.S. is a power law up to 1 TeV with an index of 3.0, without any significant structure. The paper can be found on the preprint server: arXiv:0905.0105.

H.E.S.S. observations during the Galactic Plane Scan confirm the source MGRO J1908+06 reported by Milagro, providing a >10 sigma detection and determination of the spectrum between 300 GeV and 20 TeV. The paper can be found on the preprint server: arXiv:0904.3409.

This week (Apr 27 - Mai 1) the H.E.S.S. collaboration headed over to Yerevan in Armenia for the spring collaboration meeting. A recent picture of the collaboration members can be found in the collaboration section.

The globular cluster 47 Tucanae is one of the largest globular clusters known, with a mass of a about a million solar masses, at a distance of 4 kpc. 47 Tucanae contains at least 23 millisecond pulsar and some 200 X-ray sources, hence VHE gamma-ray emission can be expected. In 13 h of H.E.S.S. observations, no signal was detected. The upper limit of 2% of the Crab flux results in constraints on the number and emission characteristics of millisecond pulsars. The paper can be found on the preprint server: arXiv:0904.0361.

Centaurus A (Cen A) is the nearest active radio galaxy and suspected as a source of ultra high energy cosmic rays. In more than 120 h of H.E.S.S. observations, a VHE gamma ray signal from Cen A is observed at a level of 0.8% of the Crab flux. The discovery of VHE gamma-ray emission from Cen A reveals particle acceleration in the source to >TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters. The paper has been published in the Astrophysical Journal Letters (preprint server: arXiv:0903.1582). See also H.E.S.S. press release in English, German, French, MPI-K press release, news l'Observatoire de Paris, and H.E.S.S. source of the month.

In a first joint campaign between H.E.S.S. and the LAT instrument on the Fermi satellite, including also the RXTE X-ray instrument and the ATOM optical telescope, the emission from the active galaxy PKS 2155-304 was tracked for 11 days over a wide energy range, including seamless gamma-ray coverage from 200 MeV to several TeV. An unusual pattern of variability is observed: the flux of VHE gamma rays correlates with the optical flux, but not with the X-ray flux. The paper can be found on the preprint server: arXiv:0903.2924. See also H.E.S.S. press release in English, German, French, NASA press release, MPG press release.

Detection of VHE gamma rays from GRBs would help in pinning down the mechanisms of particle acceleration in these objects. H.E.S.S. runs since 2003 a program of GRB follow-up observations, so far without positive detections. The paper reports flux limits for 22 GRBs observed in 2003-2007, it can be found on the preprint server: arXiv:0901.2187.