Discovery of VHE gamma-rays from the BL Lac object PKS 0548-322

PKS 0548-322 (z=0.069) is a ``high-frequency-peaked BL Lac object and a candidate very high energy (VHE, E>100 GeV) gamma-ray emitter, due to its high X-ray and radio flux. Observations at the VHE band provide insights into the origin of very energetic particles present in this source and the radiation processes at work. We report observations made between October 2004 and January 2008 with the H.E.S.S. array, a four imaging atmospheric-Cherenkov telescopes. Contemporaneous UV and X-ray observations with the Swift satellite in November 2006 are also reported. PKS 0548-322 is detected for the first time in the VHE band with H.E.S.S. We measure an excess of 216 gamma-rays corresponding to a significance of 5.6 standard deviations. The photon spectrum of the source is described by a power-law, with a photon index of Gamma=2.86 +/- 0.34 (stat) +/- 0.10 (sys). The integral flux above 200 GeV is 1.3 % of the flux of the Crab Nebula, and is consistent with being constant in time. Contemporaneous Swift/XRT observations reveal an X-ray flux between 2 and 10 keV of F_{2-10 keV}=2.3 +/- 0.2 x 10^{-11} erg.cm^{-2}. s^{-1}, an intermediate intensity state with respect to previous observations. The spectral energy distribution can be reproduced using a simple one-zone synchrotron self Compton model, with parameters similar those observed for other sources of this type.

In-medium $omega$ mass from the $gamma + Nb to pi^{0}gamma + X$ reaction

Data on the photoproduction of $omega$ mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from the analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 cite{david}, claiming a lowering of the $omega$ mass in the nuclear medium by 14$%$ at normal nuclear matter density. The extracted $omega$ line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The $omega$ signal on the $Nb$ target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a $LH_2$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $omega$ line shape to different in-medium modification scenarios is discussed.