Discovery of VHE y-rays from the BL Lacertae object PKS 0548-322

Abstract

Aims. 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. Methods. We report observations made between October 2004 and January 2008 with the HESS array, a four imaging atmospheric-Cherenkov telescopes. Contemporaneous UV and X-ray observations with the Swift satellite in November 2006 are also reported. Results. PKS 0548-322 is detected for the first time in the VHE band with HESS 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 $\Gamma=2.86\pm0.34_{\rm {stat}}\pm0.10_{\rm {sys}}$. The integral flux above 200 GeV is $\sim$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 $F_{2{-}10~\rm {keV}}=(2.3\pm0.2)\times10^{-11}~{\rm erg}~{\rm cm}^{-2}~{\rm s}^{-1}$, 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