Very High Energy Gamma Rays Frommarkarian

During remarkable aring activity in 1997 Markarian 501 was the brightest source in the sky at TeV energies, outshining the Crab Nebula by a factor of up to 10. Periods of aring activity each lasting a few days were observed simultaneously by several gamma ray telescopes. Contemporaneous multiwavelength observations in April 1997 show that a substantial fraction of the total AGN power is at TeV energies indicating that high energy processes dominate the energetics of this object. Rapid variability, on time scales of less than a day, and a at spectrum extending up to at least 10 TeV characterize this object. Results of 1997 observations by 6 telescopes are summarized and some of the implications of these results are discussed. INTRODUCTION Markarian 501 is a classical Bl Lacertae object, a sub-classiication of the Blazar class of AGN (core dominated, at-spectrum radio, highly optically polarized and optically violently variable). It is the second closest known BL Lac (z = 0:034) and like the closest, Markarian 421, it is a gamma-ray source. Both are classiied as X-ray-selected BL Lacs and show virtually no emission lines. It was rst discovered as a TeV-emitting gamma-ray source by the Whipple group in 1995 (Quinn et al. 1996). At that time, it had not been detected by OSSE, COMPTEL or EGRET on the CGRO in any observing period. At discovery the average TeV emission level was 8% of the Crab Nebula and there was some evidence for daily variability. In 1996 the emission level increased and there was stronger evidence for variability (Quinn et al. 1997). The source was connrmed as a TeV source by the HEGRA group (Bradbury et al. 1997).