Very High Energy Gamma Rays from PKS 2155−304

Abstract

The close X-ray-selected BL Lac PKS 2155-304 has been observed using the University of Durham Mark 6 very high energy (VHE) gamma-ray telescope during 1996 September/October/November and 1997 October/November. VHE gamma rays with energy more than 300 GeV were detected from this object with a time-averaged integral flux of (4.2±0.7_stat±2.0_sys)×10⁻¹¹ cm^-2 s^-1. There is evidence for VHE gamma-ray emission during our observations in 1996 September and 1997 October/November, with the strongest emission being detected in 1997 November, when the object was producing the largest flux ever recorded in high-energy X-rays and it was detected as a source of gamma rays of energy greater than 100 MeV. The VHE and X-ray fluxes show evidence of a correlation.