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 > 300 GeV were detected with a time-averaged integral flux of ( 4.2 \pm 0.7 _ { stat } \pm 2.0 _ { sys } ) \times 10 ^ { -11 } ~ { } { cm } ^ { -2 } ~ { } { s } ^ { -1 } .
There is evidence for VHE gamma ray emission during our observations in 1996 September and 1997 October/November .
The strongest emission was detected in 1997 November , when the object was producing the largest flux ever recorded in high-energy X-rays and was detected in > 100 MeV gamma-rays .
The VHE and X-ray fluxes are correlated .