Detection of Gamma Rays withE> 100 MeV from BL Lacertae

Abstract

We present evidence for the first detection of gamma rays from the extragalactic object BL Lacertae. Observations taken with EGRET on the Compton Gamma Ray Observatory between 1995 January 24 and 1995 February 14 indicate a 4.4 σ excess from the direction of BL Lacertae. The corresponding flux is (40 ± 12) × 10-8 photons cm-2 s-1 above 100 MeV. The combination of all previous observations where BL Lacertae was in EGRET's field of view result in a 2.4 σ excess and a corresponding 95% confidence upper limit of 14 × 10-8 photons cm-2 s-1, indicating that its gamma-ray emission is variable, at least on timescales of several months. Observations of BL Lacertae between 22 and 375 GHz were also taken between 1995 January 24 and 1995 February 14, and the flux levels for those measurements are similar to the historical average values for this object. A deep exposure on BL Lacertae with the Whipple Observatory 10 m gamma-ray telescope shows no evidence of emission above 350 GeV during a period 9 months after the EGRET observations. The 99.9% confidence flux upper limit derived from these observations is 0.53 × 10-11 photons cm-2 s-1, which implies a large reduction in the gamma-ray emission of BL Lacertae between EGRET and Whipple Observatory energies. This reduction should result from processes intrinsic to BL Lacertae because it is near enough to Earth that intergalactic background IR fields should not significantly reduce the flux of gamma rays to which the Whipple Observatory telescope is sensitive.