Observations of a Correlated Gamma-Ray and Optical Flare for BL Lacertae

BL Lacertae was detected by the EGRET instrument on the Compton Gamma Ray Observatory at the 10.2 σ level with an average flux of (171 ± 42) × 10 −8 photons cm -2 s -1 , at energies greater than 100 MeV, during the optical outburst of 1997 July. This flux is more than 4 times the previously highest level. Within the July 15-22 observation there was a dramatic factor of 2.5 increase in the gamma-ray flux on July 18.75-19.08, apparently preceding, by several hours, a brief optical flare. The gamma-ray flux decreased to its previous level within 8 hr, and the optical flux decreased to its prior level in less than 2 hr. The gamma-ray photon spectral index of 1.68±0.12 indicates that the spectrum during the 7 day observation was harder than the previous detection.