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

BL Lacertae was detected by the EGRET instrument on CGRO at the 10.2 σ level with an average flux of 171 ± 42 × 10 photons cms at energies > 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 hrs and the optical flux decreased to its prior level in less than 2 hrs. 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. Subject headings: Gamma Rays: observations — galaxies: active — BL Lacertae Objects: individual (BL Lacertae)