GeV Gamma-Ray Variability from the TeV Blazar PKS 2155-304

W. Thomas Vestrand

P. Sreekumar

J. Gregory Stacy


We present EGRET observations of a major outburst of GeV gamma-ray emission from the BL Lac PKS 2155-304. During those observations the > 100 MeV flux exceeded the upper limit for the preceding PKS 2155-304 observation by a factor of five and the highest flux previously measured by a factor of three. The > 100 MeV gamma-ray flux also showed evidence for variability on a timescale of 3.5 days. An outburst of TeV gamma-rays was also detected contemporaneously from PKS 2155-304 by a ground-based imaging Cherenkov telescope. ToO x-ray observations with RXTE and BeppoSAX, that were initiated by the detection of the GeV outburst, found the x-ray emission to be at the highest flux levels yet observed. Unlike the major TeV/x-ray outbursts detected from Mrk 421 and Mrk 501, PKS 2155-304 therefore shows strong associated variability at GeV energies. We briefly discuss the implications for our understanding of TeV blazars.

File translated from TEX by TTH, version 2.32.
On 2 Aug 1999, 16:04.