A Gamma‐Ray Flare from PKS 2255−282

Abstract

A gamma-ray flare from the quasar PKS 2255-282 was detected in 1997 December by the EGRET instrument on board the Compton Gamma Ray Observatory (CGRO). Previously undetected at gamma-ray energies, the source remained bright for 2 weeks of observations with weak evidence of gamma-ray variability within the flare observation. The average flux was (1.6±0.3)×10−6 cm−2 s−1 above 100 MeV with a hard power-law index of 1.7±0.3. This places the outburst among the brightest detected by EGRET. A later Rossi X-Ray Timing Explorer (RXTE) observation revealed a simple power-law spectrum with photon index 1.8 and 2-10 keV flux of 1.44×10−11 ergs cm−2 s−1. This paper describes the CGRO and RXTE observations and data analysis, the source identification, and gamma-ray variability. We also consider the broadband spectrum, albeit on the basis of nonsimultaneous multiwavelength measurements, and the nature of the outburst from this source in the context of other known gamma-ray blazars.