An outburst of geV gamma-ray emission from centaurus X-3

We present Compton Gamma Ray Observatory (CGRO) EGRET measurements of an outburst of greater than 100 MeV gamma-ray emission from the massive X-ray binary system Centaurus X-3 that occurred during an interval of rapid spin-down by the X-ray pulsar. For the 1994 October outburst, the phase-averaged 30 MeV- 10 GeV emission is best fitted by a power law with index a 5 1.81 H 0.37 and an integral flux above 100 MeV of (9.2 H 2.3) 3 1027 photons cm22 s21, corresponding to a phase-averaged luminosity in GeV gamma rays of 15 3 1036 ergs s21. Our phase analysis, employing contemporaneous X-ray pulse observations by CGRO BATSE, indicates modulation of the gamma-ray emission at the pulsar's spin frequency with a significance level higher than 99.5%. Straightforward interpretation of the EGRET measurements requires at least sporadic acceleration of GeV particles within the Cen X-3 binary system. The observations also suggest that Galactic X-ray binary systems may constitute a class of highly variable GeV gamma-ray sources.