On the hard X-ray variability of Centaurus A

Abstract

The radio galaxy Centaurus A has been observed by the imaging telescope SIGMA on board the GRANAT satellite on three occasions during a 1 yr period. Hard X-ray emission was detected each time at a position compatible with the position of the galaxy and from no other source in the region. A comparison between the observations indicates a flux increase by a factor of 3 over a 1 yr time scale but even more interesting is a similar decrease which was observed in just 4 days; this is the first report of short-time scale variability at hard X-ray frequencies; it agrees well with soft X-ray measurements. If due to a flaring component, we estimate the duration of a typical event to be 8-10 days (rise and decay time) with a frequency of ∼45 events per year