On the Hard X-Ray Variability of Centaurus A

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. The amount of energy involved is enormous as photons from a few keV to a few hundreds of keV (and maybe more) are involved in the intensity variation ({DELTA}L_(40-200keV) ~ 3 x 10^42^ ergs s^-1^) and the region responsible for the process is probably the nucleus of the galaxy. Despite the intensity variation of a factor of 2 or 3, the shape of the spectra observed is compatible with no change and is well fitted by a power law with an average α close to 2 up to about 100 keV. We have compared this value with a set of other measurements and found that they are compatible with a Gaussian distribution for a having a mean value of 1.64 and standard deviation of 0.28. Our 1990 data compare well with contemporaneous data at low energies. However, if extrapolated to higher energies in order to meet the SAS 2 and COS B upper limits the broad-band spectra require a break or steepening above 100 keV both in the high- and low-intensity states. Marginal evidence for such a break is suggested by our data.