ON THE HARD X-RAY VARIABILITY OF CENTAURUS-A

The radio galaxy Centaurus A has been observed by the imaging telescop e 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 comp atible with the position of the galaxy and from no other source in the region. A comparison between the observations indicates a flux increa se by a factor of 3 over a 1 yr time scale but even more interesting i s a similar decrease which was observed in just 4 days; this is the fi rst report of short-time scale variability at hard X-ray frequencies; it agrees well with soft X-ray measurements. If due to a flaring compo nent, we estimate the duration of a typical event to be 8-10 days (ris e and decay time) with a frequency of approximately 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 intensit y variation (DELTAL(40-200kcV) approximately 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 sha pe of the spectra observed is compatible with no change and is well fi tted by a power law with an average alpha close to 2 up to about 100 k eV. We have compared this value with a set of other measurements and f ound that they are compatible with a Gaussian distribution for alpha h aving a mean value of 1.64 and standard deviation of 0.28. Our 1990 da ta 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 abov e 100 keV both in the high- and low-intensity states. Marginal evidenc e for such a break is suggested by our data.