HARD X-RAY VARIABILITY OF X-RAY NOVA PERSEI 1992 - GRANAT/SIGMA RESULTS

Nova Persei 1992 (GRO J0422+32) was observed by Granat/SIGMA in 1992 August-September, being at that time the brightest hard X-ray source detected by SIGMA. We have analyzed the source hard (40-150 keV) X-ray variability on timescales from days to seconds. From 1992 August 15 until September 25 the 40-150 keV decreased monotonically from 2.9 to 1.0 crab following an exponential decay law with an e-folding time of 43 days. On timescales of hours the source was remarkably stable: the rms variations with respect to the exponential fit are approximate to 3%. Power density spectra (PDSs) were obtained in the 10(-4)-10 Hz frequency range. The power density is nearly constant below 0.03 Hz and decreases as a power law f(-1) above this frequency, with an index alpha similar or equal to 0.9. A significant decrease of power density below the frequency of about 3 x 10(-3) Hz was seen during the first half of observations. A strong peak of quasi-periodic oscillations was detected at the frequency of 0.3 Hz, with associated rms variations of intensity similar to 12%. The total fractional rms variations of the source flux in the 10(-3)-10(-1) Hz range were similar to 15% during the first observations and increased to similar to 25% as the source flux decreased. A shot noise model has been applied to describe the strong flickering of GRO J0422+32, and parameters of this model (shot amplitude, rate, and typical duration) have been determined from the analysis of the PDSs and the counts distribution.