NOISE VARIABILITY OF THE HARD X-RAY TRANSIENT NOVA PERSEI

The hard X-ray transient Nova Persei (GRO J0422+32) was observed with the SIGMA telescope continuously for 2 months, soon after its discover y in 1992 August. The temporal analysis of the 40-300 keV emission of this source showed a very high level of noise variability (approximate ly 20%). Such variability has previously only been observed at hard X- ray/soft gamma-ray energies from an accreting X-ray source, in the cas e of the black hole candidate Cygnus X-1. The Fourier power distributi on in the 2 x 10(-4)-1.25 x 10(-1) Hz range is flat up to a break freq uency (approximately 0.03 Hz) and then steepens to a power law with an index of approximately -0.8 around 0.1 Hz. These results can be inter preted in terms of the shot noise model. The photons in the shots repr esent a significant fraction (approximately 20%) of the source hard X- ray flux. The derived shot duration range from approximately 1 to a fe w seconds, depending on the shot shape. The shot shape and duration do not show any correlation with the hard X-ray source flux, as the spec tral shape did not apparently evolve during the outburst, so probably they are only weakly correlated with the accretion rate. While the sou rce flux decreased uniformly from around 3 to 1 Crab unit in the perio d 1992 August 15 until September 25, the fractional integrated root me an square variability increased from about 15% to 30%. We have studied this correlation with the help of a ''source total flux-source noise flux'' diagram which reveals a regular but rather complex dependence w ith at least three different regimes.