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 discovery in 1992 August. The temporal analysis of the 40-300 keV emission of this source showed a very high level of noise variability (approximately 20%). Such variability has previously only been observed at hard X-ray/soft gamma-ray energies from an accreting X-ray source, in the case of the black hole candidate Cygnus X-1. The Fourier power distribution in the 2 x 10(-4)-1.25 x 10(-1) Hz range is flat up to a break frequency (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 interpreted in terms of the shot noise model. The photons in the shots represent a significant fraction (approximately 20%) of the source hard X-ray flux. The derived shot duration range from approximately 1 to a few 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 spectral shape did not apparently evolve during the outburst, so probably they are only weakly correlated with the accretion rate. While the source flux decreased uniformly from around 3 to 1 Crab unit in the period 1992 August 15 until September 25, the fractional integrated root mean 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 with at least three different regimes.