Simulation of Aperiodic and Periodic Variabilities in X-RAY Sources

L. Burderi, M. Guainazzi, N. R. Robba

Istituto di Fisica, Universitá di Palermo, Via Archirafi 36, I- 90133 Palermo, Italy

Introduction

The Power Spectrum Density (PSD) has been extensively used to study the periodic (Harmonic Lines, HL) and the aperiodic (Red Noise, RN) features of the temporal variability of the High Mass X Ray Binaries (HMXRB). Some authors have proposed a very simple Shot Noise model (SN) in order to explain the RN feature (Terrel and Olsen 1970). In this scenario the plasma instabilities that can occur during the accretion process (Rayleigh-Taylor, Kelvin-Helmoltz, Alfvén zone instabilities above the accretion column) could clump the matter in blobs (Elsner and Lamb 1977,1984; Arons and Lea 1976); the shot emission associated with the accretion of these blobs is the cause of the aperiodic variability. The timescale associated with the instabilities mentioned above cover a very wide range (roughly from milliseconds to tens of seconds). Due to the casual occurrence of the blobs, as a consequence of the instability phenomena, these emissions are supposed to occur randomly (stochastically) in time.

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