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Astron. Astrophys. 354, 847-852 (2000)

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1. Introduction

There is some evidence that X-ray photons from astronomical sources can not be fully described by a Poisson process. A deterministic modulation of the photon series is detected, which is reflected in observed wavelet spectra of photon counts (Liszka & Holmström 1999; hereafter referred to as Paper I). A part of the short-term variations of photon counts recorded by the ROSAT most likely contains deterministic information corresponding to the properties of the source. Wavelet spectra are a useful tool in studying these short-term temporal variations.

In particular, for active galactic nuclei (AGN) there is an indication of a physical source of deterministic variations of the X-ray flux. Pacholczyk & Stoeger (1994) propose "building blocks" in the X-ray photon flux from AGN resulting from discrete events in the source such as accretion disk flares or smaller black holes passing through the accretion disks of larger black holes. The present study indicates evidence for such building blocks.

The latest results from the Rossi X-ray Timing Explorer (RXTE) also indicate the presence of quasi-regular oscillations of the X-ray flux from some sources, both galactic and extragalactic. The oscillations may be generated by relativistic effects (Nowak et al. 1997) or by oscillations of the accretion disk.

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000