It is usually assumed that photon events, as X-ray photons observed by a satellite, may be described by the theory of point processes (cf e. g. Matsuo, Teich and Saleh, 1983). The simplest and most common process is the homogeneous Poisson point process. Photons are usually generated in a Poisson process and in addition to that, the detection of photons is a Poisson process too. In the case of ROSAT satellite there is an additional complication. The spacecraft itself wobbles with a period of 400 seconds in order to avoid that the same parts of the sensor mosaic are constantly receiving photons from the same sources on the sky. The final result is a photon flux controlled by several Poisson processes and modulated by the spacecraft wobble. In order to study the dynamical properties of a source it is necessary to study the spectrum of variations of the Poisson parameter (intensity of the process), at least above the wobble frequency. For frequencies from the wobble frequency and below, a method based on the statistical analysis of the image has been proposed earlier (Liszka, 1996b).
The present study will deal with temporal variations of which are much shorter than the wobble period. Of course, nothing may be said about variations shorter than the shortest . Another question is whether the recorded photon events really may be described by a Poisson process. Time series of photon counts are analyzed using the wavelet technique and the principal component method.
IRF Scientific Report 239
January 1997.