The present report describes a study of long-term temporal variations of the X-ray emission from NGC5548 recorded by the ROSAT PSPC instrument. The ROSAT satellite was primarily designed to study the spatial structure of X-ray sources. The temporal structure of the X-ray photon events will be influenced by the temporal variations of the source, the spacecraft wobble and the photon counting process in the instrument. Since the first and last process are Poisson processes, the resulting series of photon events will be a compound Poisson process modulated by the spacecraft wobble.
The present study has shown that the temporal variations of the spatial structure of the PSPC image may, especially for low photon energies, be related to real properties of the source.
Extraction of the long-term temporal variations was done dividing the core of the image into four equal areas. Average time between two photons and its variance in all four areas were analyzed using the principal component method. In-phase variations of the variables were believed to be due to real temporal variations, while the spacecraft wobble generated components with opposite phase in areas properly oriented with respect to the direction of wobble. A method for decomposition of the components is presented.
IRF Scientific Report 236
December 1996.