AbstractsAstronomy & Space Science

This thesis discusses the physics of two accreting X-ray pulsars, A 0535+26 and Her X-1. Based on observations from different X-ray satellites, the timing and spectral behavior of the two systems is studied. A 0535+26 is a Be/X-ray binary system, i.e. it regularly undergoes normal and giant outbursts, separated by phases of quiescence. In April 2010, Swift/BAT detected a brightening of A 0535+26. The maximum and the decay of the outburst were observed by the RXTE and INTEGRAL observatories. We analyzed the light curves, pulse profiles and the pulsed fraction of the source and performed a spectral analysis using the RXTE and INTEGRAL data. Our analysis reveals variability of the continuum parameters with source flux, in particular a hardening of the continuum with increasing source flux on short time scales (i.e. for single pulsation cycles) and long time scales (i.e. on time scales of many pulsation cycles). The spectra averaged over many pulsation cycles of A 0535+26 show a variation of the cyclotron line centroid energy but no correlation with the luminosity of the source. However, our short time scale analyses (i.e. based on fragments of the pulsation cycle) indicate a positive correlation of the cyclotron line energy with the source flux. Some accreting X-ray pulsars show spectral changes on long time scales and on the time scale of single pulsation cycles. Depending on the type of spectral variations with flux, the sources seem to form distinct groups. This behavior is probably reflected in the accretion regimes that may result in a particular pulsar depending on its luminosity: the so-called sub- and super-critical accretion regimes. Based on the observed spectral properties, we argue that A 0535+26 operates at the sub-critical accretion regime during the analyzed outburst. The second part of the thesis describes the analysis of the X-ray binary system Her X-1/HZ Her. The light curve of Her X-1 shows different periodicities including the 35 day super-orbital period with two on-states (high X-ray flux) and two off-states (low X-ray flux). The source's pulse period shows a long-term spin-up, interrupted by short intense spin-down periods. A possible explanation for this behavior can be ejection of matter during the spin-down episodes. In this work, we have analyzed XMM-Newton observations taken during main-on states of the source and studied the spectral changes in the RGS and EPIC data. We compared the derived amount of hot, photoionized material for different spin-up/-down regimes. A change in the amount of matter around the neutron star is expected to be indicated by the width and intensity of the emission lines from the photoionized plasma and of the iron emission line. We do not see a correlation between the line parameters and the locally measured pulse period change, and thereby cannot supply evidence of increased disk outflows within this analysis. The analysis of EPIC-pn spectra on short time scales (i.e. based on fragments of the pulsation cycle) reveals a unprecedented variability of the…