Accretion regimes in the X-ray binary systems A 0535+26 and Her X-1/HZ Her [thesis]

Daniela Bohl, Universitätsbibliothek Tübingen, Universitätsbibliothek Tübingen
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
more » ... nalyzed 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 t [...]
doi:10.15496/publikation-420 fatcat:bmqy4ihjsbcmfewzyykkebbwyu