G. T. B. Ramsay
ROSAT observations of AM Her Stars
1995 (supervisor: K. O. Mason)

I have examined a sample of 19 AM Her stars observed using the ROSAT satellite. I fit model spectra to the ROSAT data to determine the energy balance ratio of these systems. I find that although the majority of the systems show a soft X-ray excess over the standard model of Lamb & Masters, there is a spread in the soft X-ray excess for systems observed in the high state. Further, I find that the value of the soft X-ray excess is correlated with the magnetic field strength of the white dwarf and of the radius at which material couples onto the magnetic field of the white dwarf. There were another 3 systems which were found to be in an intermediate state. One of these systems was found to show a soft X-ray deficiency over the standard model. For one of the systems observed in a low state, a much lower thermal bremsstrahlung temperature compared to that expected in the high state was found.

To account for these observations I propose a new picture of the accretion region in AM Hers. In the high state, dense blobs penetrate the white dwarf photosphere releasing most of the kinetic energy as soft X-rays. The proportion of dense blobs in the accretion flow is determined by the magnetic field of the white dwarf. As the accretion rate decreases, the proportion of the accretion region in which a shock forms decreases until in the low state no shock forms. In the portion of the accretion region in which no shock forms, the accretion rate is such that the temperature of the thermalised radiation is lower by a factor of 2-3 compared to the high state.

Using the ROSAT data and previously published data, I find that the length of the bright phase of the two-pole systems can vary quite considerably. Further, in some systems the phase at which the bright phase commences varies over time, whilst the end of the bright phase does not. In one system the converse is found. I suggest that varying the extent of the accretion region can account for the variable bright phase. Further, it is the sign of the angle that the magnetic field of the white dwarf makes with the secondary star which influences if it is the leading or trailing edge which varies in its extent.

I also make distance estimates to the symbiotic star RX Pup and the super-soft source GQ Mus using the BVI_c reddening distance technique. This continues the work I first started during the course of my MSc at the University of St. Andrews.