X-ray observations of cataclysmic variables

Abstract

Cataclysmic variables are close binary systems where mass is accreted onto a white dwarf through an accretion disc. Approximately half the gravitational energy in the disc is released in a boundary layer as X-rays. X-rays originate from matter under the extreme and energetic conditions and provide a unique view of high energy processes. X-ray luminosities are sensitive to the accretion rate through the disc and to the conditions in the inner accretion zone. Accretion discs are wide spread throughout astronomy. The extraction of potential energy from accreted material is known to be the principal source of power in several types of system; quasars, galactic nuclei, binary X-ray sources, cataclysmic variables (CVs) and proto-planetary discs. CVs provide probably the best opportunity to study the accretion process in isolation. Previous X-ray analysis of dwarf novae relied upon relatively short snap shot observations, which are unable to provide a full picture of the outburst cycle evolution. Multiple outbursts with far greater temporal accuracy and coverage than has ever been observed before are presented in this thesis. Pointed observations using the proportional counter array on the Rossi X-ray Timing Explorer of SS Cygni, U Gem and SU UMa are analysed. The behaviour in the optical band is similar for the three systems in this thesis, however, a large distinction is seen in the X-ray band. The hard X-ray outburst flux in SS Cygni and SU UMa are quenched below the quiescent flux, while in U Gem it is unusually faint in quiescence brightening in outburst. The hard quiescent X-ray spectrum is replaced by an intense soft X-ray component in outburst for all dwarf novae. Analysis of U Gem suggests that X-rays originate from the inner accretion disc with a scale height not much greater than the disc thickness. The start of the hard X-ray outburst is delayed behind the optical rise, this delay is roughly consistent for the three systems presented. This indicates that the origin of the heating wave in the accretion disc and the time it takes to propagate to the boundary layer are similar for these systems. The hard X-ray recovery also has a range of times, with the peak occurring as the optical flux reaches quiescence suggesting the cooling front reaches the boundary layer at the same time in relation to the end of the optical outburst. The spectra for all three systems presented in this thesis are well described by a thermal plasma model with sub-solar abundances and are consistent with higher reflection during the hard X-ray suppression. This indicates that the disc is likely to be truncated in quiescence. SS Cygni has a wide range of quiescent accretion rates. However, the X-ray flux in SS Cygni and U Gem always increases when the boundary layer transitions from both optically thick to thin, into outburst, and optically thin to thick, out of outburst. This is surprising, the flux is expected to decrease when the critical accretion rate is reached suggesting that the critical accretion rate when the boundary layer transitions is not fixed. The quiescent X-ray flux in SU UMa decreases and, with SS Cygni, is between 2 − 3 orders of magnitude higher than predictions by the disc instability model