X-ray Diagnostics of Accretion Plasmas in Selected Soft Polars

Abstract

Accretion processes occur during diverse stages of stellar evolution and on diverse scales, as for example in star formation, in active galactic nuclei, or in close binaries with a compact late-type component; the X-ray binaries and cataclysmic variables. Magnetic cataclysmic variables of AM Her type, also called polars, comprise a white dwarf with a strong magnetic field, accreting matter from a Roche-lobe filling low-mass secondary. The accretion stream is channeled along the field lines towards the magnetic poles of the white dwarf. Forming a strong shock at high temperatures above the star surface, it is decelerated and cooled by optical cyclotron radiation and by bremsstrahlung in the hard X-ray regime. Soft X-ray and ultraviolet radiation arise from the heated photosphere, where the hard emission is reprocessed. Although the two X-ray flux components would be expected to balance each other according to the standard picture, several systems show a distinct dominance of soft over hard X-radiation. In these polars, cyclotron emission is believed to be the main cooling process, associated with inhomogeneous and time-variable accretion.The work concentrates on the two soft magnetic cataclysmic variables AI Tri and QS Tel. The systems have been observed during high and intermediate high states of accretion with the X-ray satellite XMM-Newton and with optical telescopes; basis for a detailed photometric and spectroscopic analysis. In order to account for the complex structure of the X-ray emitting regions, the usual black body and plasma models have been expanded to multi-temperature approaches in the spectral fits. A high soft-to-hard X-ray flux ratio characterizes both objects. Highly variable on short time scales, their light curves reflect the inhomogeneous, "blobby" column accretion. Emission from one main accretion region shapes the observational data of the long-period polar AI Tri. The region is eclipsed by the accretion stream for a tenth of the orbital cycle. The best spectral fit yields temperatures of up to $44.0^{+3.3}_{-3.2}$ eV for the soft component and between $0.8^{+0.4}_{-0.2}$ keV and $20.3^{+8.7}{-5.1}$ keV for the plasma component with nearly solar element abundances. During two observations, AI Tri was found in an irregular mode of accretion with a considerably softer X-ray spectrum and a phase shift of the optical light curves by $\Delta\phi\simeq0.2$. From the second analyzed polar, QS Tel, hardly any counts above energies of 2 keV are detected. Known to switch between one-pole and two-pole accretion, it was found in a stage dominated by emission from the X-ray bright pole during the XMM-Newton observation. The second pole, mainly seen at optical and ultraviolet wavelengths, showed less activity. Rich metal lines turn up in the X-ray spectra of QS Tel, which are composed of cool plasma and black body components at temperatures below 5 keV and around $19.5^{+3.7}_{-4.2}$ eV respectively. The comparison of different fits to the spectra of the two objects demonstrates the need for models considering the wide range of densities and temperatures in the accretion plasmas. The impact of the choice of the spectral models on the soft-to-hard ratios in polars is discussed.