Cataclysmic VariablesThe collective properties of the X-ray emission from non-magnetic cataclysmic variables have been studied by van Teeseling et al. (1996, A&A 315, 467) and by Richman (1996, ApJ 462, 404) using archival ROSAT PSPC data. Among their significant findings are: an anti-correlation between the binary inclination and the X-ray luminosity; an apparent anti-correlation between the primary mass (estimated using optical data) and the X-ray temperature; and an anti-correlation between the X-ray temperature and the optical/UV luminosity, which is believed to track the accretion rate. The first of these implies that there is significant intrinsic absorption of X-rays in high inclination non-magnetic CVs; the latter two have not been explained satisfactorily yet. Similarly, several recent papers have studied the hard X-ray spectra of magnetic CVs, using Ginga (Cropper et al 1998, MNRAS 293, 222), ASCA (Hellier et al. 1998, MNRAS 297, 526; Ezuka & Ishida 1999, ApJS 120, 277), and RXTE (Ramsay 2000, MNRAS 314, 403) data. The shock temperatures obtained from continuum fits and from line ratios indicate the white dwarf masses in magnetic CVs are higher than in the non-accreting ones. However, the thermal Fe K- lines have been found to be narrow in some systems but broad in others, probably due to resonant trapping of line photons, which can escape only after being Compton down-shifted, suggesting caution in interpreting line ratio data. As for the continuum, a study of BY Cam by Done & Magdziarz (1998, MNRAS 298, 737) has found that complex absorption may have skewed the previous fits. Further archival studies of additional data (e.g., of BeppoSAX data) and/or using more sophisticated models are clearly necessary; the same goes for non-magnetic systems, for which several groups have embarked on a systematic study of the archival ASCA data.
Among magnetic CVs, the AM Her subclass has a pronounced, blackbody-like, soft X-ray/EUV component from the white dwarf surface. Mauche (1999, ASP Conf. Ser. 157, 157) has performed a study of EUVE spectra of 11 AM Her systems, and found that the blackbody provides a better phenomenological description of the spectra than stellar atmosphere models. However, he has also found weak absorption edges of Ne VI in several systems. This clearly indicates the inadequacy of the current generation of models: perhaps a class of irradiated stellar atmosphere models needs to be tried. Further analyses of archival EUVE data, including combined temporal/spectral analyses of brighter sources, seem worthwhile.
The hard X-rays from the intermediate polar FO Aqr show pronounced spin and orbital modulations; Beardmore et al. (1998, MNRAS 297, 337) have shown that the light curves are highly variable from epoch to epoch, using archival EXOSAT, Ginga, and AQSCA observations. They have developed a model in which the light curve morphology changes in response to the variable mass-accretion rate. They and other groups are embarking on similar comparative studies of X-ray light curves of individual objects. In fact, such a comparison is now widely recognized to be a powerful and easy to use tool; as a result, many papers on new observations routinely present direct comparison with archival data.
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