Recent Advances in X-ray Observations of Cataclysmic Variables

A personal selection of noteworthy X-ray results on CVs are presented, with emphasis on XMM-Newton and Chandra observations. Progressing roughly from broad-band view to narrow-band, high spectral resolution studies, I summarize: the energy balance of polars; X-ray confirmation of IPs; eclipses in non-magnetic CVs; search for magnetism in " non-magnetic " CVs; multi-temperature plasma emission from the boundary layer; complex absorption in magnetic CVs; temperature and density diagnostics; and X-ray radial velocity studies. Over the past 25 years or so, X-ray observations have played a major role in the research into CVs. It would only be a slight overstatement to say that the class of magnetic CVs owes its existence to X-ray astronomy, as a high fraction of magnetic CVs are discovered and/or confirmed through X-ray observations. The role X-rays have played for non-magnetic CVs is less central, but nevertheless important. This is because, according to simple arguments, half the accretion luminosity of CVs ought to be emitted in the X-ray band. Unfortunately, this expectation has not been fulfilled observationally in most cases — we must be missing something, either theoretically or observationally. It is an exciting era in X-ray astronomy, with the recent launches of Chan-dra and XMM-Newton, and the upcoming launch of Astro-E2. For example, not only is the Chandra image of the Galactic Center region breath-taking, a significant fraction of the thousands of point sources may be (magnetic) CVs (Muno et al. 2003). In this review, I will define " X-rays " as the range 0.1–25 keV, or roughly 125–0.5 ˚ A. This is a factor of 250 in wavelength, equivalent to 1000Å (UV) to 25 µm (mid-IR). It is therefore not surprising to find several distinct spectral components that contribute to the X-ray emission from CVs. Single X-ray instruments often cover a factor of 25 in wavelength; this is a strength of X-ray astronomy. On the other hand, " high resolution spectroscopy " is a relative term. In terms of spectral resolution, ROSAT PSPC data are comparable to UBVRI photometry. CCD-based instruments (ASCA SIS, Chandra ACIS, and XMM-Newton EPIC) have typical resolution at 6 keV of 2%, and therefore may be compared to narrow-band photometry (∆λ ∼100Å at 5000Å, for example).