CVs
ASCA has observed most of the X-ray bright magnetic CVs (~ a dozen each of polars and IPs). An early success was the detection of H- and He-like K-alpha emission lines of Mg, Si, S, Ar, and Fe in the spectrum of the atypical IP, EX Hya (Ishida, Mukai, & Osborne 1994, PASJ 46, L81), the first direct observational evidence for multi-temperature plasma in a CV. This spectrum was later used to derive M_WD = 0.48 Solar masses (Fujimoto & Ishida 1997, ApJ 474, 774). Unfortunately, a later work suggests that in EX Hya the accretion column may be tall (Allan et al. 1998, MNRAS 295, 167), invalidating the clean relationship between T_s and M_WD assumed by Fujimoto & Ishida.This type of work now has been expanded to many other magnetic CVs, the majority of which are believed to have negligible shock heights. Cropper et al. (1998, MNRAS 293, 222) analyzed the collection of Ginga data, where T_s can be inferred from the continuum slope particularly above 10 keV. Ezuka & Ishida (1998 ApJ, submitted) instead used ASCA data, relying heavily on the ionization temperature for iron, and derived M_WD for 9 IPs. Clearly, this is an important use of X-ray spectroscopy to derive such a fundamental (and elusive) parameter of the binaries, even though current results cannot yet be taken to be definitive.
There are other complications in the X-ray spectra of magnetic CVs that offer both challenges and opportunities. One is the characterization of the intrinsic absorption. Although it has long been realized that a single nH does not adequately describe the intrinsic absorption, ASCA is the first mission with the spectral resolution and wide bandpass required to show the true complexity of the absorption, which probably involves a continuous distribution of nH (e.g., Done & Magdziarz 1998, MNRAS, in press). The details of this distribution depend on the mass accretion rate and the geometry of the post-shock region.
Another potential complication is the optical depth in the post-shock region. Hellier, Mukai & Osborne (1998, MNRAS, in press) present a systematic analysis of the Fe K lines in magnetic CVs. In AO Psc and probably several other systems, the thermal Fe K lines are considerably broadened; they find that Compton scattering in the post-shock region is a likely mechanism. If confirmed, this result offers yet another clue in understanding the physical conditions within the post-shock region of magnetic CVs, and has implications for the feasibility of future higher resolution observations (e.g., with the XRS on Astro-E).
Progress on an understanding of the X-ray spectra of non-magnetic CVs has been hampered somewhat by the lack of a simple model for the X-ray emission region--instead of the columnar configuration of the emission region of a magnetic CV, the highly sheared boundary layer between the accretion disk and the white dwarf surface in a non-magnetic CV is a much more complicated structure. Preliminary analyses of the ASCA spectra of non-magnetic CVs (e.g., Nousek et al. 1994, ApJ 436, L19; Watson & Osborne 1996, HEAD meeting) used two- and three-temperature thermal plasma models, but these must be understood to be simply phenomenological descriptions of the data. More recently, a more physical understanding of the X-ray emission region has been assembled from a number of pieces of evidence uncovered by ASCA. First, the ASCA eclipse light curve of HT Cas (Mukai et al. 1997, ApJ 475, 812) demonstrates that the X-ray emission region is very compact. Second, the discovery of the 6.4 keV Fe K-alpha emission line in the spectrum of SS Cyg (Kitamura et al. 1996, Waseda meeting) demonstrates that, like magnetic CVs, the thermal plasma in non-magnetic CVs is in close proximity to ``cold'' plasma. Done & Osborne (1997, MNRAS 288, 649) have brought these results together in a model quite similar to that of magnetic CVs: thermal emission from cooling plasma in the boundary layer in close proximity to ``cold'' plasma in the accretion disk and the white dwarf surface. In addition to the 6.4 keV line, they include in their model the high-energy continuum reflection component expected from a dense reprocessing site. The early multi-temperature thermal plasma fits have thus been replaced by a model with a continuous distribution of temperatures, plus reprocessing from the disk and white dwarf surface. In their analysis of Ginga and ASCA spectra of SS Cyg, Done & Osborne find that the fit to the data is improved over the simple two- and three-temperature thermal plasma models, and they derive the power-law slope of the temperature-emissivity distribution, the abundances of the thermal plasma, and the effective solid angle of the reprocessing site.
Last modified: Tuesday, 26-Jun-2001 14:22:36 EDT
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