Ray and EUV Emission from Nonmagnetic CVs

Observations performed with the Einstein and ROSAT satellites led to the consensus that the X-ray emission from nonmagnetic CVs falls increasingly short of the predictions of simple BL theories as the accretion rate and the luminosity of the accretion disk increase [65,89,92]. The observed HellXX4686,1640 A line fluxes suggest the presence of a higher intrinsic soft X-ray flux than actually seen and led to the conclusion that part of the EUV flux is hidden from view. Such interpretation is supported by the fact that the FX/FV ratio decreases with increasing inclination. In quiescence, a thermal X-ray component with a temperature of a few kiloelec-tronvolts dominates. The analysis of this source in the eclipsing CVs HT Cas and OY Car demonstrated that the observed emission arises from very close to the white dwarf. In the XMM-Newton EPIC light curve of OY Car in Fig. 12.2, the length of the X-ray totality is identical to that of the optical light of the white dwarf, but its ingress and egress times are significantly shorter than that of the white dwarf. This

Fig. 12.2 Folded XMM-Newton eclipse light curve of OY Car (June 2000). The dash-dotted vertical lines indicate the optical contact points marking ingress and egress of the white dwarf [96, private communication]

difference suggests that the X-ray emission arises from a region on the white dwarf, which is displaced toward its upper rotational pole [96]. Having noted (see (12.1)) that a moderate magnetic field of the white dwarf can lift the flow out of the plane, the offset of the X-ray source appears plausible. The special geometry may also prevent excessive internal absorption of the X-ray emission by the accretion disk at the inclination of OY Car of i ~ 83°.

Wavelength dependent X-ray light curves through dwarf nova outbursts represent an independent powerful approach to study the BL emission. Results include the X-ray, EUV, and optical coverage of the low-inclination long-period system SS Cyg [97] and the worldwide campaign to observe the unexpected 2001 superoutburst of the eclipsing short-period system WZ Sge [42,55]. Earlier outburst observations of OY Car [56], U Gem [50], and VW Hyi [95] complement the picture. In outburst, the rise of the optical light precedes that of the EUV by the time the heat wave needs to transit the disk. The optically thin X-ray source is quenched and replaced by the optically thick EUV component. The fainter X-ray emission during outburst cannot be uniquely assigned to a single source, but contains components from regions with different excitation conditions absorbed by different column densities. This may include emission from an accretion disk corona, possibly heated by magnetic activity [97]. At the end of the outburst, the EUV subsides, the collisionally excited thermal BL X-rays reappear, and the system returns to the emission of the quiescent state. HST observations of the cooling white dwarf in the 2001 outburst of WZ Sge indicate that the entire white dwarf is heated, but initially its temperature distribution is not uniform, with the equatorial belt likely being hotter [51].

The intense EUV component in outburst finds a comparatively simple explanation as the optically thick BL emission. It is directly observable in low-inclination systems, while at high inclination only radiation scattered in the intense disk wind is seen [56]. With appropriate object-dependent wind parameters, the seemingly quite different EUV outburst spectra of the high-inclination system WZ Sge and of the low-inclination system SS Cyg can be successfully modeled (Fig. 12.3) [54,55]. The mass loss rate by the wind in SS Cyg approaches the maximum rate

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