The quiescent X-ray emission of X-ray novae and the coronal emission of late-type stars

The X-ray luminosities and spectra of F-M stars of luminosity classes IV-V are analyzed. In dwarfs with rotational velocities of about 100 km/s, such as the optical components of low-mass X-ray novae with black holes, hot pla sma can be confined in coronal loops even in the presence of Fairly weak ma gnetic fields. Thus, the soft X-ray emission of such systems in their quies cent state (to 10(31) erg/s) could be associated with the coronal emission of the optical component/dwarf. Two systems studied with subgiants (V1033 S co and V404 Cyg) have X-ray luminosities 2 x 10(32)-2 x 10(33) erg/s. The X -ray emission of a solar-type corona cannot provide such luminosities. Howe ver, a transition to a non-solar corona is possible in rapidly rotating sub giants-a dynamical corona whose X-ray emission can be one to two orders of magnitude higher than observed for more slowly rotating late-type subgiants in the solar neighborhood. This suggests that the quiescent X-ray emission of these two systems is provided by emission from the corona of the subgia nt optical component. (C) 2000 MAIK "Nauka/Interperiodica".