Two Types of X-ray Spectra in Cataclysmic Variables

Abstract

We present Chandra HETG spectra of seven cataclysmic variables. We find that they divide unambiguously into two distinct types. Spectra of the first type are remarkably well fit by a simple cooling flow model, which assumes only steady-state isobaric radiative cooling. The maximum temperature and the normalization, which provides a highly precise measurement of the accretion rate, are the only free parameters of this model. Spectra of the second type are grossly inconsistent with a cooling flow model. They instead exhibit a hard continuum, and show strong H-like and He-like ion emission but little Fe L-shell emission, which is consistent with expectations for line emission from a photoionized plasma. Using a simple photoionization model, we argue that the observed line emission for these sources can be driven entirely by the hard continuum. The physical significance of these two distinct types of X-ray spectra is also explored.