X-ray lines from hot flows around white dwarfs: Application to SS cygni

Rather than accreting via a disk, some white dwarfs (WDs) in quiescent dwarf novae (DN) could accrete via an advection-dominated accretion flow (ADAF) possibly responsible for the X-ray bremsstrahlung emission observed. Such a hot accretion flow is also expected to produce characteristic thermal line emission. Using SS Cyg as a specific example, we show that knowing a priori the inclination and the WD mass in quiescent DN makes X-ray line diagnostics powerful probes of the flow structure in these systems. Current X-ray instruments can discriminate, from their width, between lines emitted from a flow with a Keplerian rotation rate and those emitted at a substantially sub-Keplerian rate. This could be used to observationally test the property of energy advection, which is at the origin of the partial radial pressure support by the hot gas and the sub-Keplerian rotation rate in an ADAF.