THE EXOSAT MEDIUM-ENERGY (ME) SAMPLE OF DWARF NOVAE

We present our analysis of the EXOSAT medium energy (ME) archival data on dwarf novae. Many dwarf novae were observed with EXOSAT; unfortuna tely, a significant fraction resulted in nondetection or marginal dete ction with the ME instrument. Our analysis of the brightest dwarf nova , SS Cyg, shows little correlation between the count rate and the hard ness ratio, contrary to an earlier report. This is likely to be the re sult of a slight error in background subtraction performed. The appare nt lack of correlation favors a compact boundary layer between the acc retion disk and the white dwarf as the hard X-ray emitting region, rat her than an extended ''corona.'' According to our spectral fit, all dw arf nova data are compatible with a single-temperature (2->20 keV) bre msstrahlung continuum with an iron emission line near 6.7 keV. However , several sources show significant epoch-to-epoch variability, the ori gin of which is poorly understood at the moment. Better hard X-ray dat a are needed to advance our understanding of dwarf novae. With this kn owledge, we investigate the contribution of dwarf novae to the Galacti c ridge X-ray emission. They are found to be spectrally similar; using a reasonable estimate of the spatial distribution of dwarf novae, the se systems appear to be a major, if not the dominant, component of the ridge emission.