On the Correlated X‐Ray and Optical Evolution of SS Cygni

We have analyzed the variability and spectral evolution of the prototypedwarf nova system SS Cygni using RXTE data and AAVSO observations. A series ofpointed RXTE/PCA observations allow us to trace the evolution of the X-rayspectrum of SS Cygni in unprecedented detail, while 6 years of optical AAVSOand RXTE/ASM light curves show long-term patterns. Employing a technique inwhich we stack the X-ray flux over multiple outbursts, phased according to theoptical light curve, we investigate the outburst morphology. We find that the3-12 keV X-ray flux is suppressed during optical outbursts, a behavior seenpreviously, but only in a handful of cycles. The several outbursts of SS Cygniobserved with the more sensitive RXTE/PCA also show a depression of the X-raysduring optical outburst. We quantify the time lags between the optical andX-ray outbursts, and the timescales of the X-ray recovery from outburst. Theoptical light curve of SS Cygni exhibits brief anomalous outbursts. Duringthese events the hard X-rays and optical flux increase together. The long-termdata suggest that the X-rays decline between outburst. Our results are ingeneral agreement with modified disk instability models (DIM), which invoke atwo-component accretion flow consisting of a cool optically thick accretiondisk truncated at an inner radius, and a quasi-spherical hot corona-like flowextending to the surface of the white dwarf. We discuss our results in theframework of one such model, involving the evaporation of the inner part of theoptically thick accretion disk, proposed by Meyer & Meyer-Hofmeister (1994).Comment: 24 pages, 8 figures, 2 tables, accepted for publication in Ap