The dwarf nova SS Cygni: what is wrong?

Abstract

Context. Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was used to measure the distance to SS Cyg to be 166 ± 12 pc, it became apparent that at this distance the disc instability model fails to explain the absolute magnitude during outburst. It remained, however, an open question whether the model or the distance have to be revised. Recent observations led to a revision of the system parameters of SS Cyg and seem to be consistent with a distance of d > ∼ 140 pc. Aims. We re-discuss the problem taking into account the new binary and stellar parameters measured for SS Cyg. We confront not only the observations with the predictions of the disc instability model but also compare SS Cyg with other dwarf novae and nova-like systems. Methods. We assume the disc during outburst to be in a quasi stationary state and use the black-body approximation to estimate the accretion rate during outburst as a function of distance. Using published analysis of the long term light curve we determine the mean mass transfer rate of SS Cyg as a function of distance and compare the result with mass transfer rates derived for other dwarf novae and nova-like systems. Results. At a distance of d > ∼ 140 pc, both the accretion rate during outburst as well as the mean mass transfer rate of SS Cyg contradict the disc instability model. More important, at such distances we find the mean mass transfer rate of SS Cyg to be higher or comparable to those derived for nova-like systems. Conclusions. Our findings show that a distance to SS Cyg > ∼ 140 pc contradicts the main concepts developed for accretion discs in cataclysmic variables during the last 30 years. Either our current picture of disc accretion in these systems must be revised or the distance to SS Cyg is ∼100 pc.

An intrinsic source of reddening in the cataclysmic variable SS Cygni

“…Further multifrequency data of SS Cygni showed the incompatibility of its behavior with that A&A 525, A147 (2011) of NMCV, and strongly favored its classification as that of an intermediate polar (see, e.g. Giovannelli et al 1985;Giovannelli & Martinez-Pais 1991;Kjurkchieva et al 1998;Marchev et al 1999;Gaudenzi et al 2002;Long et al 2005;Schreiber et al 2003;Schreiber & Lasota 2007).…”

Relativistic slim disks with vertical structure

“…In the case of dwarf-novae, for example, these determinations assume that outbursts are described well by the thermal-viscous instability model. However, the case of the epitome dwarf-nova SS Cyg shows that such a description might be inadequate (Schreiber & Lasota 2007;Smak 2010), putting presumably observational determinations into doubt. Therefore, one cannot exclude that the numerical simulations are correct after all.…”

The decline in irradiation from the white dwarf in old novae

“…The possibility of short-term irradiation-induced mass transfer in some dwarf novae, whereby radiation from the accretion disc during superoutburst results in an enhancement of the mass transfer rate, has been used by Schreiber et al (2004) to explain the light curves of SU UMa stars and by Schreiber & Lasota (2007) to solve the distance problem associated with SS Cyg. Recent theoretical calculations by Viallet & Hameury (2007) indicate that such effects, however, are thought to only last for 10 to 20 orbital periods and are unlikley to be able to account for the duration of long outbursts.…”