X-ray Observations of Eclipsing Dwarf Novae
2009 October 19
The outbursts of dwarf novae are interpreted in the framework of the disk instability theory. According to this, the accretion disks in CVs have two possible states, one at low viscosity, accretion rate and luminosity, and one at high viscosity, accretion rate and luminosity. If the mass transfer rate from the secondary is higher than the accretion rate in the low viscosity state, but less than that in the high state, a limit cycle behavior ensues: the dwarf nova outbursts. In quiescence, the disk is unable to accrete all the material transfered from the secondary. When a critical surface density is reached, it becomes hot and luminous, triggering an outburst.
This works well in the general outline, but there are problems with this basic version of the disk instability model. The quiescent X-ray luminosity of dwarf novae (a direct indicator of the accretion rate onto the white dwarf) is much higher than theory predicts. Also, the interoutburst intervals of many dwarf novae (notably WZ Sge) are too long. One possible modification of the theory is that there is a central hole in the quiescent accretion disk.
This can in principle be directly tested using X-ray observations of eclipsing dwarf novae in quiescence, because the eclipse light curves allows one to infer the size and the shape of the emitting region. The ASCA observation of the deeply eclipsing dwarf nova, HT Cas, indicated that the X-ray emitting region is compact. The increased collecting area of XMM-Newton has allowed better studies of, e.g., OY Car.
A schematic diagram showing a possible geometry of V893 Sco. The white dwarf (the photosphere with the X-ray emitting boundary layer) is shown at several phases around mid-eclipse, partially eclipsed by the secondary which is shown as a stationary object.
Recently, we have discovered the first partial X-ray eclipse in V893 Sco. We belive that a combined study of partial and total X-ray eclipses allow us to constrain the geometry of the X-ray emitting region, much better than by studying either individually.
Optical (top) and X-ray (bottom) eclipse light curves of V893 Sco, together with the light curves predicted by the above geometry. For the X-ray light curve, we show both a model light curve for the boundary layer as well as that for a spherical corona.
So, where do we go from here? More in-depth studies of V893 Sco (both in the optical and in the X-rays) are certainly needed. But it may be worth searching for other partially eclipsing systems. For that purpse, we present a table of eclipsing dwarf novae, extracted from RKcat version 7.1.2.
|Name||Period (d)||Quiescent Mag.||Ecl. Depth||Double?||X-ray Obs||X-ray Ecl.|
|U Gem||0.176906||13.9||1.6||XMM, Chandra, ASCA||No|
|WZ Sge||0.056688||15.0||0.6||ROSAT, ASCA, XMM||No|
|Z Cha||0.074499||15.5||1.8||Y||ROSAT, XMM, Swift||Total|
|V893 Sco||0.075962||15.5||ROSAT, Suzaku, XMM||Partial|
|V455 And||0.056309||16.1||0.4||Swift, XMM||?|
|HT Cas||0.073647||16.4||2.0||Y||ROSAT, ASCA, XMM||Total|
|OY Car||0.063121||16.7||1.3||Y||ROSAT, ASCA, XMM, Swift||Total|
|CW Mon||0.1766||16.8||ROSAT, Swift||?|
|AKO 9||1.109099||17.3||0.4||(in 47 Tuc)||?|
|SDSS J170213.26+322954.1||0.100082||17.4||1.7||Y||ROSAT, Swift||?|
|SDSS J092638.71+362402.4||0.01966||19.1g||1.2||Y||Chandra, XMM||?|
Objects are listed from the brightest to the faintest (using the quiescent, out-of-eclipse magnitudes). The periods are coded in blue for dwarf novae above the period gap (and in red for one AM CVn/dwarf nova object). The eclipse depth and the presence of a double eclipse (i.e., two distinct ingress/egress of the white dwarf and the bright spot are seen) can be used to judge if the eclipse is total or not. Based on a quick browse search, I indicate if a pointed imaging X-ray observation in quiescence exists or not, and (in the few well-known cases) whether an X-ray eclipse has been observed.