1.6.1 Dwarf Novae
The first dwarf nova to be discovered, and the prototype of its sub-class, was U Gem, discovered by Hind & Hansen in 1855. It has an optical range of magnitude 14 at quiescence rising to magnitude 9 during outburst (Szkody & Mattei, 1984). Its high inclination results in absorption dips in the light curves making it possible to study the origin of the X-ray emission and the physical structure of the disc (Mason et al., 1988; Szkody et al., 1996). The X-rays in U Gem are also brighter during outburst and not quenched as is seen in other DN making it an interesting system to observe (Swank et al., 1978; Cordova & Mason, 1984; Mattei et al., 2000).
Belonging to the same class, SS Cygni, was discovered in 1896 (Pickering & Fleming, 1896) and has undoubtedly been one of most observed variable stars in the night sky. Amateur
astronomers have continuously observed SS Cygni over the past 100 years without missing a single outburst. This is due largely to the fact that it is the brightest of the U Gem dwarf novae class with an optical range of mv ∼ 12−8 mag. Both these systems with their close proximity (96.4±4.6 pc for U Gem and 166.2±12.7 pc for SS Cygni (Harrison et al., 1999)) and regular outbursts between 50 and 100 days make them ideal candidates for studying.
SU Ursae Majoris (SU UMa) is also the prototype of its sub-class. It was discovered by Ceraski in 1908 (Ceraski, 1908). Unlike U Gem and SS Cygni this system resides below the period gap with a period of Porb=109.9±0.1 minutes (Thorstensen et al., 1986). In addition to exhibiting U Gem type outbursts SU UMa also displays the superoutburst and superhump phe- nomenon described in Section 1.5.3. Its variations occur on a shorter time-scale than either SS Cygni or U Gem. Narrow outbursts occur every 11 to 17 days lasting 1−3 days. Superoutbursts occur every 3−10 normal outburst cycles, approximately every 153 to 260 days with a duration of 10−18 days. SU UMa typically varies from a minimum of magnitude 15 to a maximum of magnitude 10.8 during superoutburst.
The outbursts observed in these systems are thought to be triggered by a thermal-viscous instability in the accretion disc surrounding the white dwarf primary (Osaki, 1996). See section 1.5.3 for further discussion.
1.6.2 X-ray Studies
SS Cygni was the first dwarf novae to be detected in the X-ray band. Detected by Rappaport et al. (1974) in soft X-rays, between 0.15−0.28 keV and 0.4−0.85 keV, during a scanning rocket flight. It was later detected again by Heise et al. (1978) where it was observed to emit both soft (0.16−0.284 keV) and hard (1−7 keV) X-ray components. Observations by Ricketts et al. (1979) and Watson et al. (1985) provided a more detailed picture of the X-ray behaviour and showed that the soft X-ray emission was well correlated with the optical outburst, whilst the hard X- rays were equally strongly anti-correlated and quenched to below quiescent levels (discussed in Section 1.4.3). The most complete multi-wavelength coverage of an outburst was presented by Wheatley et al. (2003) who analysed the flux evolution through an entire outburst, resolving the rise to outburst. Using X-ray, extreme UV (EUV) and optical bands the correlation of optical and
EUV and the anti-correlation with the hard X-ray band was seen in great detail. The observed quiescent luminosity corresponded to an accretion rate about two and a half orders of magnitude higher than predicted by the DIM (Meyer & Meyer-Hofmeister, 1994). In studying archival SS Cygni data, from the Ginga and ASCA satellites, Done & Osborne (1997) found single and multi-temperature plasma models with line emission and a reflection component to represent the data well. They claimed there to be a larger contribution from the reflection component in the softer outburst spectra supporting models where the inner disc is truncated (Disc truncation is discussed further in Section 1.7).
Observations of eclipsing system HT Cas during quiescence provides evidence for hard X-ray emission arising from the boundary layer (Mukai et al., 1997). The observed X-ray eclipses were found to be short and compatible with total eclipses indicating an X-ray emis- sion region of<1.15 times the size of the white dwarf. Currently, it is not known where hard X-rays originate from in outburst. Patterson & Raymond (1985a) suggested that the hard X-rays are produced in an optically thin region surrounding an optically thick boundary layer. How- ever, observations of OY Car during superoutburst did not detect an eclipse in the soft X-ray band providing evidence for an extended X-ray source (Naylor et al., 1988; Pratt et al., 1999). Observations in the EUV band by Mauche & Raymond (2000) also show a lack of eclipse in outburst, they suggest the extended source originates from optically thick radiation being scat- tered into the line of sight by an accretion disc wind. Observations of VW Hyi over a two month period by van der Woerd & Heise (1987), found the spectral shape during outburst to be remark- ably constant while the fluxes changed by two orders of magnitude. They concluded that the observations were consistent with a spectrum comprising of multiple components, possibly with emission from an optically thin extended region surrounding the white dwarf.
The spectral parameters of 32 cataclysmic variables observed with the Einstein satellite were determined by Eracleous et al. (1991). An optically thin thermal bremsstrahlung model was found to describe the data well, indicating the X-ray emitting region is a hot optically thin boundary layer region. Later, all available non-magnetic CVs from the ROSAT PSPC archive were presented by van Teeseling et al. (1996) finding that there was no correlation between the temperature and X-ray luminosity, emission measure or X-ray luminosity. This is difficult to
explain if the X-rays are emitted by the boundary layer. Also the ratio of X-ray to UV+optical flux was found to be anti-correlated with the accretion rate1, which is not consistent with simple boundary layer models. However, evidence for anti-correlation between the observable emission measure and orbital inclination in the X-rays indicate that the emitting region is very close to the white dwarf.
Work by Yoshida et al. (1992) on SS Cygni and U Gem data found the energy spectrum, during quiescence, to be well reproduced by a thermal bremsstrahlung continuum. Baskill et al. (2005) presented 34 non-magnetic cataclysmic variables in outburst and quiescence, including SS Cygni, U Gem and SU UMa. Like SS Cygni, the brighter systems favoured the inclusion of an emission line at 6.4 keV from neutral iron. The outburst emission from the dwarf novae was found to be weighted towards lower temperatures. Using high resolution data taken from the Chandra satellite using the High Energy Transmission Grating (HETG) Mukai et al. (2003) found seven systems to be well fit with either a cooling flow model or a photoionised continuum with all having strong H and He like ion emission. The presence of an iron fluorescence line and a number of thermal emission lines from a broad range of ions in the spectra of six dwarf novae was found by Rana et al. (2006). The prominent fluorescent iron line, seen in many non-magnetic CVs, indicates the presence of a significant reflection component. The equivalent width of this line is also consistent with a reflection origin. The presence of a strong FeXXV
triplet at 6.7 keV is a common feature in the hard X-ray spectra of non-magnetic CVs. The triplet is present in outburst and quiescence indicating plasma temperatures of& 3×107 K. The ratio of the FeXXVI/XXVlines indicates a higher ionisation temperature during quiescence than in outburst. H and He like emission from a number of ions was also found by Okada et al. (2008) and emission lines were found to be narrower during quiescence suggesting that the lines arise from the entrance of the boundary layer. The line emission from ten CVs observed during quiescence using XMM-Newton indicate that X-rays are emitted from a cooling plasma settling onto the white dwarf, excluding the presence of an extended X-ray emitting corona (Pandel et al., 2005).
1Accretion rates were calculated from bolometric fluxes based on flux measurements in the energy range 0.1
−2.4 keV and used distances given by Warner (1987).