Close Binary Supersoft XRay Sources CBSS

One of the important imprints left behind by ROSAT is the observational definition of a class of supersoft X-ray sources with blackbody temperatures kT < 100 eV [21,24,87]. A prime property of most SSS seems to be a short duty cycle of X-ray bright states as evidenced by the fact that observations of M31 with Chandra and XMM-Newton failed to find most SSS discovered previously with ROSAT [24]. On the other hand, some CBSS, as CAL83 and CAL87 in the Large Magellanic Cloud were discovered already with Einstein [49] and have been observed as soft X-ray

5 Carbon ignition in a CO white dwarf or oxygen ignition in an ONeMg white dwarf leads to a deflagration supernova event if the deflagration wave is sufficiently fast, whereas a collapse and neutron star formation occur when the deflagration wave is too slow. In addition, electron capture on 24Mg and 20Ne in an ONeMg white dwarf leads to a collapse if it starts before oxygen ignition [60].

sources for more than two decades. Hence, the long-term time variability differs for individual sources and a full discussion requires optical identification and dedicated observations.

CAL 83 and CAL 87 are known to reside in binaries with periods of order day. Members of this subclass are also RX J0513.9-6951 in the LMC, the Galactic binaries QR And (RX J0019.8+2156) and MR Vel (RX J0925.7-4758), and probably V Sge commonly classified as a long-period CV and the recurrent nova U Sco with outburst intervals of 8-43 yrs. Some other CVs have properties similar to V Sge and, hence, more potential or temporary CBSS may hide among the known CVs [83]. An attractive explanation of the properties of all these stars is thermal time scale mass transfer from a secondary more massive than the white dwarf [7, 37, 88], although one has to caution that for most objects the masses of the secondaries have not yet been measured. Nevertheless, they seem to fit into the picture described in the last section, are more or less variable, and have accretion rates around 10-7 MQ yr-1, except for USco, which may have M as low as 10-8 MQ yr-1, given the ~20-yr outburst period and a very high ejection velocity (compare Fig. 12.9). For CAL 83 and USco, there is some spectroscopic evidence that the donor is helium-rich, which points to an origin of the white dwarf from a path other than the usual AGB evolution [7,27].

A particularly detailed long-term time-variability study has been performed on the LMC CBSS RX J0513.9-6951. Figure 12.10 shows the result of the ROSAT HRI and optical monitoring throughout the X-ray bright interval along with the optical data folded over the cycle length [72]. The monitoring revealed the existence of quasi-regular X-ray and optical high and low states of opposite sense, suggesting that the underlying clockwork is a relaxation process. About 120 days of an optical high and X-ray off state are followed by a sharply defined ~40-day optical low and X-ray on state. The oscillating behavior of RX J0513.9-6951 has been interpreted as a limit cycle brought about by variations of the accretion rate around Mcrit. The Reinsch et al. model [72] is based on the viscous time scales for the irradiation-induced drainage of the outer accretion disk and the refilling of the inner disk hole created by the expanded white dwarf envelope. In this model, the mass transfer rate from the secondary may stay constant, with the accretion disk acting as a buffer. The more recent Hachisu & Kato model [28] starts from the similarity between CBSS and novae and takes account of the strong wind observed in several CBSS in the form of blue and red-shifted Balmer satellite lines. It associates the cyclic variations of the accretion rate with the interaction between the wind and the atmosphere of the secondary star, which modulates the transfer rate. Both models are typical relaxation scenarios. Other CBSS, like CAL 83 show a similar, but less regular optical/X-ray variability [23].

Another subclass of CBSS is that of symbiotic binaries, which have orbital periods of the order year and a giant donor of about solar mass or less. Examples are SMC3 (=RX J0048.4-7332) [38], the Einstein source 1E 1339.8+2837 in the globular cluster M3, and the stars AG Dra and RR Tel in the Galactic disk [21,37]. These

RX J0513-69 HRI monitoring

JD - 2450000

observed optical minima observed optical minima

0 20

Fig. 12.10 The X-ray and optical light curves of RX J0513-69 throughout an outburst cycle (from [72])

0 20

Fig. 12.10 The X-ray and optical light curves of RX J0513-69 throughout an outburst cycle (from [72])

sources also fit into the picture presented in the last section and are quite likely candidates for the SSS discovered in elliptical galaxies and in the bulges of spirals [12].

Other SSS do not fit into these groups because they have well established periods of only a few hours, typical of CVs, as the SMC source 1E 0035.4-7230 with P = 4.1 h [21,37] and possibly the LMC source RX J0439.8-6809, which has a proposed orbital period of 3.4 h [21, see, however, [90]]. The accretion rate required to power these stars exceeds those typical of CVs and may be induced by the intense irradiation of the secondary star [91]. Still other sources display a transient behavior and do not fit into any of these patterns.

Progress in our understanding of the dynamical processes in CBSS and a more physical classification of the individual observed SSS requires more specific observations and substantial theoretical effort. In particular, time-dependent highresolution X-ray spectroscopy is a promising avenue. Such observations have become possible with the transmission and reflection gratings on board of Chandra and XMM-Newton, respectively. The interpretation of such observations requires

Fig. 12.11 Observed Chandra LETGS/HRC-S spectra of V4743 Sgr (N Sgr 2002 No. 3 (top), RX J0513-69 (second and third from top, multiplied by factors of 10 and 8, respectively), CAL 83 (second from bottom), and CAL 87 (bottom). The spectra have been binned into 0.1 A bins and gently smoothed with a box car over 3 bins. They are not corrected for interstellar absorption. Noisy parts have been omitted in the lower two spectra

Fig. 12.11 Observed Chandra LETGS/HRC-S spectra of V4743 Sgr (N Sgr 2002 No. 3 (top), RX J0513-69 (second and third from top, multiplied by factors of 10 and 8, respectively), CAL 83 (second from bottom), and CAL 87 (bottom). The spectra have been binned into 0.1 A bins and gently smoothed with a box car over 3 bins. They are not corrected for interstellar absorption. Noisy parts have been omitted in the lower two spectra the extension of the art of NLTE-modeling of expanding atmospheres to the soft X-ray regime [46,67,71]. Figure 12.11 compares the observed spectra of the CBSS RX J0513.9-6951 [9], CAL 83 [46] and CAL 87 6 with that of the X-ray bright phase of V4743 Sgr (N Sgr 2002) [59]. V4743 Sgr shows an absorption line spectrum with a few emission features superimposed [67, 71]. The upper of the two spectra of RX J0513.9-6951 is the mean of two exposures near X-ray maximum, the lower was taken during the rise from the X-ray off phase. The spectral structure of RX J0513.9-6951 and CAL83 suggests that strong blends and the dynamic properties of the atmosphere combine to produce what is observed. CAL 87, on the other hand, shows an emission line spectrum. Combined with the X-ray faintness of the eclipsing system, this result suggests that we do no see the accreting white dwarf directly, but only scattered photons from the wind or from the accretion disk rim. This interpretation is supported by the general similarity of the absorption and emission line spectra of the CBSS observed at different inclinations with the corresponding

6 Chandra LETG/HRS-C observation ID 1896, August 13, 2001, PI J. Greiner.

spectra of dwarf novae in outburst shown in Fig. 12.3. At present, the interpretation of these spectra is just beginning. In the near future, high-resolution X-ray spectroscopy combined with advanced atmospheric modeling will substantially improve our understanding of the dynamical processes in CVs, novae, and CBSS -and quite certainly raise new questions as well.

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