X-ray novae are a subclass of binaries formed by a compact primary - a black hole or a neutron star - and a F-M type dwarf or subgiant. They are characterized by undergoing strong X-ray and optical outbursts every 10-50 years. (e.g. Bradt & McClintock 1983). These outbursts appear to be connected with an increase in the accretion rate on to the primary which can be caused by an instability in the outer disc (Lin & Taam 1984) or by a sudden increase in the mass-transfer from the secondary (Hameury et al. 1986). The X-ray spectra of these particular objects are similar to that of Cyg X-1, one of the first black hole candidates (Sunyaev et al. 1991). Indeed, many of the best black hole candidates have been found in X-ray novae. We may list, among others, A0620-00 = V 616 Mon (McClintoc & Remillard 1986), XN Mus 1991 = GRS 1121-68 (Remillard et al. 1992), V 404 Cyg = GS 2023+338 (Casares et al. 1992).
The optical radiation of X-ray novae is mostly associated with the reprocessing of the inner disc hard radiation by the outer disc and secondary star, as suggested by the numerous emission lines in their spectra (Della Valle et al. 1991). Nevertheless, it is possible to observe Balmer lines in absorption in the optical spectra during the outburst decay (Suleimanov 1996). Such absorption lines have been seen, for example, in the spectra of GRO J0422+32 (Shrader et al. 1994 ; Callanan et al. 1995) and XN Vel 1993 (Della Valle et al. 1997). The study of disc absorption lines can provide a powerful diagnostic of the physical conditions of the accretion disc as well as detailed information on its chemical composition. In particular, the formation of disc absorption lines may be relevant to the investigation of the origin of the high Li abundances found in the secondary stars of several X-ray novae (Martín et al. 1992, 1994). One possible explanation for these high abundances, first suggested by Martín et al. (1992), is that Li would be originated by collisions and spallation reactions in the accretion disc around the compact object or in the secondary itself. This scenario can be tested through the detection of emission lines at 470 keV associated with de-excitation of the 7 Li nuclei produced by reactions. In this respect, the observed emission line at 476 15 keV in the spectrum of XN Mus 1991 (Sunyaev et al. 1992) and the recent detection of Li in the secondary of this system (Martín et al. 1996) are remarkable. An alternative method of testing this scenario would be the detection of high Li abundances in the accretion disc itself. In this paper, we examine the formation of the Li I 6708 Å resonance line in the spectra of X-ray novae during outburst decay. It is found that detection is feasible provided the abundance in the disc is as high as in the atmospheres of the secondaries log N (Li) 2-3.5 (in the usual scale where log N (H) = 12).
The formation of spectral lines in accretion discs has been investigated by many authors. For example, the works by Herter et al. (1979), Mayo et al. (1979), Williams (1980) and la Dous (1989) were devoted to the modelling of absorption lines in the spectra of accretion discs around white dwarfs. These authors assumed that each disc ring radiates as a stellar atmosphere with the same effective temperature and surface gravity log g. However, this approach does not explain the emission cores of the absorption lines, and to surmount this problem Tylenda (1981) took into account self-irradiation of the disc and irradiation by the boundary layer, and Cheng & Lin (1989, 1992) took into consideration optically thin parts and a "chromosphere-like" temperature distribution in the accretion disc atmosphere. The emission spectra of low-mass X-ray binaries (LMXB) accretion discs irradiated by X-rays have been calculated by Ko & Kellman (1994) with the use of the photon escape probability method.
In our work we have chosen a simple method of considering the self-irradiation of the disc which takes into account the available constraints on the parameters of accretion discs (Sect. 2). This approach does not allow the exact calculation of spectral lines, but it does to estimate the maximum possible equivalent width of any absorption line component. Our results for the Li I 6708 and the H lines have been obtained in Sect. 3 and the discussion and conclusions are presented in Sect. 4.
© European Southern Observatory (ESO) 1998
Online publication: January 27, 1998