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Astron. Astrophys. 354, L37-L40 (2000)

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2. The transient binary SSS RX J0513.9-6951

The luminous transient soft X-ray source RX J0513.9-6951 (henceforth RX J0513) discovered in the ROSAT all-sky survey (Schaeidt et al. 1993) has been optically identified as a high mass-transfer accreting binary system in the LMC (Cowley et al. 1993; Pakull et al. 1993) with an orbital period of 0.76 days (Crampton et al. 1996). Optical monitoring has revealed that RX J0513 undergoes recurrent low states at quasi-regular intervals, in which the optical brightness drops by [FORMULA] 1 magnitude (Reinsch et al. 1996; Southwell et al. 1996). The optical low-states are accompanied by a turn-on of the system in the soft X-ray range (Reinsch et al. 1996; Schaeidt 1996).

The optical low states last for [FORMULA] 40 days and repeat about every 140-180 days. Such short time scales cannot be explained by the limit-cycle behaviour sketched by van den Heuvel et al. (1992) or by recurrent burning models (Fujimoto 1982). Within the framework of a shell-burning white dwarf an alternative explanation has been suggested by Pakull et al. (1993): The rather sudden changes in the soft X-ray flux are the result of the direct response of the white dwarf to slight changes in the mass transfer rate. On the horizontal shell-burning branch, a small increase of the accretion rate may significantly affect the effective radius of the white dwarf envelope (Kato 1985). An increase of the photospheric radius by e.g. a factor of 4 implies that the effective temperature drops by a factor of 2. Given the extreme sensitivity of the ROSAT PSPC and HRI count rates on temperature, this in turn implies that the source may become undetectable although the bolometric luminosity remains roughly the same (e.g. Heise et al. 1994). In this model of an expanding and contracting envelope the sudden drop of the optical flux, the colour variation, and the temporarily increased soft X-ray flux can be quantitatively described by variations in the effective temperature of the hot central star and variations in the irradiation of the accretion disk (Reinsch et al. 1996).

This model is supported by an independent estimate of the neutral hydrogen column density obtained with recent HST UV spectroscopy which constrains the X-ray luminosity during the on-state of RX J0513 to [FORMULA] erg s-1, i.e. somewhat below the Eddington limit, and confirms that the radius of the soft X-ray source is consistent with the radius of a non-expanded white dwarf (Gänsicke et al. 1998).

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000