Astron. Astrophys. 342, L45-L48 (1999)

7. Quiescent X-ray luminosity and accretion rate

V592 Her is in the pointed ROSAT PSPC observation of  Her (obs. id. 201228p), obtained on 1-4 September 1992, but was not detected in this 5563 s observation, with a upper limit to the count rate in channels 11-240 of cts s^-1. For a moderately absorbed ( cm^-2) 1-10 keV bremsstrahlung spectrum (typical for a non-magnetic CV; Van Teeseling & Verbunt 1994), this corresponds to an unabsorbed 0.1-2.4 keV flux of erg cm^-2s^-1 and a upper limit for the 0.1-2.4 keV luminosity of erg s^-1. With an estimated ultraviolet+optical flux of erg cm^-2s^-1, we note that the upper limit for the X-ray flux gives a ratio of `bolometric' X-ray to ultraviolet+optical flux of , consistent with a dwarf nova in quiescence (Van Teeseling et al. 1996). Assuming that roughly half of the total accretion energy is released in observable X-rays (which is only true for very low accretion rates and also depends on the orbital inclination) the corresponding mass-accretion rate onto the white dwarf is

We can crudely estimate the disk luminosity and the mass-transfer rate by assuming that the optical flickering is due to a classical "bright spot" radiating at the canonical temperature of K: if the bright-spot R-band flux is of ( mag), then the bright-spot luminosity is erg s^-1. This bright-spot luminosity is produced by the gravitational potential drop between the L₁-point and the outer disk radius, the latter roughly equal to the 3:1 resonance radius cm needed to produce the observed superhump phenomena (Warner 1995, p. 207, with hr). We then derive an estimated mass-transfer rate (Warner 1995, p. 83)

Given the fact that the accretion disk is not in a steady-state, that the inclination and white dwarf mass are not known, that Eq. (2) is based on very crude approximations, and that the inner disk may also contribute to the flickering, this is not in contradiction with Eq. (1).

We can also estimate the mean long-term transfer rate from the outburst energy and recurrence time: With , (as appropriate for a blue outburst disk; Paczyski & Schwarzenberg-Czerny 1980), and d, we obtain a mean outburst luminosity of and, for a recurrence time yr, a transfer rate

The mean mass transfer rate inferred for short-period CVs is (Patterson 1984), but this is dominated by systems with hydrogen burning secondaries. With a degenerate brown dwarf secondary the mass-transfer rate driven by gravitational radiation at hr and for is

With a relatively massive white dwarf , , , and are all consistent at .

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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