BeppoSAX does not discriminate between a continuous (cooling flow) and a discrete temperature distribution. Our observation of a decreasing count rate, followed by a constant count rate during quiescence is in contradiction with the disk instability models. These models predict a slightly increasing mass transfer onto the white dwarf which must show up as an increase in the X-ray flux. Ad hoc modifications to disk instability models, such as interaction of the inner disk with a magnetic field of the white dwarf (Livio & Pringle 1992), evaporation of the inner disk (Meyer & Meyer-Hofmeister 1994), or irradiation of the inner disk by the white dwarf (King 1997), possibly are compatible with the decrease of ultraviolet flux (e.g. Van Amerongen et al. 1990) and X-ray flux during quiescence.
If we assume a continuous temperature distribution the upper temperature limit of our quiescence spectrum is consistent with the observations by Wheatley et al. (1996). The cooling flow model requires an accretion rate of to explain the X-ray luminosity late in quiescence. A similar result is obtained when we convert the luminosity derived from the two-temperature model to an accretion rate. Any outburst model must accommodate this accretion rate.
BeppoSAX MECS observes a significant decrease in the count rate during outburst. Our simulations show a similar decrease for the ROSAT PSPC which would have been significantly detected. The fact that the ROSAT count rate during outburst was constant (Wheatley et al. 1996) and the results from our cooling flow model fits suggest that the outburst of Sep 24 1998 behaved differently from the outburst of Nov 3 1990.
© European Southern Observatory (ESO) 1999
Online publication: September 2, 1999