THE SOFT-X-RAY TURNOFF OF NOVA MUSCAE-1983

Nova GQ Muscae 1983 was detected by ROSAT as a luminous ''supersoft'' X-ray source in 1992, nearly a decade after outburst. Further, this is the only classical postnova known to have maintained constant luminos ity on a timescale predicted counter in 1993 January and September, an d complemented with B-band photometry taken in 1993 January. By 1993 J anuary, the X-ray count rate had declined by a factor of 17, while the re was neither an appreciable decrease in the optical magnitude nor a change in the amplitude of modulation. In 1993 September the soft X-ra y flux was below the ROSAT threshold limit, implying a decrease of a f actor greater-than-or-equal-to 30 in the count rate. This decline can be interpreted by the turnoff of nuclear processes due to the complete consumption of the residual hydrogen-rich envelope. However, the opti cal luminosity of the system is not simply coupled to the X-ray lumino sity (e.g., through reprocessing).