The ROSAT detection of RS Ophiuchi at quiescence.

The recurrent nova RS Ophiuchi was detected at quiescence by the ROSAT PSPC, with net count rate 0.0104 ± 0.0015 cts ^-1. The counts, strongly peaked around 1 KeV, were not detected below 0.5 KeV. The best spectral fits of an optically thin model of thermal plasma in the admitted range of Nβ imply a temperature 1.1-1.3 KeV and flux 1.6-3.4 × 10^-13 ergs cm^-2 s^-1, corresponding to a soft X-rays luminosity Lχ≃ 2.8 × 10³¹-1.64 × 10³² erg s^-1. This poses strong constraints on the models proposed for the system. It can be ruled out that the X-ray flux is due to ongoing hydrogen burning. If it is emitted by the boundary layer of an accretion disk around a high mass white dwarf, the mass accretion rate is M^ṡ < 10^-9 M_sun yr^-1, not sufficiently high to power a thermonuclear runaway on the surface of a white dwarf every ≃20 yrs.