XMM-Newton detection of Nova Muscae 1991 in quiescence

The soft X-ray transient GU Mus has been detected by XMM-Newton in the quiescent state. The source is very faint, with a 0.5-10.0 keV unabsorbed flux of ≃1.1 × 10-14 ergs cm-2 s-1. The spectra is well fit by an absorbed powerlaw with a photon index of α = 1.6 ± 0.4, close to the value seen when the source was in the low/hard state in Aug. 1991. From our observed luminosity, it seems unlikely that the quiescent state emission is dominated by coronal X-rays from the secondary. The flux also appears to be in agreement with the ADAF model of BH-transients in quiescence.