The X-ray spectra of black hole X-ray novae in quiescence as measured by Chandra

We present Chandra observations of black hole X-ray novae V404 Cyg, A0620-00, GROJ1655-40, and XTE J1550-564 in quiescence. Their quiescent spectra can be well fitted by a power-law model with number slope alphasimilar to2. While a coronal (Raymond-Smith) model is also a statistically acceptable representation of the spectra, the best-fit temperatures of these models is similar to5 times higher than that seen in active stellar coronae. These four spectra of quiescent X-ray novae are all consistent with that expected for accretion via an advection-dominated accretion flow and inconsistent with that expected from a stellar corona. This evidence for continued accretion in quiescence further strengthens the case for the existence of event horizons in black holes. Both A0620-00 and GROJ1655-40 were fainter than in previous observations, while V404 Cyg was more luminous and varied by a factor of 2 in a few kiloseconds. A reanalysis of the X-ray data for XTE J1550-564 shows that (like V404 Cyg and A0620-00) its luminosity exceeds the maximum prediction of the coronal model by a large factor. The 0.3-7 keV luminosities of the four sources studied are in the range from similar to10(30) to 10(33) ergs s(-1).