We present Chandra observations of black hole X-ray novae V404 Cyg , A0620–00 , GRO J1655–40 and XTE J1550–564 in quiescence .
Their quiescent spectra can be well fitted by a power-law model with number slope \alpha \sim 2 .
While a coronal ( Raymond-Smith ) model is also a statistically acceptable representation of the spectra , the best fit temperatures of these models is \sim 5 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 ( ADAF ) 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 GRO J1655–40 were fainter than in previous observations , while V404 Cyg was more luminous and varied by a factor of 2 in a few ksec .
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 luminosity of the four sources studied ranges from \sim 10 ^ { 30 } -10 ^ { 33 } \hbox { erg } \thinspace \hbox { s } ^ { -1 } .