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, GRO J1655–40 and XTE J1550–564 in quiescence. Their quiescent spectra can be well fitted by a power-law model with number slope α∼ 2. While a coronal (Raymond-Smith) model is also a statistically acceptable representation of the spectra, the best fit temperatures of these models is ∼ 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∼ 1030 − 1033 ergs−1. Subject headings: binaries: close — black hole physics — stars: individual (V404 Cyg, A0620–00, GRO J1655–40, XTE J1550–564) — X-rays: stars