X-rays from Quiescent Black Holes: Accretion or Jet Powered

We propose to obtain Xray spectra of transient black hole binaries (BHBs) inquiescence. The spectra are expected to provide a severe test of the Comptonorigin of Xrays from such sources, as expected from accretionbased models. Atvery low accretion rates, a Comptonized spectrum is expected to appear stronglycurved in a loglog plot. However, there is already tentative evidence for thepowerlaw shape of the quiescent Xray spectrum. If confirmed, it would favor ajet origin of the Xray emission. However, the quality of existing data is stillquite poor. The deep observations proposed here will allow a detailed look at the roles of accretion flows and jets in quiescent BHBs and thus cast light on the issues of accretionjet coupling and particle acceleration in such systems. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]