  We report results from a systematic study of X-ray emission from black hole
transients in quiescence. In this state mass accretion is thought to follow the
geometry of an outer optically thick, geometrically thin disc and an inner
optically thin, geometrically thick radiatively inefficient accretion flow
(RIAF). The inner flow is likely also coupled to the jets near the black hole
that are often seen in such systems. The goal of the study is to see whether
the X-ray emission in the quiescent state is mainly powered by the accretion
flow or the jets. Using data from deep XMM-Newton observations of selected
black hole transients, we have found that the quiescent X-ray spectra are, to a
high precision, of power-law shape in the cases of GRO J1655-40 and V404 Cyg.
Such spectra deviate significantly from the expected X-ray spectrum of the RIAF
at very low accretion rates. On the other hand, they can naturally be explained
by emission from the jets, if the emitting electrons follow a power-law
spectral distribution (as is often assumed). The situation remains ambiguous in
the case of XTE J1550-564, due to the relatively poorer quality of the data. We
discuss the implication of the results.
