Journal Article

Origin of X-ray emission from transient black hole candidates in quiescence

Gabor Pszota, Hui Zhang, Feng Yuan and Wei Cui

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 389, issue 1, pages 423-428
Published in print September 2008 | ISSN: 0035-8711
Published online August 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13587.x
Origin of X-ray emission from transient black hole candidates in quiescence

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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 by the jets. Using data from deep XMMNewton 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.

Keywords: radiation mechanisms: non-thermal; radiation mechanisms: thermal; stars: individual: XTE J1550-564; stars: individual: GRO J1655-40; stars: individual: V404 Cyg; X-rays: binaries

Journal Article.  3925 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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