Origin of X-ray Emission from Transient Black Hole Candidates in Quiescence

TL;DR

This study investigates the origin of X-ray emission in quiescent black hole transients, finding evidence that jets, rather than accretion flows, are the primary source of X-ray emission in certain cases based on spectral analysis.

Contribution

It provides the first systematic analysis of X-ray spectra in quiescent black hole transients, highlighting the dominant role of jets in X-ray emission at low accretion rates.

Findings

X-ray spectra of GRO J1655-40 and V404 Cyg are power-law shaped.

Spectra deviate from RIAF predictions at low accretion rates.

Jet emission models explain the observed spectra well.

Abstract

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.