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

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 by 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.

Correlation between the photon index and X-ray luminosity of black hole X-ray binaries and active galactic nuclei: observations and interpretation

“…Therefore, when the luminosity is below a critical value L crit , we predict the Xray emission will be dominated by the synchrotron emission from the jet . This prediction has been confirmed for observations of the faint LLAGNs and the quiescent state of BHBs (e.g., Wu, Yuan & Cao 2007;Pellegrini et al 2007;Wrobel et al 2008;Pszota et al 2008;Yuan, Yu & Ho 2009;de Gasperin et al 2011;Younes et al 2012;Xie, Yang & Ma 2014).…”

Jet-dominated quiescent states in black hole X-ray binaries: the case of V404 Cyg

“…However, it is challenging to reliably determine the spectral shape of BHXTs in their quiescent states, mainly because of their faintness. Pszota et al (2008) recently carried out deep high-quality XMM-Newton observations of three BHXTs, the exposure time for each source is more than 40 ks. Two of them (GRO J1655−40 and V404 Cyg) are confirmed to have precisely powerlaw X-ray spectra, without any possible curvatures.…”

“…Interested readers are referred to Hynes et al (2009) for details of data reduction and archival compilation. The quiescent state of this source was also modelled under the jet model by Pszota et al (2008) with a focus on the high-quality XMM-Newton data, and under pure ADAF model by Narayan et al (1997).…”

“…In other words, the X-ray emission of the quiescent state of BHXTs is likely comes from the jet. This prediction has passed several observational tests, mainly through spectral fitting (Pszota et al 2008;YCN05). However, these tests suffer the shortage of poor observational data (the data are non-simultaneous and/or limited to narrow wavebands), and thus lead to some debates.…”

Limits on the quiescent radio emission from the black hole binaries GRO J1655−40 and XTE J1550−564

“…During this initial outburst the system was observed simultaneously at X-ray and radio wavelengths (Harmon et al 1995) clearly showing relativistic ejection events following X-ray flares from the system. The system has also recently been observed in X-rays during quiescence (Pszota et al 2008) where power-law fits gave a flux of 6 × 10 −14 erg s −1 cm −2 (0.5-10 keV). GRO J1655−40 remained in quiescence when we observed, making it an ideal target for exploring the lower limits of the Fundamental Plane.…”