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Jets and the accretion flow in low-luminosity black holes

Gardner, E.; Done, C.

Jets and the accretion flow in low-luminosity black holes Thumbnail


E. Gardner


The X-ray spectra of black hole binaries (BHB) in the low/hard state (LHS) first harden as the flux decreases, then soften. This change in behaviour has been variously attributed to either the X-rays switching from being produced in the flow to being dominated by the jet, or to the flow switching seed photons from the disc to self-generated seed photons from cyclo-synchrotron. Here, we build a simple truncated disc, hot inner flow, plus standard conical synchrotron jet model to explore what this predicts for the X-ray emission mechanism as a function of mass accretion rate. We find that the change in X-ray spectral index can be quantitatively (not just qualitatively) explained by the seed photon switch in the hot flow, i.e. this supports models where the X-rays are always produced by the hot flow. By contrast, standard conical jet models are as radiatively inefficient as the hot flow so there is no transition in X-ray production mechanism with ṁ. Including the effects of electron cooling allows the jet X-rays to drop more slowly with accretion rate and hence overtake the X-rays from the hot flow; however, this produces a corresponding change in the radio–X-ray correlation, which is not observed. We argue that the unbroken radio–X-ray correlation down to quiescence rules out the jet transition model as an explanation for the trend in X-ray spectral index. Our favoured model is then a truncated disc with an inner, hot, radiatively inefficient flow which always dominates the hard X-rays, coupled to a conical synchrotron jet which produces the radio emission. However, even this has issues at low m˙m˙ as the low optical depth and high temperature of the flow means that the Compton spectrum is not well approximated by a power law. This shows the need for a more sophisticated model for the electron distribution in the hot flow.


Gardner, E., & Done, C. (2013). Jets and the accretion flow in low-luminosity black holes. Monthly Notices of the Royal Astronomical Society, 434(4), 3454-3462.

Journal Article Type Article
Acceptance Date Jul 9, 2013
Online Publication Date Jul 26, 2013
Publication Date Oct 1, 2013
Deposit Date Mar 16, 2016
Publicly Available Date Mar 18, 2016
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 434
Issue 4
Pages 3454-3462
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Published Journal Article (419 Kb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society ©: 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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