Modelling Spectral and Timing Properties of Accreting Black Holes: The Hybrid Hot Flow Paradigm

Abstract

The general picture that emerged by the end of 1990s from a large set of optical and X-ray, spectral and timing data was that the X-rays are produced in the innermost hot part of the accretion flow, while the optical/infrared (OIR) emission is mainly produced by the irradiated outer thin accretion disc. Recent multiwavelength observations of Galactic black hole transients show that the situation is not so simple. Fast variability in the OIR band, OIR excesses above the thermal emission and a complicated interplay between the X-ray and the OIR light curves imply that the OIR emitting region is much more compact. One of the popular hypotheses is that the jet contributes to the OIR emission and even is responsible for the bulk of the X-rays. However, this scenario is largely ad hoc and is in contradiction with many previously established facts. Alternatively, the hot accretion flow, known to be consistent with the X-ray spectral and timing data, is also a viable candidate to produce the OIR radiation. The hot-flow scenario naturally explains the power-law like OIR spectra, fast OIR variability and its complex relation to the X-rays if the hot flow contains non-thermal electrons (even in energetically negligible quantities), which are required by the presence of the MeV tail in Cyg X-1. The presence of non-thermal electrons also lowers the equilibrium electron temperature in the hot flow model to 100 keV, making it more consistent with observations. Here we argue that any viable model should simultaneously explain a large set of spectral and timing data and show that the hybrid (thermal/non-thermal) hot flow model satisfies most of the constraints.

The black hole binary V404 Cygni: a highly accreting obscured AGN analogue

“…Low temperature seed photons required to produce the direct Comptonized spectrum are normally found in black hole binaries at low luminosities (see, e.g., Dunn et al 2011). Such seed photons would come, in our case, from either a (cool) heavily absorbed accretion disk truncated at large radii (e.g., Done et al 2007) and/or from synchrotron self-Compton emission by non-thermal electrons in the hot Comptonizing medium (see, e.g., Poutanen & Veledina 2014). …”

The 2015 Decay of the Black Hole X-Ray Binary V404 Cygni: Robust Disk-Jet Coupling and a Sharp Transition Into Quiescence

“…One is the relatively rapid X-ray spectral softening. In a hot accretion flow, synchrotron self-Compton (SSC) processes are important for generating X-ray emission (see, e.g., recent reviews by Poutanen & Veledina 2014;Malzac 2016), and the X-ray spectral softening toward quiescence is generally expected to be driven by a lower optical depth to inverse Compton scatterings and/or a lower flux of seed photons as Ṁ decreases (e.g., Esin et al 1997;Tomsick et al 2001;Sobolewska et al 2011;Niedźwiecki et al 2014). However, for V404 Cygni, our monitoring campaign demonstrates that such a decrease in optical depth/seed photon flux cannot be accompanied by a large change in the the X-ray efficiency q, since the slope of the radio/X-ray correlation does not change at a detectable level (i.e., the uncertainty on the best-fit =  m 0.54 0.03 implies s » 0.08 q ).…”

Understanding the periodicities in radio and GeV emission from LS I +61°303

“…Finally, optical observations by Mendelson & Mazeh (1989) showed two variations in orbital phase: one variation at periastron and another smaller variation at apastron. This optical emission could come from the hot accretion flow (optical synchrotron emission) as studies of X-ray binaries suggest (Poutanen & Veledina 2014). …”