Colours of black holes: infrared flares from the hot accretion disc in XTE J1550–564

Abstract

Outbursts of the black hole (BH) X-ray binaries are dramatic events occurring in our Galaxy approximately once a year. They are detected by the X-ray telescopes and often monitored at longer wavelengths. We analyse the X-ray and optical/infrared (OIR) light-curves of the BH binary XTE J1550-564 during the 2000 outburst. By using the observed extreme colours as well as the characteristic decay time-scales of the OIR and X-ray light curves, we put strong constraints on the extinction towards the source. We accurately separate the contributions to the OIR flux of the irradiated accretion disc and a non-thermal component. We show that the OIR non-thermal component appears during the X-ray state transitions both during the rising and the decaying part of the outburst at nearly the same X-ray hardness but at luminosities differing by a factor of 3. The line marking the quenching/recovery of the non-thermal component at the X-ray hardness-flux diagram seems to coincide with the 'jet line' that marks the presence of the compact radio jet. The inferred spectral shape and the evolution of the non-thermal component during the outburst, however, are not consistent with the jet origin, but are naturally explained in terms of the hybrid hot flow scenario, where non-thermal electrons emit synchrotron radiation in the OIR band. This implies a close, possibly causal connection between the presence of the hot flow and the compact jet. We find that the non-thermal component is hardening during the hard state at the decaying stage of the outburst, which indicates that the acceleration efficiency is a steep function of radius at low accretion rate.

Expanding hot flow in the black hole binary SWIFT J1753.5−0127: evidence from optical timing

“…The ratio L d /Ls ∝ L −1/2 X increases as the X-ray luminosity decreases. A similar trend was observed in the BH binary XTE J1550-564, the spectral evolution of which allowed the contributions of these two components to be separated (Poutanen, Veledina & Revnivtsev 2014). Hence, the results of our timing analysis are in agreement with expectations from spectral studies.…”

Superhump period of the black hole X-ray binary GX 339−4

“…Such flares have also been detected in other sources and are believed to arise from the appearance of an additional component, probably of non-thermal origin (Callanan et al 1995;Jain et al 2001;Buxton & Bailyn 2004;Poutanen et al 2014). On the other hand, the ONIR spectra during the soft state seem to approximately agree with the blackbody spectrum, suggesting a major contribution of the irradiated accretion disc.…”

Physical Constraints from Near-infrared Fast Photometry of the Black Hole Transient GX 339–4

“…Therefore, under this same scenario, O-IR flux changes cannot be attributed solely to a change in the size of the hot-inflow (see e.g. Poutanen et al 2014).…”