Modelling the ‘outliers’ track of the radio–X-ray correlation in X-ray binaries based on a disc–corona model

Abstract

The universal radio-X-ray correlation (F R ∝ F b X , b ∼ 0.5 − 0.7) has been found for a sample of black-hole X-ray binaries (BHBs) in their low/hard states, which can roughly be explained by the coupled model of jet and radiatively inefficient advection dominated accretion flow. However, more and more 'outliers' were found in last few years, which evidently deviate from the universal radio-X-ray correlation and usually show a much steeper correlation with an index of ∼ 1.4. Based on simple physical assumptions, the radiatively efficient accretion flows are speculated to exist in these 'outliers'. In this work, we test this issue by modelling the 'outliers' track based on the radiatively efficient disc-corona model and the hybrid jet model. We find that our model predicts a steeper radio-X-ray correlation with slopes 1.2 for the typical viscosity parameter of α ∼ 0.05 − 0.2. In particular, the slope is ∼ 1.4 for the case of α ∼ 0.1, which is consistent with the observational results of H1743−322 very well. Our results suggest that the 'outliers' track may be regulated by the disc-corona model.

The radio/X-ray domain of black hole X-ray binaries at the lowest radio luminosities

“…Recently, MeyerHofmeister & Meyer (2014) proposed an explicit form of such an efficient solution (see also Cao, Wu & Dong 2014;Huang, Wu & Wang 2014) by arguing that thermal photons from a weak, cool disc in the innermost regions of an otherwise inefficient accretion flow would be responsible for enhancing the seed photons available for Comptonisation, and hence the hard X-ray flux (in the context of this model, the 'radio quiet' track corresponding to the initial decline of the 2008 outburst of H1743-322 is better described as 'X-ray bright'). The condensation of hot, optically thin accreting gas into an inner, cool, keplerian disc is predicted above a critical mass accretion rate if thermal conduction and Compton cooling are properly accounted for in the equations of energy and mass exchange Liu, Meyer & Meyer-Hofmeister 2006;Liu et al 2007).…”

A disc corona–jet model for the radio/X-ray correlation in black hole X-ray binaries

“…In this way the accretion energy is released in the corona as thermal energy and eventually emitted away via inverse Compton scattering. By studying the energy balance of the disc and the corona, Huang et al (2014) solved the structure of the cold accretion disc. However, they can not self-consistently determine the temperature of the corona T and the Compton scattering optical depth τ, which are two very important quantities for determining the shape of the Compton emergent spectrum.…”

“…Theoretically, T and τ should be solved independently for determining the Compton emergent spectrum. In Huang et al (2014), it is argued that the observationally inferred value of τ is in the range of ∼ 0.1 − 0.8, then through their calculation, τ = 0.5 is fixed to solve T for fitting this radio/X-ray correlation. In our model, we do not fix the value of τ.…”

A model for the radio/X-ray correlation in three neutron star low-mass X-ray binaries 4U 1728-34, Aql X-1ad, and EXO 1745-248

“…It is suggested that the correlation of L R ∝ L ∼1.4 X may be resulted by the transition of the accretion flow from the radiatively inefficient accretion flow, ADAF, to the radiatively efficient accretion flow, disc-corona system, with increasing the mass accretion rate (Coriat et al 2011;Gallo et al 2012, for discussions), which is later quantitatively interpreted by several authors with the disc-corona + jet model (e.g. Qiao & Liu 2015;Huang et al 2014;Meyer-Hofmeister & Meyer 2014). However, we note that the statistical significance of the aforementioned two tracks, i.e., the 'universal' correlation and the steeper correlation is still questioned (Gallo et al 2014(Gallo et al , 2018.…”