Relevance of jet emitting disc physics to microquasars: application to Cygnus X-1

Abstract

Context. Interpretation of the X-ray spectra of X-ray binaries during their hard states requires a hot, optically thin medium. There are several accretion disc models that account for this aspect. However, none is designed to simultaneously explain powerful jets detected during these states. Aims. A new quasi-Keplerian hot accretion disc solution, a jet emitting disc (JED hereafter), which is part of a global disc-jet MHD structure producing stationary super-alfvénic ejection, is investigated here. Its radiative and energetic properties are then compared to the observational constraints found in Cygnus X-1. Methods. We solve the disc energy equation by balancing the local heating term with advection and cooling by synchrotron, bremsstrahlung, and Comptonization processes. The heating term, disc density, accretion velocity, and magnetic field amplitude were taken from published self-similar models of accretion-ejection structures. Both optically thin and thick regimes are considered in a one-temperature, gas-supported disc. Results. Three branches of solutions are found to be possible at a given radius, but we only investigate the hot, optically thin and geometrically slim solutions. These solutions give simultaneously and consistently the radiative and energetics properties of the discjet system. They are able to reproduce the global accretion-ejection properties of Cygnus X-1 very well, namely its X-ray spectral emission, jet power, and jet velocity. About half of the released accretion power is used to produce two mildly relativistic (v/c 0.5) jets, and for a luminosity of about 1% of the Eddington luminosity, the JED temperature and optical depth are close to what is observed in the hard state of Cygnus X-1. Conclusions. The accretion and ejection properties of JEDs agree with the observations of the prototypical black hole binary Cygnus X-1. The JED solutions are likely to be relevant to the whole class of microquasars.

Absorption lines from magnetically driven winds in X-ray binaries

“…The F97 MHD solutions have been used in Ferreira et al (2006) and Petrucci et al (2010), to describe accretion disks giving rise to jets in the Hard States of BHBs. Winds, on the other hand are seen in the Soft state of the BHBs when radio jets are absent.…”

Is the disc thermal state controlling the Blandford & Znajek/Blandford & Payne jet dichotomy?

“…It is well known that, for a given accretion rate and at a given radius, the thermal equilibrium of non-ejecting discs has basically three branches: an optically thick cold branch, an optically thin warm branch and an intermediate, thermally unstable branch (see [29] and references therein). The same has been shown to hold for JEDs [30]. However, the energy budget in JEDs writes…”

Dark jets in the soft X-ray state of black hole binaries?

“…Hot and geometrically thick accretion flows are generally believed to be more efficient at launching jets than the thin discs of the soft state (Meier 2001;Sikora & Begelman 2013;Avara et al 2016). However recent studies of accretion disc coupled to a jet through the Blandford & Payne (1982) mechanism indicate that thin discs can eject a larger fraction of the accretion power than geometrically thick discs (Petrucci et al 2010). In fact, from a theoretical point of view, the thermal disc-dominated state may harbour an even more powerful jet than the hard state.…”