A unified accretion-ejection paradigm for black hole X-ray binaries. II. Observational signatures of jet-emitting disks

TL;DR

This paper investigates the spectral signatures of jet-emitting disks (JEDs) in black hole X-ray binaries, developing a detailed plasma code to model their thermal equilibria and emission, highlighting their role in luminous hard states.

Contribution

It introduces a comprehensive model for JED spectral signatures, incorporating a two-temperature plasma code and analyzing thermal equilibria, hysteresis, and spectral states in accretion disks.

Findings

JEDs exhibit three thermal equilibria with hysteresis cycles.

JEDs can reproduce luminous hard states in X-ray binaries.

Standard outburst cycles are not reproduced by JEDs.

Abstract

We elaborate on the paradigm proposed in Ferreira et al. (2006), where the increase and decrease in the disk accretion rate is accompanied by a modification of the disk magnetization $\mu \propto B_z^2/\dot{m}_{in}$, which in turn determines the dominant torque allowing accretion. For $\mu>0.1$, the accretion flow produces jets that vertically, carry away the disk angular momentum (jet-emitting disk or JED). The goal of this paper is to investigate the spectral signatures of the JED configurations. We have developed a two-temperature plasma code that computes the disk local thermal equilibria, taking into account the advection of energy in an iterative way. Our code addresses optically thin-to-thick transitions, both radiation and gas supported regimes and computes in a consistent way the emitted spectrum from a steady-state disk. The optically thin emission is obtained using the BELM code, which provides accurate spectra for bremsstrahlung and synchrotron emission processes as well as for their local Comptonization. For a range in radius and accretion rates, JEDs exhibit three thermal equilibria, one thermally unstable and two stables. Due to the existence of two thermally stable solutions, a hysteresis cycle is naturally obtained. However, standard outbursting X-ray binary cycles cannot be reproduced. Another striking feature of JEDs is their ability to reproduce luminous hard states. Showing that when the loss of angular momentum and power in jets is consistently taken into account, accretion disks have spectral signatures that are consistent with hard states, even up to high luminosities. The reproduction of soft states being well performed by standard accretion disks (SAD), this study argues for the existence of hybrid disk configuration: JED and SAD. A study of such hybrid configuration will be presented in a forthcoming paper III.