2D MHD Simulations of the State Transitions of X-Ray Binaries Taking into Account Thermal Conduction

Kenji E Nakamura,Mami Machida,Ryoji Matsumoto

doi:10.3390/galaxies7010022

Kenji E Nakamura, Mami Machida + Show 1 more

Open Access

https://doi.org/10.3390/galaxies7010022

Copy DOI

Abstract

Thermal conduction plays an important role in bimodal accretion flows consisting of high-temperature flow and cool flow, especially when the temperature is high and/or has a steep gradient. For example, in hard-to-soft transitions of black hole accretion flows, thermal conduction between the high-temperature region and the low-temperature region is appropriately considered. We conducted two-dimensional magnetohydrodynamic (MHD) numerical simulations considering anisotropic heat conduction to study condensation of geometrically thick hot accretion flows driven by radiative cooling during state transitions. Numerical results show that the intermediate region appears between the hot corona and the cool accretion disk when we consider heat conduction. The typical temperature and number density of the intermediate region of the 10 Mo black hole at 10Rg (Rg = 3.0 x 106 cm is the Schwarzschild radius) are 4 x 1010 < T [K] < 4 x 1012 and 5 x 1015 < n [cm-3] < 5 x 1717, respectively. The thickness of intermediate region is about half of the radius. By comparing two models with or without thermal conduction, we demonstrate the effects of thermal conduction.

Highlights

Black hole X-ray binary systems consisting of either a high-mass X-ray binaries (HMXB) such as “Cyg X-1” or a low-mass X-ray binaries (LMXB) such as “GX339-4” show two quasi-steady X-ray spectral states—hard state and soft state—and state transitions between them
We must include the effect of thermal conduction, when we study the accretion disk surrounded by hot corona
The implicit method is necessary to calculate our model because the time scale of thermal conduction is much shorter than one of the other time scales in high-temperature regions

Summary

Introduction

Black hole X-ray binary systems consisting of either a high-mass X-ray binaries (HMXB) such as “Cyg X-1” or a low-mass X-ray binaries (LMXB) such as “GX339-4” show two quasi-steady X-ray spectral states—hard state and soft state—and state transitions between them. X-ray satellites and other instruments have revealed the hidden nature of X-ray binaries, such as existence of the intermediate state, jet ejections and quasi-periodic oscillations [1]. Accretion flows in the hard state are explained by optically thin high-temperature accretion flows, “advection-dominated accretion flows (ADAFs)” [2,3]. The soft state is explained by the optically thick cool accretion disks,. To understand the structural change of accretion flows during state transitions; two-dimensional or three-dimensional numerical studies are necessary. Hard-to-soft state changes has been studied by Machida et al by performing three-dimensional

Methods

Results

Conclusion