The Spectral Features of Disk and Corona with Mass Evaporation in the Low/Hard State

TL;DR

This paper models the spectral features of accretion flows with disk and corona, incorporating magnetic fields and Compton scattering, to explain observed anti-correlations in X-ray binary spectra.

Contribution

It introduces a comprehensive model including magnetic fields and Compton scattering to predict spectral features and their relation to accretion rates in low/hard states.

Findings

The model reproduces the anti-correlation between Eddington ratio and X-ray photon index.

Spectral features vary with accretion rate and disk truncation radius.

At low accretion rates, spectra resemble pure ADAF.

Abstract

We investigate the spectral features of accretion flows composed of an outer cool, optically thick disk and inner hot, optically thin, advection dominated accretion flows (ADAF) within the framework of disk and corona with mass evaporation (Liu et al. 2002a). In this work, both the magnetic field and Compton scattering of soft photons from the disk by electrons in the corona are included to calculate the evaporation rates at different distances. The disk is truncated at the distance where the evaporation rate equals to the accretion rate ($\dot m_{\rm evap}(r_{\rm tr})=\dot m$). For a series of accretion rates, the corresponding truncation radii are calculated out, with which we are able to calculate the emergent spectra from the inner ADAF + outer disk + corona. At very low accretion rates, the spectra are similar to that of a pure ADAF because the disk is truncated at large distances. The disk component becomes important at high accretion rates since the truncation occurs at small distances. An anti-correlation between the Eddington ratio $\xi \equiv L_{\rm 0.5-25 \,keV}/L_{\rm Edd}$ and the hard X-ray photon index $\Gamma_{\rm 3-25 \,keV}$ at low/hard states is predicted by the model. Comparing the theoretical results with observations, we find that our model can reproduce the anti-correlation between the Eddington ratio $\xi$ and the hard X-ray photon index observed for the X-ray binary XTE J1118+480.