Truncation of the Inner Accretion Disk around a Black Hole at Low Luminosity

TL;DR

This study provides direct observational evidence that the inner accretion disk around a black hole recedes at low luminosities, supporting models where the disk is replaced by different accretion flows at low accretion rates.

Contribution

First direct measurement showing the inner accretion disk recedes at very low luminosity in a black hole binary, extending iron line studies to 0.14% of Eddington luminosity.

Findings

Inner disk radius increases by over 27 times at low luminosity.

Inner radius exceeds 35 Rg at 0 degrees inclination.

Inner radius exceeds 175 Rg at 30 degrees inclination.

Abstract

Most black hole binaries show large changes in X-ray luminosity caused primarily by variations in mass accretion rate. An important question for understanding black hole accretion and jet production is whether the inner edge of the accretion disk recedes at low accretion rate. Measurements of the location of the inner edge (Rin) can be made using iron emission lines that arise due to fluorescence of iron in the disk, and these indicate that Rin is very close to the black hole at high and moderate luminosities (near 1% of the Eddington luminosity, Ledd). Here, we report on X-ray observations of the black hole GX 339-4 in the hard state by Suzaku and the Rossi X-ray Timing Explorer (RXTE) that extend iron line studies to 0.14% Ledd and show that Rin increases by a factor of >27 over the value found when GX 339-4 was bright. The exact value of Rin depends on the inclination of the inner disk (i), and we derive 90% confidence limits of Rin > 35 Rg at i = 0 degrees and Rin > 175 Rg at i = 30 degrees. This provides direct evidence that the inner portion of the disk is not present at low luminosity, allowing for the possibility that the inner disk is replaced by advection- or magnetically-dominated accretion flows.