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

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 (R _in) can be made using iron emission lines that arise due to fluorescence of iron in the disk, and these indicate that R _in is very close to the black hole at high and moderate luminosities (gsim1% of the Eddington luminosity, L _Edd). 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 that extend iron line studies to 0.14% L _Edd and show that R _in increases by a factor of >27 over the value found when GX 339 - 4 was bright. The exact value of R _in depends on the inclination of the inner disk (i), and we derive 90% confidence limits of R _in > 35R_g at i = 0° and R _in > 175R_g at i = 30°. 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.