The Truncated Disk from Suzaku Data of GX 339-4 in the Extreme Very High State

Abstract

We report on the geometry of accretion disk and high-energy coronae in the strong Comptonization state (the very high/steep power law/hard intermediate state) based on a Suzaku observation of the famous Galactic black hole GX 339-4. These data were taken just before the peak of the 2006-2007 outburst, and the average X-ray luminosity in the 0.7-200 keV band is estimated to be 2.9 × 10³⁸ erg s^-1 for a distance of 8 kpc. We fit the spectrum with both simple (independent disk and corona) and sophisticated (energetically coupled disk and corona) models; all fits imply that the underlying optically thick disk is truncated significantly before the innermost stable circular orbit around the black hole. We show this directly by a comparison with similar broadband data from a disk-dominated spectrum at almost the same luminosity observed by XMM-Newton and RXTE 3 days after the Suzaku observation. During the Suzaku observation, the quasi-periodic oscillation (QPO) frequency changes from 4.3 Hz to 5.5 Hz, while the spectrum softens. The energetically coupled model gives a corresponding 5% ± 8% decrease in the derived inner radius of the disk. While this is not significant, it is consistent with the predicted change in QPO frequency from the Lense-Thirring precession of the hot flow interior to the disk and/or a deformation mode of this flow, as a higher QPO frequency implies a smaller size scale for the corona. This is consistent with the truncated disk extending further inward toward the black hole.