The truncated disk from Suzaku data of GX 339-4 in the extreme very high state

TL;DR

This study uses Suzaku data to analyze the accretion disk and corona geometry in GX 339-4 during a very high state, revealing a truncated disk significantly inside the innermost stable orbit and linking QPO frequency changes to disk-corona dynamics.

Contribution

It provides direct evidence of disk truncation in the very high state and explores the relationship between QPO frequency variations and disk-corona geometry changes.

Findings

The accretion disk is truncated well before the innermost stable orbit.

QPO frequency increases from 4.3 Hz to 5.5 Hz during observation.

A possible link between QPO frequency and inner disk radius change.

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 2.9E38 erg/s 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, but 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 similarly 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 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 derived inner radius of the disk. While this is not significant, it is consistent with the predicted change in QPO frequency from 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 inwards towards the black hole.