Is the black hole in GX 339-4 really spinning rapidly?

TL;DR

This study reanalyzes Suzaku data of GX 339-4, showing that the black hole's spin may not be as high as previously thought, due to data analysis considerations affecting the interpretation of the accretion disk's inner radius.

Contribution

It demonstrates that careful data analysis can lead to alternative interpretations of black hole spin, challenging previous claims of rapid spin based on spectral features.

Findings

The Fe-K line profile depends on data extraction regions.

The inner disk radius is inferred to be 5-14 times the gravitational radius.

A non-maximal black hole spin is consistent with the data.

Abstract

The wide-band Suzaku spectra of the black hole binary GX 339-4, acquired in 2007 February during the Very High state, were reanalyzed. Effects of event pileup (significant within ~ 3' of the image center) and telemetry saturation of the XIS data were carefully considered. The source was detected up to ~ 300$ keV, with an unabsorbed 0.5--200 keV luminosity of ~3.8 10^{38} erg/s at 8 kpc. The spectrum can be approximated by a power-law of photon index 2.7, with a mild soft excess and a hard X-ray hump. When using the XIS data outside 2' of the image center, the Fe-K line appeared extremely broad, suggesting a high black hole spin as already reported by Miller et al. (2008) based on the Suzaku data and other CCD data. When the XIS data accumulation is further limited to >3' to avoid event pileup, the Fe-K profile becomes narrower, and there appears a marginally better solution that suggests the inner disk radius to be 5-14 times the gravitational radius (1-sigma), though a maximally spinning black hole is still allowed by the data at the 90% confidence level. Consistently, the optically-thick accretion disk is inferred to be truncated at a radius 5-32 times the gravitational radius. Thus, the Suzaku data allow an alternative explanation without invoking a rapidly spinning black hole. This inference is further supported by the disk radius measured previously in the High/Soft state.