Is the Black Hole in GX 339-4 Really Spinning Rapidly?

Abstract

The wide-band Suzaku spectra of the black hole (BH) 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 X-ray Imaging Spectrometer (XIS) data were carefully considered. The source was detected up to ~300 keV, with an unabsorbed 0.5-200 keV luminosity of 3.8 × 10³⁸ erg s^-1 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 BH spin as already reported by Miller et al. based on the same 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 a marginally better solution appears which suggests that the inner disk radius is 5-14 times the gravitational radius (1σ), though a maximally spinning BH 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 BH. This inference is further supported by the disk radius measured previously in the High/Soft state.