Studying the inner accretion disk of GX 339-4 with NuSTAR and Swift
Abstract
The latest outburst of the famous transient black-hole binary GX 339-4 at the end of 2013 was monitored by NuSTAR and Swift. Here we present the spectral analysis of four observations following the rise of the outburst and a final observation at the end of the outburst. During the outburst GX339-4 never left the low-hard state. The NuSTAR data provide excellent data quality to study the weak reflection component in this state. The iron line shows a significant broadening which is best described by relativistic effects close to the black-hole. If we assume a standard disk with an emissivity index of q=3 and a maximally spinning black-hole, the accretion disk seems to be truncated before reaching the ISCO, as expected in the low-hard state. However, a statistically equally good fit can be achieved with a black-hole spin of a=0.42\pm0.01. If we assume a lamp-post geometry of the corona and self consistently describe the emissivity, the data can be explained with a corona located several r_g above the black-hole. In that scenario a maximally spinning black-hole and an accretion disk extending all the way to the ISCO are found. All scenarios show a weak dependence on flux, with the strongest relativistic effects measured at the highest flux phases. We discuss the physical implications of these models.
- Publication:
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AAS/High Energy Astrophysics Division #14
- Pub Date:
- August 2014
- Bibcode:
- 2014HEAD...1420504F