The complex accretion geometry of GX 339-4 as seen by NuSTAR and Swift

TL;DR

This study analyzes NuSTAR and Swift X-ray observations of GX 339-4 during a failed outburst, revealing complex accretion geometry, reflection features, and insights into disk and corona properties with improved modeling techniques.

Contribution

It introduces a modeling approach allowing different photon indices for the incident and observed continua, reducing iron abundance estimates and providing new insights into accretion geometry.

Findings

No evidence for disk truncation beyond 100 r_g.

Best-fit inclination angle between 40-60 degrees.

Detection of a broadened iron line with a narrow core.

Abstract

We present spectral analysis of five NuSTAR and Swift observations of GX 339-4 taken during a failed outburst in summer 2013. These observations cover Eddington luminosity fractions in the range ~0.9-6%. Throughout this outburst, GX 339-4 stayed in the hard state, and all five observations show similar X-ray spectra with a hard power-law with a photon index near 1.6 and significant contribution from reflection. Using simple reflection models we find unrealistically high iron abundances. Allowing for different photon indices for the continuum incident on the reflector relative to the underlying observed continuum results in a statistically better fit and reduced iron abundances. With a photon index around 1.3, the input power-law on the reflector is significantly harder than that which is directly observed. We study the influence of different emissivity profiles and geometries and consistently find an improvement when using separate photon indices. The inferred inner accretion disk radius is strongly model dependent, but we do not find evidence for a truncation radius larger than 100 r_g in any model. The data do not allow independent spin constraints but the results are consistent with the literature (i.e., a>0). Our best-fit models indicate an inclination angle in the range 40-60 degrees, consistent with limits on the orbital inclination but higher than reported in the literature using standard reflection models. The iron line around 6.4 keV is clearly broadened, and we detect a superimposed narrow core as well. This core originates from a fluorescence region outside the influence of the strong gravity of the black hole and we discuss possible geometries.