Spectral analysis of the XMM-Newton data of GX 339-4 in the low/hard state: disc truncation and reflection

TL;DR

This study analyzes XMM-Newton data of GX 339-4 in the low/hard state, revealing disc truncation and reflection features, and suggests the soft X-ray component is affected by irradiation rather than a standard blackbody disc.

Contribution

It provides a comprehensive spectral analysis using reflection models, exploring disc truncation, irradiation effects, and geometry assumptions in the low/hard state of GX 339-4.

Findings

Inner disc radius is large in coronal models.

Disc ionization anti-correlates with spectral hardness.

Relativistic broadening effects are relatively weak.

Abstract

We analyse all available observations of GX 339--4 by XMM-Newton in the hard spectral state. We jointly fit the spectral data by Comptonization and the currently best reflection code, relxill. We consider in detail a contribution from a standard blackbody accretion disc, testing whether its inner radius can be set equal to that of the reflector. However, this leads to an unphysical behaviour of the disc truncation radius, implying the soft X-ray component is not a standard blackbody disc. This appears to be due to irradiation by the hard X-rays, which strongly dominate the total emission. We consider a large array of models, testing, e.g., the effects of the chosen energy range, of adding unblurred reflection, and assuming a lamppost geometry. We find the effects of relativistic broadening to be relatively weak in all cases. In the coronal models, we find the inner radius to be large. In the lamppost model, the inner radius is unconstrained, but when fixed to the innermost stable orbit, the height of the source is large, which also implies a weak relativistic broadening. In the former models, the inner radius correlates with the X-ray hardness ratio, which is consistent with the presence of a truncated disc turning into a complete disc in the soft state. We also find the degree of the disc ionization to anti-correlate with the hardness, leading to strong spectral broadening due to scattering of reflected photons in the reflector in the softest studied states.