A relativistically smeared spectrum in the neutron star X-ray Binary 4U 1705-44: Looking at the inner accretion disc with X-ray spectroscopy

TL;DR

This paper presents X-ray spectroscopy of the neutron star binary 4U 1705-44, revealing a relativistically smeared iron emission line and absorption edge, allowing precise measurement of the inner accretion disc's properties.

Contribution

The study provides the first detailed measurement of the inner disc radius, inclination, and ionization state using relativistic iron line profiles in 4U 1705-44.

Findings

Detection of a relativistically smeared iron line at 6.7 keV

Inner disc radius measured at approximately 14 gravitational radii

Inclination angle of the disc is about 39 degrees

Abstract

Iron emission lines at 6.4-6.97 keV, identified with fluorescent Kalpha transitions, are among the strongest discrete features in the X-ray band. These are therefore one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper we present a recent XMM observation of the X-ray burster 4U 1705-44, where we clearly detect a relativistically smeared iron line at about 6.7 keV, testifying with high statistical significance that the line profile is distorted by high velocity motion in the accretion disc. As expected from disc reflection models, we also find a significant absorption edge at about 8.3 keV; this feature appears to be smeared, and is compatible with being produced in the same region where the iron line is produced. From the line profile we derive the physical parameters of the inner accretion disc with large precision. The line is identified with the Kalpha transition of highly ionised iron, Fe XXV, the inner disc radius is Rin = (14 \pm 2) R_g (where R_g is the Gravitational radius, GM/c^2), the emissivity dependence from the disc radius is r^{-2.27 \pm 0.08}, the inclination angle with respect to the line of sight is i = (39 \pm 1) degrees. Finally, the XMM spectrum shows evidences of other low-energy emission lines, which again appear broad and their profiles are compatible with being produced in the same region where the iron line is produced.