Spectral properties of MXB 1658-298 in the low/hard and high/soft state

TL;DR

This study analyzes the spectral properties of the low mass X-ray binary MXB 1658-298 during different accretion states, revealing state-dependent spectral features and a link to Fe K absorption, using broadband observations from Swift/XRT and NuSTAR.

Contribution

First comparison of soft and hard spectral states during an outburst in MXB 1658-298, highlighting state-dependent spectral features and absorption characteristics.

Findings

Spectral states correlate with different spectral models and luminosities.

Fe K absorption features are present only in the soft state.

A link exists between spectral state and ionized Fe absorption.

Abstract

We report results from a broadband spectral analysis of the low mass X-ray binary MXB 1658-298 with the Swift/XRT and NuSTAR observations made during its 2015-2017 outburst. The source showed different spectral states and accretion rates during this outburst. The source was in low/hard state during 2015; and it was in high/soft state during the 2016 NuSTAR observations. This is the first time that a comparison of the soft and hard spectral states during an outburst is being reported in MXB 1658-298. Compared with the observation of 2015, the X-ray luminosity was about four times higher during 2016. The hard state spectrum can be well described with models consisting of a single-temperature blackbody component along with Comptonized disc emission, or three component model comprising of multi-colour disc, single-temperature blackbody and thermal Comptonization components. The soft state spectrum can be described with blackbody or disc blackbody and Comptonization component, where the neutron star surface (or boundary layer) is the dominant source for the Comptonization seed photons. We have also found a link between the spectral state of the source and the Fe K absorber. The absorption features due to highly ionized Fe were observed only in the soft state. This suggests a probable connection between accretion disc wind (or atmosphere) and spectral state (or accretion state) of MXB 1658-298.