NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901

TL;DR

This study presents extensive simultaneous XMM-Newton and NuSTAR observations of the neutron star transient AX J1745.6-2901, revealing state-dependent spectral features, absorption characteristics, and improved continuum modeling in outburst states.

Contribution

It provides the first comprehensive analysis of multiple outbursts with simultaneous broad-band spectra, enhancing understanding of spectral states and absorption features in this neutron star LMXB.

Findings

Soft state spectra show disk black body and boundary layer emission with Fe K absorption.

Hard state spectra exhibit a power-law with no high-energy cutoff, indicating a hot corona.

Fe K absorption is strong in the soft state and weakens in the hard state.

Abstract

AX J1745.6-2901 is a high-inclination (eclipsing) transient neutron star (NS) Low Mass X-ray Binary (LMXB) showcasing intense ionised Fe K absorption. We present here the analysis of 11 XMM-Newton and 15 NuSTAR new data-sets (obtained between 2013-2016), therefore tripling the number of observations of AX J1745.6-2901 in outburst. Thanks to simultaneous XMM-Newton and NuSTAR spectra, we greatly improve on the fitting of the X-ray continuum. During the soft state the emission can be described by a disk black body ($kT\sim1.1-1.2$ keV and inner disc radius $r_{DBB}\sim14$ km), plus hot ($kT\sim2.2-3.0$ keV) black body radiation with a small emitting radius ($r_{BB}\sim0.5-0.8$ km) likely associated with the boundary layer or NS surface, plus a faint Comptonisation component. Imprinted on the spectra are clear absorption features created by both neutral and ionised matter. Additionally, positive residuals suggestive of an emission Fe K$\alpha$ disc line and consistent with relativistic ionised reflection are present during the soft state, while such residuals are not significant during the hard state. The hard state spectra are characterised by a hard ($\Gamma\sim1.9-2.1$) power law, showing no evidence for a high energy cut off ($kT_e>60-140$ keV) and implying a small optical depth ($\tau<1.6$). The new observations confirm the previously witnessed trend of exhibiting strong Fe K absorption in the soft state, that significantly weakens during the hard state. Optical (GROND) and radio (GMRT) observations suggest for AX J1745.6-2901 a standard broad band SED as typically observed in accreting neutron stars.