On the Fe K absorption - accretion state connection in the Galactic center neutron star X-ray binary AX J1745.6-2901

TL;DR

This study analyzes 38 XMM-Newton observations of the neutron star binary AX J1745.6-2901, revealing Fe K absorption features that are state-dependent and linked to accretion processes, similar to black hole binaries.

Contribution

It demonstrates a clear connection between Fe K absorption features and the accretion state in a neutron star binary, expanding understanding of wind behavior in such systems.

Findings

Fe K absorption appears only in the soft state.

Absorption features are not due to continuum changes.

High column densities and turbulent velocities are inferred.

Abstract

AX J1745.6-2901 is a high-inclination (eclipsing) neutron star Low Mass X-ray Binary (LMXB) located less than ~1.5 arcmin from Sgr A*. Ongoing monitoring campaigns have targeted Sgr A* frequently and these observations also cover AX J1745.6-2901. We present here an X-ray analysis of AX J1745.6-2901 using a large dataset of 38 XMM-Newton observations, including eleven which caught AX J1745.6-2901 in outburst. Fe K absorption is clearly seen when AX J1745.6-2901 is in the soft state, but disappears during the hard state. The variability of these absorption features does not appear to be due to changes in the ionizing continuum. The small Kalpha/Kbeta ratio of the equivalent widths of the Fe xxv and Fe xxvi lines suggests that the column densities and turbulent velocities of the absorbing ionised plasma are in excess of N_H ~ 10^23 cm^-2 and v_turb >~ 500 km s^-1. These findings strongly support a connection between the wind (Fe K absorber) and the accretion state of the binary. These results reveal strong similarities between AX J1745.6-2901 and the eclipsing neutron star LMXB, EXO 0748-676, as well as with high-inclination black hole binaries, where winds (traced by the same Fe K absorption features) are observed only during the accretion-disc-dominated soft states, and disappear during the hard states characterised by jet emission.