# Chandra-HETGS Characterization of an Outflowing Wind in the accreting millisecond pulsar IGR J17591-2342

## TL;DR

This study uses high-resolution X-ray spectroscopy to characterize an outflowing wind in the accreting millisecond pulsar IGR J17591-2342, revealing a high-velocity outflow and potential calcium lines, suggesting a neutron star binary origin.

## Contribution

First detailed high-resolution spectral analysis of IGR J17591-2342 revealing an outflow and possible calcium lines, advancing understanding of its nature and origin.

## Key findings

- Detection of an outflow with velocity 2800 km/s at high significance
- Identification of potential calcium lines in the spectra
- Agreement between NICER and HETGS continua after accounting for dust scattering

## Abstract

IGR J17591-2342 is an accreting millisecond X-ray pulsar discovered in 2018 August in scans of the Galactic bulge and center by the INTEGRAL X-ray and gamma-ray observatory. It exhibited an unusual outburst profile with multiple peaks in the X-ray, as observed by several X-ray satellites over three months. Here we present observations of this source performed in the X-ray/gamma-ray and near infrared domains, and focus on a simultaneous observation performed with the Chandra-High Energy Transmission Gratings Spectrometer (HETGS) and the Neutron Star Interior Composition Explorer (NICER). HETGS provides high resolution spectra of the Si-edge region, which yield clues as to the source's distance and reveal evidence (at 99.999% significance) of an outflow with a velocity of $\mathrm{2\,800\,km\,s^{-1}}$. We demonstrate good agreement between the NICER and HETGS continua, provided that one properly accounts for the differing manners in which these instruments view the dust scattering halo in the source's foreground. Unusually, we find a possible set of Ca lines in the HETGS spectra (with significances ranging from 97.0% to 99.7%). We hypothesize that IGR J17591-2342 is a neutron star low mass X-ray binary at a distance of the Galactic bulge or beyond that may have formed from the collapse of a white dwarf system in a rare, calcium rich Type Ib supernova explosion.