Chandra-HETGS Characterization of an Outflowing Wind in the Accreting Millisecond Pulsar IGR J17591–2342

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 a variety of X-ray satellites over the course of three months. Here we present observations of this source performed in the X-ray/gamma-ray and near infrared domains. We focus on a simultaneous observation performed with the Chandra-High Energy Transmission Gratings Spectrometer (HETGS) and NICER. With the former instrument, we obtain high resolution spectra of the Si-edge region, which provide clues as to the source's distance and reveal evidence of an outflow with a velocity of 2800kms⁻¹. We show that there is good agreement between the NICER and Chandra-HETGS continua, provided that one properly accounts for the differing manners in which these two instruments view the dust scattering halo in the foreground of this source. Unusually, we find a possible set of Ca lines in the HETGS spectra. 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.