The radio-bright accreting millisecond X-ray pulsar IGR J17591-2342

Abstract

IGR J17591$-$2342 is a 527-Hz accreting millisecond X-ray pulsar that was discovered in outburst in 2018 August. In this paper, we present quasi-simultaneous radio and X-ray monitoring of this source during the early part of the outburst. IGR J17591$-$2342 is highly absorbed in X-rays, with an equivalent hydrogen absorption along the line of sight, $N_{\mathrm{H}}$, of $\approx$4.4$\times$10${}^{22}$\,cm${}^{-2}$, where the Galactic column density is expected to be $\approx$1 -- 2$\times$10${}^{22}$\,cm${}^{-2}$. The high absorption suggests that the source is either relatively distant ($>$6\,kpc), or the X-ray emission is strongly absorbed by material local to the system. Radio emission detected by the Australia Telescope Compact Array shows that, for a given X-ray luminosity and for distances greater than 3\,kpc, this source was exceptionally radio loud when compared to other accreting neutron stars in outburst ($L_{\mathrm{X}}>10^{33}$\,erg\,s${}^{-1}$). For most reasonable distances, IGR J17591$-$2342 appeared as radio luminous as actively accreting, stellar-mass black hole X-ray binaries.

8 pages, 2 figures. Accepted for publication by ApJL