The first mm detection of a neutron star high-mass X-ray binary

TL;DR

This paper reports the first millimeter detection of a neutron star high-mass X-ray binary, providing insights into stellar wind properties and highlighting the potential of multi-wavelength campaigns for future research.

Contribution

It presents the first millimeter-wave observation of a neutron star HMXB, comparing wind emissions between different systems and discussing future multi-wavelength measurement strategies.

Findings

Detected radio emission from SFXT IGR J18410-0535 at 100 GHz.

SFXT shows a flat or inverted spectrum suggesting wind emission.

Undetected SgXB indicates possible differences in wind properties.

Abstract

Neutron stars accreting from OB supergiants are often divided between persistently and transiently accreting systems, called Supergiant X-ray Binaries (SgXBs) and Supergiant Fast X-ray Transients (SFXTs). This dichotomy in accretion behaviour is typically attributed to systematic differences in the massive stellar wind, binary orbit, or magnetic field configuration, but direct observational evidence for these hypotheses remains sparse. To investigate their stellar winds, we present the results of pilot 100-GHz observations of one SFXT and one SgXB with the Northern Extended Millimetre Array. The SFXT, IGR J18410-0535, is detected as a point source at $63.4 \pm 9.6$ $\mu$Jy, while the SgXB, IGR J18410-0535 remains undetected. Radio observations of IGR J18410-0535 imply a flat or inverted low-frequency spectrum, arguing for wind emission and against non-thermal flaring. Due to the uncertain SFXT distance, however, the observations do not necessarily imply a difference between the wind properties of the SFXT and SgXB. We compare the mm constraints with other HMXBs and isolated OB supergiants, before considering how future mm campaigns can constrain HMXB wind properties by including X-ray measurements. Specifically, we discuss caveats and future steps to successfully measure wind mass loss rates and velocities in HMXBs with coordinated mm, radio, and X-ray campaigns.