Do Neutron Star Ultra-Luminous X-Ray Sources Masquerade as Intermediate Mass Black Holes in Radio and X-Ray?

TL;DR

This study investigates whether neutron star ultraluminous X-ray sources (ULXs) can be mistaken for intermediate mass black holes in radio and X-ray observations, finding that neutron star ULXs generally lack significant radio emission and are unlikely to be misclassified.

Contribution

First comprehensive radio analysis of neutron star ULXs showing they rarely produce detectable radio emission, reducing misclassification risk as intermediate mass black holes.

Findings

Only one neutron star ULX shows confident radio detection.

Most neutron star ULXs do not emit significant radio signals.

Radio counterparts are unlikely to lead to misidentification as black holes.

Abstract

Ultraluminous X-ray sources (ULXs) were once largely believed to be powered by super-Eddington accretion onto stellar-mass black holes, although in some rare cases, ULXs also serve as potential candidates for (sub-Eddington) intermediate mass black holes. However, a total of eight ULXs have now been confirmed to be powered by neutron stars, thanks to observed pulsations, and may act as contaminants for radio/X-ray selection of intermediate mass black holes. Here we present the first comprehensive radio study of seven known neutron star ULXs using new and archival data from the Karl G. Jansky Very Large Array and the Australia Telescope Compact Array, combined with the literature. Across this sample there is only one confident radio detection, from the Galactic neutron star ULX Swift J0243.6+6124. The other six objects in our sample are extragalactic, and only one has coincident radio emission, which we conclude is most likely contamination from a background HII region. We conclude that with current facilities, neutron star ULXs do not produce significant enough radio emission to cause them to be misidentified as radio/X-ray selected intermediate mass black hole candidates. Thus, if background star formation has been properly considered, the current study indicates that a ULX with a compact radio counterpart is not likely to be a neutron star.