Radio counterparts of ultraluminous X-ray sources in nearby galaxies

TL;DR

This study identifies 21 radio counterparts of ultraluminous X-ray sources in nearby galaxies, revealing diverse structures and possible different physical origins, including quasars, star-forming regions, and accreting black holes with jets.

Contribution

It provides the first systematic crossmatch of ULX catalogs with radio surveys, classifying their radio morphologies and spectra, and suggesting new candidate populations like SS 433/W50-like objects.

Findings

Three sources have double-lobed radio structures, possibly quasars.

Five sources are associated with star-forming regions, with ambiguous radio emission origins.

Thirteen sources are unresolved radio sources, some with steep spectra indicating jet/wind nebulae.

Abstract

Radio surveys of ultraluminous X-ray sources (ULXs) allow us to find supercritically accreting compact objects (SS 433/W50 like systems) or stripped nuclear black holes in nearby galaxies. We identified 21 such objects by crossmatching a ULX catalog with the Rapid ASKAP Continuum Survey (RACS) and Very Large Array Sky Survey (VLASS). They may have a diverse population. (i) Three have a double lobed radio structure with a compact core found in two of them, and could be quasars. (ii) Five are associated with extended radio structure and star forming regions in optical, where the radio emission is likely due to star forming activities, although the steep radio spectrum up to several GHz casts doubt on that. Two of them show X-ray variability suggesting that they are ULXs embedded in star forming regions. (iii) Thirteen are associated with an unresolved radio source, with a steep spectrum seen in eight, a flat or inverted spectrum seen in two. Those with a steep spectrum are arguably candidates for SS 433/W50 like objects, with radio emission due to optically thin synchrotron radiation in a surrounding jet/wind powered nebula. Remarkable cases include NGC 925 ULX1 and NGC 6946 ULX1, which are associated with an optical nebula. Those with a flat or inverted spectrum could be accreting black holes with a compact jet, while the black hole mass is estimated to be several $10^6 - 10^8$ $M_\odot$ based on the fundamental plane. Redshift measurements are needed to firmly determine the association with their apparent host galaxy.