# A Systematic Search for Near-Infrared Counterparts of Nearby Ultraluminous X-ray sources (II)

## TL;DR

This study systematically searches for near-infrared counterparts to ultraluminous X-ray sources within 10 Mpc, identifying candidate counterparts and analyzing their properties to aid in understanding the nature of ULXs.

## Contribution

It provides the first large sample of NIR candidate counterparts to ULXs, including potential red supergiants, and discusses their implications for future spectroscopic follow-up.

## Key findings

- Detected NIR counterparts for 15 of 42 ULXs observed.
- Identified 7 ULXs with counterparts consistent with red supergiants.
- Some counterparts exhibit extended morphology or AGN-like colors.

## Abstract

We present the results of our continued systematic search for near-infrared (NIR) candidate counterparts to ultraluminous X-ray sources (ULXs) within 10 Mpc. We observed 42 ULXs in 24 nearby galaxies and detected NIR candidate counterparts to 15 ULXs. Fourteen of these ULXs appear to have a single candidate counterpart in our images and the remaining ULX has 2 candidate counterparts. Seven ULXs have candidate counterparts with absolute magnitudes in the range between -9.26 and -11.18 mag, consistent with them being red supergiants (RSGs). The other eight ULXs have candidate counterparts with absolute magnitudes too bright to be a single stellar source. Some of these NIR sources show extended morphology or colours expected for Active Galactic Nuclei (AGN), strongly suggesting that they are likely stellar clusters or background galaxies. The red supergiant candidate counterparts form a valuable sample for follow-up spectroscopic observations to confirm their nature, with the ultimate goal of directly measuring the mass of the compact accretor that powers the ULX using binary Doppler shifts.