The first glimpse of ULXs through the near-infrared images captured by the JWST

Abstract

ABSTRACT This work presents the first results of near-infrared (NIR) counterparts of ultraluminous X-ray sources (ULXs) in the case of NGC 1672 by using JWST observations. Through advanced astrometry, unique counterparts were identified for four of the eight ULXs (ULX-1, ULX-4, ULX5, and ULX-8) located in NGC 1672, while multiple counterparts were identified for the remaining ULXs. The NIR observations show clues of warm dust or circumbinary disc around ULX-5 and ULX-8. In the case of ULX-5, optical spectral energy distribution is a well-fitted blackbody with 4300 K while NIR excess is well-fitted by a blackbody with a temperature of 1600 K. The optical-NIR photometric results show that the possible donor star of ULX-5 may be a K–M type red supergiant (RSG) whose optical emission is affected by the presence of a circumbinary disc or warm dust. Additionally, the counterpart of ULX-4 could be an active galactic nuclei (AGN) or star cluster due to its high K-band magnitude. Thanks to the good enough resolution of the JWST images, in past studies, most of the point-like and/or bright NIR counterparts of ULXs in distant galaxies observed appear to be likely blended sources, so most likely, many of them do not have the red colour that an RSG could have. The significant improvement in sensitivity and resolution supplied by JWST will lead to a new perspective on the ambiguous nature of ULXs donors and environments necessitating a significant reassessment of earlier infrared studies of ULXs.