Optical counterparts of ULXs in NGC 1672

Abstract

ABSTRACT In this work, we deploy archival data from Hubble Space Telescope, Chandra, XMM–Newton, and Swift-XRT, to probe the nature of nine candidate ultraluminous X-ray sources (ULXs) in NGC 1672. Specifically, our study focuses on using the precise source positions obtained via improved astrometry based on Chandra and Hubble Space Telescope observations to search for and identify potential optical counterparts for these ULXs. Unique optical counterparts are identified for two of the ULX candidates, i.e. X2 and X6; for three of the candidates, i.e. X1, X5, and X7, we found two potential counterparts for each source within the respective error radii. No optical counterparts were found for the remaining four sources. The spectral energy distribution of X2 is fitted to a blackbody spectrum with a temperature of ∼104 K and the spectral class of the source is determined to be B7−A3, a supergiant donor star. We used colour–magnitude diagrams to investigate ages of the counterparts. Of all the sources studied, X9 exhibits the most variability whereby the X-ray flux varies by a factor of ∼50 over a time period spanning 2004–2019, and also traces a partial q-curve-like feature in the hardness–intensity diagram, hinting at possible spectral transitions.