Optical Counterparts of ULXs in NGC 1672

TL;DR

This study uses multi-mission archival data to identify optical counterparts of ULXs in NGC 1672, revealing two unique counterparts, potential donor star types, and significant X-ray variability in one source, advancing understanding of ULX nature.

Contribution

It provides the first identification of optical counterparts for several ULXs in NGC 1672 using precise astrometry and multi-wavelength data, including spectral and variability analysis.

Findings

Unique optical counterparts identified for X2 and X6.

X2's spectral energy distribution suggests a supergiant donor star.

X9 shows extreme variability with flux changing by a factor of 50.

Abstract

In this work, we deploy archival data from {\it HST}, {\it Chandra}, {\it XMM-Newton}, and {\it Swift-XRT}, to probe the nature of 9 candidate ULXs in NGC 1672. Specifically, our study focuses on using the precise source positions obtained via improved astrometry based on {\it Chandra} and {\it HST} 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 $\sim$ 10$^{4}$K and the spectral class of the source is determined to be B7$-$A3, a supergiant donor star. We used colour magnitude diagrams (CMDs) 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 $\sim$ 50 over a time period spanning 2004 to 2019, and also traces a partial q-curve-like feature in the hardness-intensity diagram, hinting at possible spectral transitions.