Optical and X-ray Follow-Up to a Globular Cluster Ultraluminous X-ray Source in NGC 4472

TL;DR

This study presents multi-wavelength follow-up observations of a globular cluster ultraluminous X-ray source in NGC 4472, revealing no optical emission lines and low X-ray activity, with similarities to Galactic black hole binaries.

Contribution

It provides the first detailed optical and X-ray follow-up of a specific ULX in a globular cluster, offering insights into its nature and behavior compared to known black hole systems.

Findings

No significant [OIII] optical emission detected.

X-ray luminosity remained below a few 10^38 erg/s from 2019-2021.

GCU1 shows variability and properties similar to GRS 1915+105.

Abstract

NGC 4472 is home to five ultraluminous X-ray sources hosted by globular clusters. These sources have been suggested as good black hole candidates in extragalactic globular clusters$-$ a highly sought after population that may provide observational information regarding the progenitors of merging black hole binaries. In this body of work, we present X-ray and optical follow up to one of these sources, CXOUJ1229410+075744 (GCU1). We find no evidence of [OIII] optical emission in GCU1 which indicates a lack of significant evidence for super-Eddington outflows, unlike what is seen in a handful of ULXs in extragalactic GCs. X-ray monitoring from 2019-2021 shows no detected X-ray emission above a few $\times$ $10^{38}$ erg/s. Comparisons of the multi-wavelength properties to disc-dominated, near Eddington Galactic black hole low mass X-ray binaries (GRS 1915+105 and XTEJ1817-330) suggests that GCU1 may show similar behaviour to GRS 1915+105 in terms of X-ray variability and similar relationships between $L_X$ and $kT$, with GCU1 showing maximum X-ray luminosities one order of magnitude higher.