The optical counterpart to the X-ray transient IGR J18245-2452 in the globular cluster M28

TL;DR

This study identifies the optical counterpart to the X-ray transient IGR J18245-2452 in globular cluster M28, revealing its variable nature and signs of active accretion during outburst states.

Contribution

First identification of the optical counterpart to IGR J18245-2452 in M28 using multi-epoch HST data, showing variability and accretion signatures.

Findings

Optical counterpart is a variable star coincident with X-ray and radio sources.

Star is faint during quiescence and brighter with bluer color during outburst.

Halpha excess indicates active accretion during outburst.

Abstract

We report on the identification of the optical counterpart to the recently detected INTEGRAL transient IGR J18245-2452 in the Galactic globular cluster M28. From the analysis of a multi epoch HST dataset we have identified a strongly variable star positionally coincident with the radio and Chandra X-ray sources associated to the INTEGRAL transient. The star has been detected during both a quiescent and an outburst state. In the former case it appears as a faint, unperturbed main sequence star, while in the latter state it is about two magnitudes brighter and slightly bluer than main sequence stars. We also detected Halpha excess during the outburst state, suggestive of active accretion processes by the neutron star.