THE OPTICAL COUNTERPART TO THE X-RAY TRANSIENT IGR J1824–24525 IN THE GLOBULAR CLUSTER M28.

We report on the identification of the optical counterpart to the recently detected INTEGRAL transient IGR J1824–24525 in the Galactic globular cluster M28. From analysis of a multi-epoch Hubble Space Telescope data set, we have identified a strongly variable star positionally coincident with the radio and Chandra X-ray sources associated with 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 Ha excess during the outburst state, suggestive of active accretion processes by the neutron star. [ABSTRACT FROM AUTHOR]