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.Comment: Accepted for publication by ApJ; 15 pages, 4 figures, 1 tabl
