The Optical Counterpart to the X-Ray Transient IGR J1824-24525 in the Globular Cluster M28
Dalessandro, E.; Beccari, G.; Ferraro, F. R.; Lanzoni, B.; Pallanca, C.
Italy, Germany
Abstract
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 Hα excess during the outburst state, suggestive of active accretion processes by the neutron star.
Based on observations collected with the NASA/ESA Hubble Space Telescope (Prop. 19835), obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.