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

ATel #5003; Pallanca C., Dalessandro E., Ferraro F. R., Lanzoni B. (Dipartimento di Fisica e Astronomia, Bologna, Italy) and Beccari G. (ESO, Garching bei Munchen, Germany)
on 23 Apr 2013; 17:29 UT
Credential Certification: Cristina Pallanca (cristina.pallanca3@unibo.it)

Subjects: Optical, Globular Cluster, Neutron Star, Transient

Referred to by ATel #: 5031, 5069, 5086

Here we report on the possible identification of the optical counterpart to the INTEGRAL ISGRI transient source IGR J18245-2452, recently detected by Eckert et al. (ATel #4925) in the Galactic globular cluster M28. By using high-resolution HST images obtained in three different epochs (covering roughly 1 year) we identified a Main-Sequence star showing unexpectedly large luminosity variations (Delta mag~1.5-2 mag). The star coordinates are RA=18h 24m 32.496s and Dec=-24 52' 07.799''. It is located within 2.5 sigma from the SWIFT/XRT source positions, as reported by Heinke et al. (Atel #4927) and Romano et al. (ATel #4929). In addition its position is in very good agreement (within ~0.2'') with the location of the ATCA variable radio source possibly associated to IGR J18245-2452, (Pavan et al. ATel #4981), as well as with the Chandra X-ray source #23 of Becker et al. (2003; ApJ, 594, 798). In the linked Figure we show three HST images obtained in different epochs, with the candidate optical counterpart highlighted with a red circle. In two of them (2009 Apr 07, and 2010 Apr 26) the star was in a quiescent state (F435W=23.23+-0.03), while in the middle epoch (2009 Aug 08) the source turned out to be about two magnitudes brighter (F390W=20.61+-0.01). Such a large luminosity variation has been also detected in the Halpha images acquired in two different epochs (2009 Aug 08 and 2010 Apr 26). In addition, we found an indication of variability during each of the observation time-spans (of the order of a few hours). The full analysis of the images and the detailed discussion of the results will be presented in an forthcoming paper (Pallanca et al., in preparation).

HST images of the optical counterpart