Optical identification of the X-ray burster EXO 1745-248 in Terzan 5
ATel #7470; F. R. Ferraro, C. Pallanca, B. Lanzoni (DIFA-Bologna), M. Cadelano (DIFA-Bologna, INAF-OABO), D. Massari (INAF-OABO, Kapteyn-Groningen), E. Dalessandro, A. Mucciarelli (DIFA-Bologna)
on 4 May 2015; 17:03 UT
Credential Certification: Cristina Pallanca (cristina.pallanca3@unibo.it)
Subjects: Optical, Binary, Globular Cluster, Neutron Star, Transient
We report on the identification of the optical counterpart to the neutron star transient EXO 1745-248 in the stellar system Terzan 5. The identification has been performed by exploiting a set of HST ACS/WFC images acquired in Director Discretionary Time on 2015 April 20 (MJD 57133), approximately 1 month after the Swift/BAT detection of an X-ray outburst from this source (Altamirano et al. 2015, ATEL #7240). By comparing these images with previous archive data, we identified a star currently appearing ~3 magnitudes brighter than in its quiescence state. In the linked Figure 1 we show three HST images in the F814W filter, obtained in different epochs (EP1: 2003 September 17 - MJD 52899, EP2: 2013 August 18 - MJD 56522, EP3: 2015 April 20), with the optical counterpart highlighted with a red circle. In the first two epochs the star was in quiescence (F814W=21.74 +/-0.03), while in the 2015 image the source is about three magnitudes brighter (F814W=18.88 +/-0.01). The star coordinates are RA(J2000)=17:48:05.23, Dec(J2000)= -24:46:47.6. As shown in the linked Figure 2, these coordinates are in full agreement (within 0.1'') with the VLA position of the burster quoted by Tremou et al. (2015, ATEL #7262). In quiescence the optical companion is a sub giant branch star (possibly belonging to the main metal poor sub-population harbored in Terzan 5). The full analysis of the optical source is presented in a forthcoming paper (Ferraro et al. 2015, submitted to ApJL).
This research is part of the project Cosmic-Lab (web site: http://www.cosmic-lab.eu ) funded by the European Research Council (under contract ERC-2010-AdG-267675).
HST images of the optical counterpart
