Optical identification of the X-ray burster EXO 1745-248 in Terzan 5
Fig. 1: from left to right, the figure shows three HST images of the 2" x 2" region around the X-ray burster EXO 1745-248 in Terzan 5. The images have been obtained in the F814W filter with the ACS/WFC, in three different epochs: September 2003 (EP1), August 2013 (EP2), and April 2015 (EP3). The optical counterpart to the burster is highlighted with a red circle. In quiescence (EP1 and EP2) it is visible as a faint star, in the outburst state (EP3) it is ~3 magnitudes brighter. North is up, east is to the left.
The Astronomer's Telegram (ATel) text:
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)=17h 48m 05.23s, Dec(J2000)= -24o 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 in Ferraro et al. 2015, ApJL, 807, L1 (see also arXiv:1506.03219 for a PDF copy of the paper).
Fig.2: F814W HST-ACS/WFC image of the 5 "x 5" region around EXO 1745-248 acquired in EP3 (outburst state). The source positions and uncertainties obtained from the various observational campaigns are marked: the Swift/XRT 2.2" radius error circle is shown in black, the Chandra error circle in blue, the VLA measure in green, and the HST optical determination (0.2" radius) in red. The cyan square marks the star previously proposed Heinke et al. (2003, ApJ 590, 809) as the possible optical counterpart to EXO 1745-248.
From left to right, the figure shows three HST images of ~3" x 3" obtained in the filters F336W, F390W and F435W with the WFPC2, WFC3 and ACS, respectively. Each image was obtained in a different epoch as labelled in each panel. The red circle marks the candidate optical counterpart to IGR J1824-2452. The green and blue circles mark the position of the ATCA source (Pavan et al., ATel #4981) and of the Chandra X-ray source #23 (Becker et al., 2003, ApJ, 594, 798) possibly associated to IGR J18245-2452, with the circle radii corresponding to the astrometric uncertainties.
The optical counterpart to the transient IGRJ18245-2452
in the globular cluster M28
The Astronomer's Telegram (ATel) text:
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 are presented in Pallanca et al. 2013, ApJ, 773, 122 (see also arXiv:1306.6416 for a PDF copy of the paper).
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