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ATEL # 2870; M. A. P. Torres (SRON/CfA), D. Steeghs (Warwick/CfA), P. G. Jonker (SRON/CfA/RU), N. R. Ross (UCLA)
on 24 Sep 2010; 21:23 UT
Password Certification: Manuel Torres (firstname.lastname@example.org)
Subjects: Optical, Binaries, Black Holes, Neutron Stars, Transients
Referred to by ATEL #: 2991, 2997, 3011
Following the recently announced subarcsec Chandra localization for XTE J1728-295 (Chakrabarty et al. ATel #2869), we report on optical observations of the field containing this currently active X-ray transient (Atels #2823, #2824, #2825).
Our observations consist of several 60s and 180s i'-band images acquired during 2010 Sep 8 23:55 - Sep 9 00:23 UT using the MagIC camera mounted at the 6.5m Magellan Clay telescope. The data were obtained with an image quality of ~0.4" and a 0.069"/pix plate scale. Conditions were not photometric. An astrometric calibration of the frames was defined using 2MASS stars in the field of view. This delivered a positional accuracy < 0.1'.
From our images we measure a position of R.A.(J2000) = 17:28:38.86, Dec.(J2000) = -29:21:44.0 for the candidate optical counterpart suggested in Russell et al. 2010, (ATel #2827). This position is 0.5" away from the Chandra localization, but consistent within the errors. Differential photometry of this optical source with respect to nearby stars indicates no evidence for variability (< 0.01 mag) over the 28 min-long observation interval. Further visual inspection of the data and PSF fitting reveal at least two other point-like sources consistent with the 0.6' Chandra position and > 3 mag fainter than the proposed candidate.
Given the failure to detect variability(see also ATel #2827), we encourage continued photometric observations and/or the acquisition of a spectrum to confirm/find the candidate optical counterpart.