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ATEL # 1817; M.A.P. Torres (CfA), P. G. Jonker (SRON/CfA), D. Steeghs (Warwick/CfA), A. C. Seth (CfA)
on 28 Oct 2008; 22:34 UT
Password Certification: Manuel Torres (firstname.lastname@example.org)
Subjects: Optical, Binaries, Neutron Stars
Following the reported decline of the X-ray flux from the eclipsing low-mass X-ray binary EXO 0748-676 (Wolff et al. 2008, ATel #1736), we observed its optical counterpart (UY Vol) on 2008 Oct 02 08:07-08:18 UT using the LDSS3 imaging spectrograph mounted on the 6.5m Magellan Clay telescope at Las Campanas Observatory. Single 5 min Sloan r'-band and i'-band images were acquired with seeing conditions of about 1.0". Absolute photometric calibration was obtained using observations of a field containing stars calibrated in SDSS. The astrometric scale was defined using USNO B1.0 sources in the field of UY Vol.
The images reveal a single source consistent within the errors with the position reported for UY Vol by Wade et al. (1985,PASP,97,1092) and the X-ray position as derived from Chandra observations obtained on 2003 Nov 12 with the HRC-S-LETG configuration (R.A = 07:48:33.73, Dec = -67:45:07.9, J2000, 90% confidence level uncertainty of 0.6").
Aperture photometry of the optical counterpat yields the following magnitudes: r' = 22.12 +/- 0.04 and i'= 21.67 +/- 0.03. The uncertainty in the absolute flux calibration is expected to be < 0.1 mag. The above r'-band magnitude is in line with the photometry reported by Hynes & Jones (2008, ATel #1816).
The above result indicates that EXO 0748-676 has decayed by ~ 4.5 magnitudes compared to the optical brightness during X-ray activity (e.g. r~17.5; Wade et al. 1985).