Optical imaging of EXO 0748-676 in quiescence
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
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)
Subjects: Optical, Binary, Neutron Star
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).