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A 3.2 hr candidate orbital period for SWIFT J1753.5-0127

ATel #1130; C. Zurita (Instituto de Astronomia, UNAM), M. A.P. Torres (CfA), M. Durant, T. Shahbaz, H. H. Peralta, J. Casares (IAC), D. Steeghs (CfA/Warwick)
on 6 Jul 2007; 15:09 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

Subjects: Optical, Binary, Black Hole, Transient

We report time-resolved photometry of the optical counterpart to the black hole candidate SWIFT J1753.5-0127 (Palmer et al. 2005, ATel #546: Halpern et al. 2005, ATel #549). After its discovery, this source has remained in the low/hard state and bright at optical/IR wavelengths. Our analysis indicates that this X-ray transient is a short orbital period X-ray binary.

We obtained time-series photometry on 2007 Jun 3, 4, 5, 7 and 8 UT with the 1.5-m telescope and from 2007 Jun 27 to Jul 1 UT with the 0.84-m telescope, both at the Mexican Observatorio de San Pedro Martir. The data consist of R-band photometry with a 80s and 110s time resolution respectively and were acquired during ~6 hr per night. Time-resolved photometry was also acquired on the nights of 2007 July 12, 13, 14, 18, 20 and 21 on the IAC 80cm Telescope, Tenerife, Spain. All exposures were 60s in the R band, about 7 hours per night. Observing conditions were mostly good and differential photometry with respect to calibrated field stars was performed.

The light curves are not sinusoidal, but exhibit a sawtooth shape with a ~0.15 mag peak-to-peak amplitude. The deepest peak in a phase dispersion minimization periodogram is found at 3.24 +/- 0.02 hr. A similar result (3.246 +/- 0.018 hr) is obtained when calculating a Lomb periodogram.

Given the low mass ratio expected for this system, this photometric period is likely to be a superhump period, slightly larger than the orbital period. Therefore SWIFT J1753-0127 is likely the black hole X-ray binary with the shortest orbital period observed to date.