Swift J1753.5−0127: The Black Hole Candidate with the Shortest Orbital Period

Abstract

We present time-resolved photometry of the optical counterpart to theblack hole candidate Swift J1753.5-0127 which has remained in thelow/hard X-ray state and bright at optical/IR wavelengths since itsdiscovery in 2005. At the time of our observations Swift J1753.5-0127does not show a decay trend but remains stable at R=16.45 with anight-to-night variability of ~0.05 mag. The R-band light curves, takenfrom 2007 June 3 to August 31, are not sinusoidal, but exhibit a complexmorphology with remarkable changes in shape and amplitude. The bestperiod determination is 3.2443+/-0.0010 hr. This photometric period islikely a superhump period, slightly larger than the orbital period.Therefore, Swift J1753.5-0127 is the black hole candidate with theshortest orbital period observed to date. Our estimation of the distanceis comparable to values previously published and likely places SwiftJ1753.5-0127 in the Galactic halo.