[vsnet-alert 9396] SWIFT J1753.5-0127 apparently in superhumping state

[Taichi Kato]

   The object very much resembles KV UMa (= XTE J1118+480).
(see ApJ 659, 549 or astro-ph/0612575), mag about 16.
position: 17h53m28.3s, -01d27m06s

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The Astronomer's Telegram                   http://www.astronomerstelegram.org 
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Posted: Thu Jul  5 01:30:00 EDT 2007 -- Sat Jul  7 01:30:01 EDT 2007
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ATEL #1130                                                           ATEL #1130

Title:          A 3.2 hr candidate orbital period for SWIFT J1753.5-0127
Author: 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)
Queries:        mtorres at cfa.harvard.edu
Posted: 6 Jul 2007;  15:09  UT
Subjects:       Optical, Binaries, Black Holes, Transients

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  
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.