Periodic Optical Signal(s) from Swift J0732.5-1331
ATel #757; J. Patterson, J. Halpern (Columbia U.), N. Mirabal (U. Michigan), Grant Christie (CBA-Auckland), Jennie McCormick (Farm Cove Observatory), Robert Rea (CBA-Nelson), David Messier (CBA-Connecticut)
on 3 Mar 2006; 00:37 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)
Subjects: Optical, X-ray, Binary, Cataclysmic Variable
On 2006 February 17, we obtained low-resolution spectra
on the MDM 2.4m telescope of the proposed optical counterpart
of the hard X-ray source Swift J0732.5-1331
(Ajello et al., ATel #697).
The optical position is 07h 32m 37.64s, -13° 31' 09.0" (J2000.0).
The spectrum looks essentially like that of a normal G star, except for very
weak He II 4686 emission that is variable
on a time scale of 1 hour. This differs from the description of the spectrum
obtained by Masetti et al. (ATel #735) just a few days earlier
(H, He I, and He II emission lines, featureless blue continuum).
During February 15-28, time-series photometry was obtained using the
small-telescope network of the Center for Backyard Astrophysics (CBA).
Nightly light curves were nearly flat, but power-spectrum analysis showed
a stable pulsation with a period of 512.42(3) seconds, with most of the
power in the first harmonic. This fast, stable period, in conjunction with
hard X-rays and Lx/Lopt ~ 1, is the defining signature
of the intermediate polars (DQ Herculis stars).
While the pulse period is not in doubt,
we continue to puzzle over the orbital period of this (probable) cataclysmic
variable. Its 4000-7000 Ã
light is evidently dominated by the secondary,
so periodic tidal distortion of a Roche-lobe-filling secondary should be
quite easy to see in the light curve. But the upper limit on any signal
near a plausible orbital frequency is severe (0.024 mag semiamplitude),
and this casts some doubt on our interpretation. A weak candidate signal
at 11.3 hours could signify the orbital period, but it requires that the
binary be close to face-on (i < 15°).
Finding charts, Optical spectra, Power spectra, Light curves
