Orbital Period of Swift J0732.5-1331
ATel #767; J. R. Thorstensen (Dartmouth), J. Patterson, J. Halpern (Columbia U.), N. Mirabal (U. Michigan)
on 19 Mar 2006; 13:58 UT
Credential Certification: J. R. Thorstensen (john.thorstensen@dartmouth.edu)
Subjects: Optical, X-ray, Binary, Cataclysmic Variable, Variables
On the nights of 2006 March 17, 18, and 19 UT we obtained
time-series spectroscopy of the optical counterpart of this
hard X-ray source (Ajello et al., ATEL #697), using the
Hiltner 2.4m telescope at MDM Observatory and the Modular
Spectrograph, covering from 4300 to 7500 Angstroms at
3.6 A resolution. The slit was oriented at a position angle
of 43 degrees to minimize contamination from the nearby
crowding star (Marsh et al., ATEL #760), but the stars were
not cleanly resolved. In total, 45 exposures of 8 minutes
each were obtained. The observations were arranged to
span the greatest possible range in hour angle, on one
occasion reaching 4.3 airmasses.
All the spectra show substantial H-alpha emission,
which varied substantially in radial velocity,
consistent with the findings of Torres et al. (ATEL #763).
The variations are periodic; a least-squares sine fit gives 0.2335(8) d
and a semiamplitude K = 150(7) km/s, with an rms scatter of 17 km/s.
The period choice is unambiguous; the best alternative period
at 0.1895 d (corresponding to a different choice of the daily
cycle count) gives a significantly worse fit, and a
Monte Carlo simulation indicates that it has a negligible
likelihood of being the true period. The lack of ambiguity
is due to the relatively strong, clean velocity modulation and
the wide range of hour angles observed.
The radial velocity modulation almost certainly indicates the
orbital period of this object. Other observers have noted
a much shorter-period (512.42 s) modulation in the optical
(Patterson et al., ATEL #757) and in the
X-ray (Wheatley et al., ATEL #765), leading to the
classification of this object as an "intermediate polar",
or DQ-Her type cataclysmic binary. The orbital period
found here is similar to other members of this class and
supports this interpretation.
