Columbia University Astronomy and Astrophysics
CBA Center for Backyard Astrophysics


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 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 Å 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

(J2000) 07h 32m 37.64s -13° 31' 09.0" (6'x6')
POSS II B=14.56 POSS II R=13.82
POSS II R POSS II R

Swift UV-Optical Telescope (UVOT) on 2006 Jan. 6
UVW2 UVM2
UVW1 U
B V

Optical Spectra

MDM 2.4m
Figure 1: 30-minute exposure under cloudy conditions. Not flux calibrated.

MDM 2.4m
Figure 2: 30-minute exposure. Not flux calibrated.

MDM 2.4m
Figure 3: 30-minute exposure. Not flux calibrated.

Power Spectra, Light Curves

CBA Power Spectrum
Figure 4: Mean power spectrum of 11 nights of photometry, with peaks labeled with their frequency in cycles/day (±0.4). Inset: The mean light curve, which shows the dominance of the first harmonic.

CBA Power Spectrum
Figure 5: Power spectrum of the entire light curve. A candidate signal at 4.23 cycles/day appears, but is too weak (0.02 mag) to command confidence. In particular, uncorrected differential extinction of about this amount could be present. An orbital interpretation would most plausibly identify it as twice the orbital frequency; and indeed, a small peak at 2.11 cycles/day leads us to consider that as a candidate orbital frequency. Inset: The mean light curve at the candidate orbital frequency.

References

SWIFT/BAT Detections of Hard X-ray Sources: IV, M. Ajello, et al., ATel #697 (2006)

Optical Classification of Five Swift Hard X-ray Sources, N. Masetti, L. Bassani, A. J. Dean, P. Ubertini, R. Walter, ATel #735 (2006)

Periodic Optical Signal(s) from Swift J0732.5-1331, J. Patterson, J. Halpern, N. Mirabal, Grant Christie, Jennie McCormick, Robert Rea, David Messier, ATel #757 (2006)

Swift CBA World Map

MDM Observatory


Last Modified: March 2, 2006 jules@astro.columbia.edu