Optical Spectroscopy of Swift J0732.5-1331

ATel #763; M. A. P. Torres, D. Steeghs, M. R. Garcia, J. E. McClintock, P. Berlind, P. Zhao, M. Calkins (CfA), P. G. Jonker (SRON/CfA), P. J. Callanan (UCC), T. Marsh, B. T. Gaensicke (Warwick), P. Rodriguez-Gil (IAC), S. Littlefair, V. Dhillon (Sheffield)
on 13 Mar 2006; 19:28 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

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In response to the detection of the X-ray source Swift J0732.5-1331 (ATEL #697, #735, #757, #760), we observed its optical counterpart with the Mt. Hopkins 1.5m telescope equipped with the FAST spectrograph. A total of 22 spectra covering 3480-7410 A were acquired between Jan 22 and Mar 1. A 3" slit provided 6 A FWHM resolution. In addition, on March 5 we acquired 2.3 hours of spectroscopy (14 spectra) with the 600 gpm grating and a 2 arcsec slit width (4490-4895 A; 2.4 A FWHM). Exposure times were typically 600s and both J0732.5 and the nearby field star at 1.8 arcsec separation (ATEL #760) were in the slit. However, we do not resolve them.

We extracted the spectra using standard IRAF tasks. In order to obtain a less contaminated optical spectrum of J0732.5, the apertures were edited for each spectrum obtained with the 300 gpm grating, centered and sized interactively in order to obtain a Halpha profile in pure emission and therefore maximize the contribution from the object. The resultant spectra show the Balmer lines in emission from Halpha (FWHM = 22 +/- 1 A) to at least Hgamma; HeII 4686 A (FWHM = 16 +/- 1 A); HeI 5876,6678 A and (maybe) the Bowen blend in emission. The line widths were measured from the average of the 22 spectra. The strength of HeII 4686 A to Hbeta is ~ 2.0, indicating the presence of a strong source of ionising photons, typical of either magnetic CVs or novalike variables. The absorption features are totally due to background contamination from the neighbouring star (we do not have the signal-to-noise to discern absorption lines in J0732.5 itself). Our analysis thus confirms that the strong absorption line features reported in ATEL #757 are due to the nearby field star, and that the spectrum of J0732.5 is consistent with that of a magnetic CV.

The HeII radial velocities suggest a variation with an amplitude of 130 - 200 km/s. For comparison, the velocity errors obtained from the scatter of the measured velocities in the absorption spectrum of the contaminating star are about 30 km/s and 4 km/s for the 300 gpm and 600 gpm grating respectively. We were not able to determine an orbital period from our current data.

The high-excitation emission lines reported here, together with the strong hard X-ray emission (ATEL #697) and stable optical pulsations in the J0732.5 light curve (ATEL #757,#760) support an intermediate polar nature for the source. We encourage further observations to establish the orbital period and other key characteristics.

FAST spectrum and ULTRACAM finding chart