Magellan optical spectroscopy of Swift J1357.2-0933

ATel #3143; M. A.P. Torres (SRON/CfA), D. Steeghs (Warwick/CfA), P. G. Jonker (SRON/CfA/RU), M. Rauch (Carnegie)
on 3 Feb 2011; 12:21 UT
Credential Certification: P.G. Jonker (pjonker@cfa.harvard.edu)

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A 10-min spectrum of the optical counterpart to the new X-ray transient Swift J1357.2-0933 (Krimm et al. ATel #3138) was acquired with the MagE echellette spectrograph on the Magellan-Clay telescope at Las Campanas Observatory on 2011 Feb 02 09:23 UT. A 1 arcsec slit provided a spectral resolution of R~4100 in the range 3200-9200 Angstroms. Apart from the blue continuum, no Balmer lines (in either emission or absorption) are detected, nor was there evidence for HeII 4686 A or Bowen blend emission. We estimate the upper limit on the EW of a spectral feature to be < 0.1 A when considering the S/N level in the continuum. We clearly detect the CaII H&K and the NaD interstellar lines in absorption. The EW of the NaD 5890, 5896 A component were measured to be ~0.3 A and ~0.2 A respectively. No diffuse interstellar bands are obvious in the spectrum, as expected from the low reddening towards the source (see ATel #3142). Outburst spectra lacking emission lines have been reported before in a few X-ray transients (e.g. Swift J1753.5-0127, Jonker et al. arXiv:0806.388; Swift J1539.2-6227, Torres et al. ATel #1958). We note that the outburst amplitude, quiescent source properties and relative hard X-ray spectrum raise the possibility of a Galactic nova origin as an alternative scenario to the transient X-ray binary scenario. If the source is a transient X-ray binary, however, our non-detection of emission features in Swift J1357.2-0933 together with the intrinsic faintness of the source in quiescence and the large outburst amplitude in the optical/NIR bands (see Rau, Greiner & Filgas 2011, ATel #3140) suggest a short orbital period low-mass X-ray binary with a low-mass main sequence or degenerate donor star.