Optical Spectroscopy of Swift J1539.2-6227 in Outburst

ATel #1958; M. A.P. Torres (CfA), P. G. Jonker (SRON/CfA), D. Steeghs (Warwick/CfA), H.-W. Chen (Univ. of Chicago), S. A. Shectman (OCIW), D. J.E. Floyd (LCO)
on 10 Mar 2009; 16:45 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

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A single 750 seconds spectrum of the optical counterpart to the X-ray transient Swift J1539.2-6227 (Krimm et al. ATel #1855, #1893) was acquired with the MagE echellette spectrograph on the Magellan-Clay telescope at Las Campanas Observatory on 2009 Feb 23 09:28 UT. A 1'' slit provided a spectral resolution of R~4100 in the range 3100-10000 Angstroms. Previous g'-band imaging obtained on Feb 10 with IMACS at the Magellan-Baade telescope showed that the optical counterpart continues to be bright since its discovery (ATel #1893). These IMACS observations are pending of an absolute photometric calibration. For reference, the counterpart is 0.89 +/- 0.01 mag brighter in the g'-band than the field star USNO-B1.0 0275-0755633 (R1=R2=17.63,B2=18.84).

Besides a blue continuum, no Balmer lines (in emission/absorption) were detected, nor was there evidence for HeII 4686 A or Bowen blend emission. We estimate that upper limits for the EW of any spectral features are < 0.2 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 components were measured to be 1.08 +/- 0.08 A and 0.93 +/- 0.07 A respectively. The diffuse interstellar band at 5780 A is also detected with an EW of 0.28 +/- 0.04 A in line with the expected reddening towards the source (E(B-V)=0.33; Schlegel et al. 1998,ApJ,500,525). The uncertainties in the EWs were estimated by looking at the scatter in the values when selecting different wavelength intervals to reflect the local continuum level.

The non-detection of emission lines found often in optical spectra of X-ray transients is rare but not unprecedented (e.g. Swift J1753.5-0127, Jonker et al. arXiv:0806.3888). Given the lack of any emission features in the outburst spectrum combined with the faintness of the source in quiescence (non detected in the DSS and 2MASS catalogues; ATel #1893) suggests a low-mass main sequence or degenerate donor star companion to the compact accretor.