SWIFT J1626.6-5156 is a Be/X-ray binary

ATel #3300; E. Nespoli (Valencian International University, Univ. of Valencia), P. Reig (FORTH, Univ. of Crete), J. Fabregat (Univ. of Valencia), R. E. Mennickent (Univ. of Concepcion)
on 21 Apr 2011; 12:46 UT
Credential Certification: Elisa Nespoli (elisa.nespoli@uv.es)

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We report results from NIR spectroscopy of the optical counterpart to the hard X-ray transient SWIFT J1626.6-5156 (ATel #678, #679). The proposed infrared counterpart (2MASS J16263652-5156305, ATel #688, #713, #739) was observed in visiting mode at the European Southern Observatory on 2010 July 23 with the SofI spectrograph on the 3.5m NTT in La Silla, Chile (ESO proposal 085.D-0297A). The sky had thin cirri, the seeing averaged between 0.9'' and 1.3'' and the target was observed at airmass 1.12. Data were taken in the Short Wavelength - Low Resolution mode, with a resolution of R=588, covering the 1.53-2.52 micron spectral range. The retrieved S/N averaged 100.

Ours is the first available IR spectrum of the proposed counterpart to the X-ray source.

The low-resolution K spectrum is almost featureless, except for presence of both HeI 20581 Å and HI 21660 Å (Brackett-gamma) in strong emission (EW = -9.9 and -8.8 Å, respectively). Both transitions are typical of Be-stars and confine the spectral type of the companion to be earlier than B2.5 (see Clark and Steele, 2000, A&AS, 141, 65). The H-band spectrum shows much more complexity, with the evidence of the HI Br-18-11 recombination series in emission. No FeII lines (1.534, 1.600 and 1.620 micron respectively) are observed. As for the K band, all the observed features are typical of Be stars, while they are absent, or observed in absorption in normal (non-emission) OB stars.

By comparison with spectral atlases (Clark and Steele, 2000, A&AS, 141, 65, Steele and Clark, 2001, 2001, A&A, 371, 643), the optical counterpart to SWIFT J1626.6-5156 can be classified as a B0-2Ve star, and the system is definitely a Be/X-ray binary.