ANTARES neutrino detection: optical spectroscopy of X-ray counterpart candidate with SALT

ATel #7993; S. Dichiara (U. Ferrara), C. Koen (U. Cape Town), T. Koen (SALT), M. Kotze (SAAO), D. Milisavljevic (SAO), R. Margutti (NYU), C. Guidorzi (U. Ferrara)
on 4 Sep 2015; 15:10 UT
Credential Certification: Raffaella Margutti (rmargutti@cfa.harvard.edu)

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We carried out spectroscopic and photometric observations of the star USNO-B1.0 0626-0501169, associated with the X-ray emission detected by Swift-XRT within the error circle of the high-energy neutrino detected by ANTARES (Dornic et al. GCN 18231, ATel 7987, de Ugarte Postigo et al. GCN 18236, Smartt et al. ATel 7992). Two spectra started September 03, 18:42 and 18:46 UT covering the wavelength ranges 3500-6400 and 6000-8800 AA, 120 and 90s exposure, obtained with the Southern African Large Telescope (SALT) under program 2015-1-DDT-005 showed the steeply increasing continuum typical of a late K/early M star. Strong Halpha, and weaker Hbeta and Calcium emission suggest chromospheric activity.

Started September 03, 17:10 UT exposure times 60-90 s (B), 40-60 s (V) and 15-25 s for about 2.5 hours of BVR photometry with the South African Astronomical Observatory 1-m telescope showed no overt signs of variability. A one minute exposure in U of a 5X arcmin^2 field centred on the star did not reveal any strikingly bright objects. Analysis of All Sky Automated Survey (ASAS) photometry of the star, spanning ~8 years, shows marginal evidence for a 6.5 hr (or its alias of 8.9 hr) periodicity with a 3% amplitude. This would be consistent with a rapidly rotating late type star.

We conclude that the presence of this active star within the error circle of the energetic neutrino is just coincidental, in agreement with previous reports (Smartt et al. ATel 7992, de Ugarte Postigo et al. GCN 18236).