[vsnet-grb-info 16955] ANTARES neutrino detection: Optical/NIR spectroscopy of the Swift/XRT counterpart candidate from NOT

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18236
SUBJECT: ANTARES neutrino detection: Optical/NIR spectroscopy of the Swift/XRT counterpart candidate from NOT
DATE:    15/09/04 12:50:00 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte at iaa.es>

A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), H. Korhonen (U. Turku, NBI), 
M.I. Andersen (DARK/NBI), J.P.U. Fynbo (DARK/NBI), S. Schulze (PUC, 
MAS), Z. Cano (U. Iceland), D. Xu (NAOC/CAS), N.R. Tanvir (U. Leicester), 
D. Watson (DARK/NBI), D. Malesani (DARK/NBI), J. Hjorth (DARK/NBI), 
A.A. Djupvik (NOT) report on behalf of a larger collaboration:

We observed the star USNO-B1.0 0626-0501169, associated with the X-ray 
emission detected by Swift, within the error box of the high-energy neutrino 
detected by ANTARES (Dornic et al. 18231, ATel 7987, Smartt et al. ATel 7992), 
with the 2.5 m NOT telescope at La Palma (Spain). Our observation consisted 
of 3x400 s spectra with the FIES high-resolution spectrograph in the visible, 
and 4x120 s spectra with NOTCam in the near-infrared K-band.

The FIES spectra were obtained at a resolution of R = 25000 and covered the 
range between 3700 and 7100 AA, although good S/N is only obtained above 
4500 AA. The mean observing epoch was the 3rd September 2015 at 21:45 UT 
(2.59 days after the neutrino detection), and observation was performed at an 
airmass of 3. Reduction of the NIR data is still ongoing.

The visible spectrum shows a red continuum with multiple absorption features 
as well as hydrogen Balmer lines in emission. Emission lines of H-alpha and 
H-beta are resolved, having a FWHM of 75 km/s, and equivalent widths of -1.9 
and -1.0 AA, respectively. We detect no emission from HeI or [SII]. The spectrum 
shows broad and strong absorption due to NaID with a combined equivalent 
width of 7.1 AA. Detection of a strong absorption at 6708 AA seems to indicate 
the presence of Li in the object. There is no convincing evidence of molecular 
bands.

All this points to USNO-B1.0 0626-0501169 being a young accreting G-K star, 
undergoing a flaring episode that produced the X-ray emission. We also note 
that this object is close to the nearby Rho Ophiuchi star forming region, being 
probably associated with it.