A.J. Castro-Tirado (IAA-CSIC, ISA-UMA), D. Galad�-Enr�quez, F. Hoyos, A.
Guijarro (CAHA), R. S�nchez-Ram�rez, M. Fern�ndez, J. C. Tello, S. Jeong
(IAA-CSIC) and J. Ma�z-Apell�niz (CAB/CSIC-INTA), on behalf of a larger
Following the detection of a posible counterpart to the neutrino ANTARES
source by Swift (Dornic et al. GCNC 18231, ATel 7987), we conducted
optical monitoring and spectroscopic observations with the 2.2m (+BUSCA)
and 3.5m (+TWIN) telescopes at the German-Spanish Calar Alto Observatory
(Spain), under non-optimal conditions (airmass 3), starting on Sep 3 at
20:07 UT (2.52 days post ANTARES detection) of the R=12.6 star USNO-B1.0
0626-0501169 (probably a G/K-type star, Smartt et al. ATel 7992).
Marginal variability in the B-band is seen in the BUSCA data (6 x 150s
exposures). And the TWIN optical spectrum (2 x 300s) in the range
3850-4930 A and 5500-7320 A reveals H-alpha and H-beta in emission as well
as other absorption features (e.g. NaI D1+D2), confirming its association
to the X-ray source, in agreement with the findings reported by de Ugarte
Postigo et al. (GCNC 18236) and Mao et al. (GCN 18239). The non-detection
of HeI and the H-lines EWs < 5 A rules out a classical T Tauri star as the
counterpart but a weak-line T Tauri star (likely G-type) cannot be
excluded. If the X-ray emission is the tail a superflare arising from a
young stellar object such as a GRS 1100-771 detected by Granat/WATCH as a
fast (few hours long) X-ray transient (Castro-Tirado et al. 1999, A&A 347,
927), a similar episode might have been detected by space-borne
high-energy detectors in the past days.