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ANTARES neutrino detection: CAHA photometry & spectroscopy of the Swift source

ATel #7998; A. J. Castro-Tirado (IAA-CSIC, ISA-UMA), D. Galad\'i-Enr\'iquez, F. Hoyos, A. Guijarro (CAHA), R. S\'anchez-Ram\'irez, M. Fern\'andez, J. C. Tello, S. Jeong (IAA-CSIC) and J. Ma\'iz-Apell\'aniz (CAB/CSIC-INTA)
on 4 Sep 2015; 23:41 UT
Credential Certification: Alberto J. Castro-Tirado (ajct@iaa.es)

Subjects: Optical, X-ray, Neutrinos, Transient, Young Stellar Object

Referred to by ATel #: 8002, 8027, 8034, 8124

Following the detection of a possible 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), Dichiara et al. (ATel #7993) and Mao et al. (GCNC 18329). 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 cannot be excluded yet. 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) 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.