Swift J1745-26: Spectroscopy and imaging from GTC

ATel #4388; A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), R. Sanchez-Ramirez (IAA-CSIC), T. Munoz-Darias (U. Southampton), J. Gorosabel (IAA-CSIC), C. C. Thoene (IAA-CSIC), A. Cabrera-Lavers (GTC)
on 18 Sep 2012; 15:49 UT
Credential Certification: Antonio de Ugarte Postigo (adeugartepostigo@gmail.com)

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We observed the field of the new galactic transient Swift J1745-26 (Cummins et al., GCN#13774, Cummings et al. GCN#13775) with the 10.4m GTC telescope at La Palma observatory. Observations started with 10x60s exposures in i-band, obtained on 17 September 2012 between 20:20:33 and 2034:07 UT. We clearly detect the candidate reported by Rau et al. (ATel#4380) in the individual exposures. However, we note that this object is slightly outside the refined Swift/XRT error box (Sbarufatti, et al. ATel#4383, ATel#4385). Performing differential photometry we do not see any variations in its brightness greater than 0.05 magnitudes. We then performed spectroscopy of this source using the R1000B and R1000R gratings and a slit of 1", yielding a resolution of ~600 and covering the range between 3700-7000 and 5100-10000 Angstrom respectively. Observations consisted of 2x900s exposures with each grating, all at a high airmass. In the spectra we detect a broad H-alpha emission (~1000 km/s) with an equivalent width of ~15 A. There is a weak broad feature redwards of H-alpha that could correspond to HeI 6678. The profile of H-alpha is asymmetric and can be best fitted by a double gaussian with a separation of ~640 km/s. In the blue spectrum the trace is almost lost below 5000 A and we do not detect any significant features below this wavelength. The presence of these features in the spectrum is consistent with an X-ray binary nature.