TITLE: GCN CIRCULAR NUMBER: 11568 SUBJECT: The Christmas burst GRB 101225A - optical spectroscopy with GTC DATE: 11/01/14 17:35:27 GMT FROM: Antonio Deugarte at IAA-CSIC C. C. Thoene, S. Guizy, A. Castro-Tirado, J. Gorosabel (IAA-CSIC) and A. de Ugarte Postigo (DARK/NBI) report: We observed the optical counterpart of GRB 101225A (the Christmas burst, GCN 11493, Racusin et al.) with OSIRIS at the 10.4m GTC on Dec. 27, starting at 21:20 UT (51h after the trigger). We obtained 2 exposures of 1800s using grism R300B. We acknowledge the excellent support from the GTC staff. The combined and flux-calibrated spectrum shows a smooth, very blue continuum without any absorption of emission features, consistent with the spectral observations from WHT (GCN 11502, Wiersema et al.), MMT (GCN 11507, Chornock et al.) and Keck/LRIS (GCN 11563, Fynbo et al.) at different times after the trigger. The continuum is well fitted using a power law with F(nu) = nu^(0.9+/-0.5). Considering the very flat optical light curve, the apparent color changes in the optical SED, the shape of the X-ray spectrum (GCN 11501, Campana et al.) and the lack of a host galaxy (GCN 11564, Tanvir et al.), we think that this object might not be a GRB but rather a compact object from the MW or the local group. Further follow-up is encouraged to determine the nature of this peculiar object. [GCN OPS NOTE(14jan11): Per author's request, the last sentence in the first paragraph was added.]