[vsnet-grb-info 10118] The Christmas burst GRB 101225A - optical spectroscopy with GTC

[GCN Circulars]

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  <deugarte at iaa.es>

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.

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.