TITLE:   GCN CIRCULAR
NUMBER:  13069
SUBJECT: GRB 111228A: possible detection of the SN with the TNG
DATE:    12/03/16 13:03:08 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <paolo.davanzo@brera.inaf.it>

P. D'Avanzo, A. Melandri (INAF-OAB), E. Palazzi (INAF-IASF Bo), S. Campana
(INAF-OAB), M. Della Valle (INAF-OAC), E. Pian (INAF-OATs), R. Salvaterra
(INAF-IASF Mi), G. Tagliaferri (INAF-OAB) report on behalf of the CIBO
collaboration:

We observed the optical counterpart of GRB 111228A (Ukwatta et al., GCN
12737) with the Italian 3.6m TNG telescope on 2012 Feb 1.16 UT (34.5 days
post burst) and March 13.91 UT (76.2 days post burst). Observations were
carried out in the R and I bands. The counterpart is well detected in the
two epochs in both filters, with a magnitude R = 24.3 in the March
observation (calibrated using Landolt standard stars). The late value is
likely dominated by the host galaxy.

Differential photometry reveals that the source faded between the two
epochs by 0.4 +/- 0.2 and 0.9 +/- 0.1 mag in the R and I band,
respectively. The observed fading indicates that a transient component was
present during our February observation. This could be due to residual
afterglow emission, or to the presence of an emerging supernova (SN).

After subtracting the host galaxy flux, the color of the transient on
February 1 is very red, with
R-I ~ 2. This color is not typical of GRB afterglows, and is much redder
than measured at early times for this object (e.g.,  r'-i' = 0.17 AB at t
= 0.57 days: Nicuesa Guelbenzu et al., GCN 12757). Such a red spectrum, on
the contrary, is consistent with that of a type-Ic SN at z=0.72 (e.g.,
Dittman et al., GCN 12759), since the observed R band corresponds to the
rest-frame U, where severe line blanketing suppresses the SN flux.  Also,
the I-band magnitude of the transient is comparable to that of SN 1998bw
close to the peak of luminosity, placed at z=0.72.

Although a more accurate analysis of the afterglow behaviour is necessary,
based on the above arguments, we believe to have detected the emission
from the SN associated with GRB 111228A.


We acknowledge the TNG staff for their support, in particular Luca Di
Fabrizio and Daniele Carosati.