TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2544
SUBJECT: GRB 031203, possible supernova
DATE:    04/03/15 19:37:49 GMT
FROM:    David Bersier at STScI  <bersier@stsci.edu>

D. Bersier, J. Rhoads, A. Fruchter, J. M. Castro Cerón,
L.-G. Strolger, S. Malhotra (STScI), J. Gorosabel (IAA-CSIC), A. Levan
(U. of Leicester), C. Kouveliotou, S. Patel (MSFC/NASA), M. Merrill
(NOAO), E. Gawiser, M. F. Duran, V. Gonzalez, (U. de Chile)
report:

Using the CTIO 4m telescope with MOSAIC2, we have obtained late-time
I-band imaging data of the field of GRB 031203 (Gotz et al, GCN 2459)
19, 25, and 77 days after the burst.

The afterglow found in X-ray (Schartel & Calderon, GCN 2464; Tedds et
al GCN 2490; Fox et al GCN 2522), radio (Frail, GCN 2473; Soderberg et
al GCN 2483) and IR (Tagliaferri et al GCN 2476) coincides with a
galaxy at a redshift z=0.105 (Prochaska et al GCN 2482). The
brightness of the galaxy at day 25 is I=19.25 (from a preliminary
calibration).  This includes any contribution from a transient source
(afterglow and/or supernova).  Matched-psf image subtraction (Alard,
2000, A&AS, 144, 363) reveals a fading source at this position,
between days 25 and 77, whereas there is no variation between days 19
and 25.  Photometry via psf-fitting confirms this.  The change in
magnitude of the "galaxy+variable source" between days 25 and 77 is
0.26 mag.  Given the faintness of the afterglow at early times, and
the likelihood that the afterglow will have faded significantly at
the times of our measurements, we tentatively interpret this variable
source as a supernova.

The observed decay sets a lower limit on the magnitude at day 25 (near
I-band maximum in rest frame).  The true brightness of the SN must be
greater than the flux difference between the two images, which
corresponds to I=20.94 +/- 0.08.  Assuming an extinction of A_I=1.76
and a redshift z=0.105, this corresponds to a luminosity of -19.25,
which is very close to the maximum brightness of SN1998bw. If we were
to assume that the light curve of this object behaves as SN1998bw,
then the difference between these two times would underestimate the
true luminosity by 35%.