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GCN Circular 2728

Subject
GRB040912(=H3557): Candidate Optical Afterglow
Date
2004-09-20T17:09:10Z (19 years ago)
From
Nat Butler at MIT/CSR <nrbutler@space.mit.edu>
GRB040912(=H3557): Candidate Optical Afterglow

N. Butler, G. Ricker, R. Vanderspek, J. Villasenor, P. Csatorday,
P. Schechter, S. Burles (MIT); S. Malhotra (STSCI), and D. Osip (LCO),
on behalf of a larger collaboration report:

We observed the HETE WXM error region from Butler et al. (GCN 2701)
in two epochs with the 27'x27' FOV IMACS camera on the Magellan 6.5m 
Baade Telescope at Las Campanas Observatory.  Epoch1 consisted of
3x180s exposures in R band, with a mean observing time of Sep 13.15716 UT 
(t_burst + 13.57 hours).  Epoch2 consisted of 2x300s exposures in R
band, with a mean observing time of Sep 14.20236 UT (t_burst + 38.65 hours).
The 3-sigma limiting magnitudes are R=25.3 and R=25.6 for Epoch1 and 
Epoch2, respectively. 

We calibrate our astrometry and photometry against the USNO stars of
Henden et al. (GCN 2720).  We detect sources within 1" of the Chandra
positions for 8 of the 22 Chandra sources reported in Butler et al. 
(GCN 2716):

CX0#	R-magE1			R-magE2
1	.			.
2	23.95+/-0.07		24.10+/-0.07
3	.			.
4	24.29+/-0.01		24.02+/-0.08
5	.			.
6	.			.
7	22.55+/-0.04		22.53+/-0.04
8	.			.
9	.			24.4+/-0.1
10	.			.
11	.			.
12	25.00+/-0.13		24.67+/-0.09
13	22.25+/-0.04		23.27+/-0.05
14	.			.
15	.			.
16	.			.
17	.			.
18	22.38+/-0.04		22.31+/-0.03
19	.			.
20	.			.
21	25.12+/-0.14		24.56+/-0.10
22	.			.

Of the optical sources we detect, the source corresponding to 
CXO#13 has declined in brightness the most (by 1.02 pm 0.09 mag),
and we tentatively suggest this source to be the optical afterglow
of GRB040912.  The flux decay between our 2 epochs corresponds to 
a fade with temporal index alpha=-0.9+/-0.1.

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