GCN Circular 14374

B. E. Cobb (GWU), reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 130408A
(GCN 14361
Not found
 
 
, Lien et al.)  over several epochs (with mid-exposure
times of 2013-04-09 00:30 UT, 02:15 UT, 03:41 UT and 2013-04-11
02:24 UT). For each epoch, several dithered images were obtained with
total summed exposure times of 15 min in V and I and 12 min in J
and K.  For the final epoch, total exposure times were 36 min in I
and 30 min in J.

The fading afterglow of GRB 130408A (e.g. GCN 14362
Not found
 
 
, Melandri et al.;
GCN 14363
Not found
 
 
, Siegel et al.; GCN 14364
Not found
 
 
, Sudilovsky et al.) was detected
with the following magnitudes (or 3-sigma limits):

mid-exposure
time
(hours)           I mag             J mag            K mag
2.64611         18.8 +/- 0.1    17.8 +/- 0.1     > 16.1
4.38444         19.6 +/- 0.1    18.6 +/- 0.2    16.8 +/- 0.2
5.82472         20.0 +/- 0.1    18.6 +/- 0.2     > 17.2
52.53694        > 21.5            > 19.6            ...

(Optical photometry is calibrated against USNO-B1.0 stars
and IR photometry is calibrated against 2MASS stars in the field.)

Between about 2.6 hrs and 5.8 hrs post-burst, the GRB afterglow fades
with a decay rate of approximately alpha = 1.4 (where afterglow flux is
proportional to t^-alpha).