[vsnet-grb-info 4586] GRB 070420, SMARTS optical/IR afterglow observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6347
SUBJECT: GRB 070420, SMARTS optical/IR afterglow observations
DATE:    07/04/23 20:40:38 GMT
FROM:    Bethany Cobb at Yale U  <cobb at astro.yale.edu>

B. E. Cobb (Yale), part of the larger SMARTS consortium, reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 070420
(GCN 6330, Stamatikos et al.) with a mid-exposure time of
2007-04-21 01:15 UT (~18.9 hrs post-burst) and again
at 2007-04-23 00:52 UT (~66.6 hrs post-burst).  For each
set of observations, total summed exposure times amounted
to 36 minutes in I and 30 minutes in J.

The afterglow of GRB 070420 (GRB 6330, Stamatikos et al.) is detected
in the first epoch images and is observed to fade between the
first and second epochs by approximately 1 magnitude.  This indicates a 
decay rate of alpha ~ 0.7 (afterglow flux proportional to t^-alpha),
which is consistent with early-time UVOT observations (GCN 6336, Immler et al.).

We note that there is a source with constant brightness
only 1 arcseconds West and 0.7 arcseconds North of the afterglow of GRB 
070420. This source has magnitudes I~20 and J~18.5 (magnitudes are 
calibrated using USNO-B1.0 values in the optical and 2MASS stars in the IR.).
The source appears to have a stellar profile and is most likely 
just a coincident star.