[vsnet-grb-info 4497] GRB 070411: ROTSE-III Detection of Optical Counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6269
SUBJECT: GRB 070411: ROTSE-III Detection of Optical Counterpart
DATE:    07/04/11 21:21:03 GMT
FROM:    Eli Rykoff at U of Michigan/ROTSE  <erykoff at umich.edu>

E.S. Rykoff (U Mich), W. Rujopakarn (Seward), R. Quimby (U Texas),
B. E. Schaefer (Louisiana State), H. Swan (U Mich), S.A. Yost (U
Mich), report on behalf of the ROTSE collaboration:

ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia,
responded to GRB 070411 (Swift trigger 275087, GCN 6267, Moretti, et
al.).  The first image was at 20:15:25.1 UT, 172 s after the burst;
the response was delayed due to a GCN connection problem.  We took 10
5s exposures followed by 50+ 20s exposures.  The unfiltered images are
calibrated relative to USNO B1.0, and no correction has been made for
the large Galactic extinction in the direction of the burst.

We confirm the afterglow candidate reported by Moretti, et al. (GCN
6267) and Kann, et al. (GCN 6268).  The afterglow was marginally
detected in the first few coadded images (stacking 10 images at a
time).  We see the afterglow brighten significantly during our
observations, to a peak of 17.7+/-0.1 mag, around 900s after the
burst; due to the faint afterglow, our time resolution is very coarse.

start UT     exptime  tstart-tburst (s)   mag
----------------------------------------------------------
20:15:25.2   50.0       172             18.2+/-0.4
20:21:53.7   200.0      560             18.5+/-0.4
20:26:50.0   200.0      856             17.7+/-0.1
20:31:45.8   200.0      1152            18.6+/-0.4


A jpeg image is available at
http://www.rotse.net/images/gsb275087_3c031-040_key.jpg

Continuing observations are in progress.