TITLE:   GCN CIRCULAR
NUMBER:  6824
SUBJECT: GRB 070920B: ROTSE-III Early Optical Limits
DATE:    07/09/25 19:11:20 GMT
FROM:    Heather Swan at U.of Michigan/ROTSE  <hflewell@umich.edu>

H. Swan (U Mich), E.S. Rykoff (UCSB), F. Yuan (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 070920B (Swift trigger 291728; J. L. Racusin, GCN
6808), producing images beginning 8.5 s after the GCN notice time. An
automated response took the first image at 21:05:16.5 UT, 44.1 s after
the burst, under partly cloudy conditions. We took 10 5-sec, 10 20-sec
and 386 60-sec exposures. These unfiltered images are calibrated
relative to USNO A2.0 (R).  We lost internet contact with ROTSE-IIIc
shortly after the GCN trigger, thus we were unable to retrieve the burst
data until Sept. 25.

Comparison to the DSS (second epoch) reveals no new sources within the
3-sigma Swift/BAT error circle or the XRT error circle , (J. L. Racusin,
GCN 6820) for both single images and coadding into sets of 10; the field
is not crowded. Individual images have limiting magnitudes ranging from
12.8-17.8; we set the following specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
21:05:16.5   21:05:21.5         5     15.7           44.1       N
21:05:16.5   21:06:34.5        78     16.7           44.1       Y
21:29:06.7   21:34:44.6       337     17.8         1474.3       Y