GCN Circular 4860

B. E. Schaefer (Louisiana State), F. Yuan (U Mich), S.A. Yost (U Mich), R. 
Quimby (U Texas), report on behalf of the ROTSE collaboration:

ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB
060312 (Swift trigger 201391; D. Grupe et al. 2005, GCN 4859
Not found
 
 
), producing
images beginning 6.0 s after the GCN notice time. An automated response
took the first image at 01:36:33.0 UT, 20.3 s after the burst trigger. We
took 10 5-sec, 10 20-sec and 74 60-sec eposures; with ongoing imaging. The
first 25 images were taken with moderate twilight and thin clouds, but
after this there appears to be no problem with the sky conditions. These
unfiltered images are calibrated relative to USNO A2.0 (R).

Comparison to the DSS (second epoch) reveals no new sources within the 
3-sigma error circle, for both single images and coadding into sets of 10; 
the field is not crowded. In particular, we see no source at the location 
of the XRT fading transient (Grupe et al. 2005). Individual images have 
limiting magnitudes ranging from 14.4-17.5; we set the following specific 
limits.

start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd?
--------------------------------------------------------------------
01:36:33.0 01:36:38.0 5 14.6 20.3 N
01:47:39.9 01:52:22.2 282 17.1 687.2 Y
01:57:23.3 02:02:06.5 283 18.3 1270.6 Y