TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3165
SUBJECT: GRB050401: ROTSE-III Detection of Prompt Optical Counterpart
DATE:    05/04/01 18:49:02 GMT
FROM:    Eli Rykoff at U of Michigan/ROTSE  <erykoff@umich.edu>

E. S. Rykoff, S. A. Yost, D. A. Smith (Umich) report on behalf of the 
ROTSE collaboration:

ROTSE-IIIa, located at Siding Spring Observatory, Australia responded to 
Swift GRB050401 (Trigger #113120).  An automated response produced 
images beginning 6.2 seconds after the GCN trigger under good 
conditions.  The first image was taken at 01 Apr 14:20:39.2 UT, 33 
seconds after the updated burst time as reported by Barbier et al (GCN 
3162).  We took ten 5 second, ten 20 second and sixty 60 s exposures, 
and imaging is ongoing.  The images are unfiltered and were calibrated 
relative to USNO A2.0.

Individual images have limiting magnitudes ranging from 16.0-17.3. We 
compared sets of 10 co-added frames to the DSS (second epoch).  In our 
first co-added set of images we have a marginal detection of a source 
consistent with the XRT position (GCN 3161) and the candidate reported 
by McNaught & Price (GCN 3163).  The source is at 17.0+/-0.2 for a 151 
second co-add starting at 14:20:39.2 UT, with a limiting magnitude of 
17.4.  We do not detect any significant flux in our following images, to 
a limiting magnitude of 17.9/18.4 for sets of images beginning at 
14:28:22.3 UT/14:39:57 UT.

We also note that our first 5-s image was taken coincidentally with the 
end of the gamma-ray emission detected by Swift (GCN 3162).  We do not 
detect significant flux to a limiting magnitude of 16.0.