TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2776
SUBJECT: ROTSE-IIIa Observations of GRB 041006
DATE:    04/10/07 01:09:00 GMT
FROM:    Sarah Yost at U.Michigan  <sayost@umich.edu>

Yost, S. A., Smith, D. A., Rykoff, E. S., and Swan, H. report on behalf of
the ROTSE collaboration:

Although ROTSE-IIIa (Siding Spring Observatory, Australia) was temporarily
off-line when the initial alert for GRB 041006 (HETE-2 3570, GCN Circ.
#2770) was distributed, ROTSE collaboration members reactivated the system
and manually initiated a standard ROTSE response sequence (10 5-s images,
10 20-s images, and then a large number of 60-s images) 16 minutes after
the time of the burst.  The field was observed from Oct 6.52387 to 6.57424
(UTC).  The observing plan was complicated by the receipt of a HETE-2
ground alert at Oct 6.558, which reset the sequence.  ROTSE-III images are
unfiltered and calibrated against the USNO A2.0 catalog in R-band.

The optical counterpart discovered by Da Costa, Noel, and Price (GCN
Circ. #2765) is clearly visible. A preliminary analysis gives a
magnitude of 17.1+-0.4 in the first ROTSE-IIIa image.  The magnitude
of the source clearly decays over the interval of the ROTSE-IIIa
observations, and we cannot detect it during the short-exposure
images taken after the receipt of the HETE-2 ground alert.  We last
detect the source at a (preliminary) magnitude of 18.1+-0.4 in a 60-s
image taken beginning Oct 6.57263 (UTC).  Three subsequent 60-s
exposures show no evidence for the source to upper limits around 18.1.
The decay is consistent with the same power-law reported by Price, Da
Costa, and Noel (GCN Circ. #2771).

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