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GCN Circular 3135

Subject
GRB 050319: ROTSE-III Refined Analysis
Date
2005-03-22T02:19:44Z (19 years ago)
From
Robert Quimby at U of Texas/ROTSE <quimby@astro.as.utexas.edu>
R. M. Quimby, E. S. Rykoff, B. E. Schaefer, T. McKay, and S. A. Yost
report on behalf of the ROTSE collaboration:

ROTSE-IIIb, located at the McDonald Observatory, Texas, responded
automatically to Swift GRB 050319 (Krimm et al., GCN 3117, GCN 3119)
with the first 5-second exposure beginning Mar 19, 09:31:45.56 UT
(Rykoff et al., GCN 3116), 27.1 seconds after the BAT trigger. The
observing sequence consisted of ten 5-second and ten 20-second images
followed by about 70 60-second exposures before clouds set in. All
images are unfiltered and the average time between exposures was 9.5
seconds.

We measured the afterglow flux in circular apertures relative to our
deepest image, and calibrated the magnitude scale by adopting SDSS
r-band values for our fiducial reference stars. Magnitudes on this
system for our first 5, 20, and 60-second images are given below.

                     Time (UT)    mag   +/-
                    09:31:45.56  16.16  0.17
                    09:34:09.95  17.22  0.20
                    09:39:05.24  17.58  0.18

The afterglow is detected in the first 30 images, and coadding the
later exposures in groups of 5 to 10 also results in detections,
making this one of the best sampled early light curves for any
GRB. The afterglow is well fit by a t^(-alpha) power law with
alpha=0.59 +/- 0.05, consistent with later R band observations
reported in the GCN Circulars (GCNs 3120, 3121, 3124). Continuing this
trend predicts the afterglow is currently brighter than 22nd
magnitude.

No host is present in the SDSS data at the location of the afterglow,
which implies the redshift for this GRB is larger than 0.3. We also
note the IR detection reported by George et al. (GCN 3125) gives a J-R
color of about 7, and could suggest a high-z source. Spectral
observations are encouraged to accurately determine the GRB redshift.
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