TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3135 SUBJECT: GRB 050319: ROTSE-III Refined Analysis DATE: 05/03/22 02:19:44 GMT FROM: Robert Quimby at U of Texas/ROTSE 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.