GCN Circular 1474
James Rhoads (STScI), Matthew Holman (CfA), JJ Kavelaars (McMaster), Andrew Fruchter (STScI), and Andrew Levan (STScI) report: We observed the error circle of GRB 020813 (HETE Trigger #2262) with the CTIO 4m Blanco telescope and Mosaic II imager starting at 020813 04:34 UT (110 minutes after the burst) and continuing for an hour. We obtained a total of ten 300 second images, four in B band and three each in R and I bands. The weather was good with subarcsecond seeing. Preliminary analysis of the first image (a B band frame) confirms the transient reported by Price et al (GCNC 1470). Astrometric calibration using 759 USNO-A2 stars brighter than R=16 places this image on a world coordinate system. We find coordinates of 19:46:41.865 -19:36:04.76 (J2000.0) for the optical transient, with an estimated error of 0.3" on each axis based on the 0.28" RMS residuals of the USNO star positions. Crude photometric calibration using the blue USNO-A2 magnitudes of three nearby stars gives B=18.6 at 04:36 UT. This B band detection immediately implies an upper limit z < 3.8 to the GRB redshift. (The positions and adopted magnitudes of the reference stars were 19:46:43.477 -19:36:05.66 B=17.10 19:46:42.729 -19:36:06.65 B=17.20 19:46:42.187 -19:35:37.00 B=15.90 .) The third B band image, taken 14 minutes later, shows a decline of 0.07 magnitudes, corresponding to a rough decay slope of -0.55, consistent with that reported by Li et al (GCNC 1473). Analysis of the full data set is underway and will give both refined decay slopes and spectral slopes.