TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2318 SUBJECT: XRF030723, ROTSE-III Early Observations DATE: 03/07/24 19:48:57 GMT FROM: Don Smith at U michigan D. A. Smith (U. of Michigan) & R. Quimby (U. of Texas) report on behalf of the ROTSE collaboration: The ROTSE-IIIb instrument at McDonald Observatory, Texas, responded automatically to HETE-2 alert 2777, and began taking images within 5 s of when the alert was distributed. The first exposure began 47 seconds after the burst trigger time. All objects in the SXC 2' (radius) error circle that appear in more than four individual ROTSE images match to sources in the USNO 2.0 catalog. Unfiltered limiting magnitudes, calibrated to USNO R-band, for the first ten (5-s) images were around 16.5, for the second ten (20-s) images around 17.5, and for the next 40 (60-s) images around 18.0. We then co-added sets of ten frames each to drive the limiting magnitudes down to 18.1, 18.7, and 19.1, respectively. We also co-added the first twenty frames as a check against false positives in the first two co-added images. Only two non-USNO sources within the error circle appear in more than the first of the co-added, composite images. We find no sources that appear in the five later co-adds that are not in the first one. Both of the non-USNO sources in the first image are dimmer in the second and third images, and both vanish by the fourth image. Neither source appears to be in the DSS or the 2MASS J and K archival images of this field, and neither source appears in images taken the following night (to a limiting magnitude of 18.6) at 0.95 d after the burst. One of the sources is too close to a USNO-cataloged star for SExtractor to de-blend the two. The star is at coordinates 21h 49m 24s.8, -27o 40' 10".16 (J2000.0), and its derived ROTSE magnitude (which would contain emission from both sources) fades by 1.6 mag from 17.3+-0.1 to 18.9+-0.2 as the nearby source (which is about 7.5" East and 3" South) disappears. The following night, this star was measured at 18.9+-0.2. The second source was isolated, and we derive the following light curve: R.A. Dec. (J2000.0) Magnitude 21h 49m 24s.379 -27o 40' 35".04 17.9+-0.2 18.2+-0.2 18.2+-0.1 > 19.1 > 19.1 > 19.1 (Time from burst) (47.3 s) (191.9 s) (8.1 m) (19.7 m) (31.3 m) (42.9 m) We cannot at this time determine if either of these sources is related to the HETE-2 event. Images and finding charts can be found at http://grad40.as.utexas.edu/~quimby/HETE2777