TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3708 SUBJECT: GRB 050730, SMARTS optical/IR afterglow observations DATE: 05/07/31 01:07:04 GMT FROM: Bethany Cobb at Yale U B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS consortium, report: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 050730 (GCN 3704, Holland et al.) with a mid-exposure time of 2005-07-30 23:05 UT, which is ~3.1 hours post-burst. Total summed exposure times amounted to 15 minutes in I and V and 12 minutes in J and K. The candidate afterglow (Holland et al, GCN 3704, Sota et al. GCN 3705, Gomboc et al. GCN 3706) is well detected in our images. Assuming the nearby USNO-B1.0 star 0862-0270654 has a magnitude of I = 17.6 (USNO-B1.0 I2 magnitude) and J = 16.4 (2MASS magnitude), then the magnitude of the afterglow candidate is I ~ 17.2 and J ~ 16.3 in the total combine frames. The afterglow candidate also shows strong evidence of decay between successive frame. Preliminary analysis suggests that the afterglow may have decayed in I by ~ 0.5 magnitudes between the first and last frame taken over a 40 minute period. Given this evidence of decay, this source is very likely the optical afterglow of GRB 050730.