TITLE: GCN CIRCULAR NUMBER: 10244 SUBJECT: GRB 091127, SMARTS optical/IR afterglow observations DATE: 09/12/03 23:50:51 GMT FROM: Bethany Cobb at UC Berkeley B. E. Cobb (UC Berkeley) reports: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 091127 (GCN 10191, Troja et al.) over seven epochs with mid-exposure times between ~2.2 and 100.8 hours post burst. The optical afterglow of GRB 091127 (e.g. GCN 10192, Smith et al. & GCN 10193, Immler et al., GCN 10194) is observed in our imaging. In our first epoch, with a mid-exposure time of ~2.2 hours post-burst, the afterglow is clearly detected in our imaging with the following magnitudes: B = 17.21 +/- 0.02 +/- 0.20 (error in zeropoint) R = 16.70 +/- 0.02 +/- 0.23 (error in zeropoint) I = 16.61 +/- 0.02 +/- 0.30 (error in zeropoint) J = 15.56 +/- 0.05 H = 15.40 +/- 0.05 K = 14.77 +/- 0.05 (Optical photometry is calibrated against USNO-B1.0 stars in the field, so suffers from a large photometric calibration error. IR photometry is calibrated against 2MASS stars.) The afterglow remains visible in our optical imaging over all our epochs of observation. Between 2.2 and 5.4 hours post burst, the afterglow decays with a power-law of alpha ~ -0.6 (where afterglow flux is proportional to t^alpha). From 27.8 to 100.8 hours post-burst, the afterglow appears to have steepened its decay rate to alpha ~ -1.6. This agrees with the behavior observed by Haislip et. al (GCNs 10219 & 10230). mid-exposure time (hours post-burst) I mag -------------------------------------------------------- 2.2 16.61 +/- 0.02 [+/- 0.30, zp error] 3.8 16.90 +/- 0.02 [+/- 0.30, zp error] 5.4 17.18 +/- 0.03 [+/- 0.30, zp error] 27.8 19.28 +/- 0.03 [+/- 0.30, zp error] 30.6 19.34 +/- 0.03 [+/- 0.30, zp error] 51.7 21.10 +/- 0.12 [+/- 0.30, zp error] 100.8 21.38 +/- 0.07 [+/- 0.30, zp error]