TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5111 SUBJECT: GRB 060428a, SMARTS optical/IR afterglow detection DATE: 06/05/10 23:56:25 GMT FROM: Bethany Cobb at Yale U B. E. Cobb (Yale), part of the larger SMARTS consortium, reports: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 060428a (GCN 5014, Mangano et al.) with a mid-exposure time of 2006-04-29 00:17 UT, which is ~20.9 hours post-burst and again at 2006-04-30 23:46 UT (~68.4 hours post-burst). For each observation total summed exposure times amounted to 36 minutes in I and 30 minutes in J. In our first epoch images, a source is visible in both I and J within the X-ray error region reported by Mangano et al. (GCN 5018). This source may correspond to the V-band source detected by De Pasquale & Mangano (GCN 5024) in the Swift UVOT imaging. The coordinates are: RA: 8:14:10.8 DEC: -37:10:11.4 The source is absent in our second epoch images. Its transient nature (see magnitudes below) suggests that the source is the afterglow of GRB 060428a. time post-burst I magnitude J magnitude ------------------------------------------------------ 20.9 hours 21.14 +/- 0.16 18.98 +/- 0.14 68.4 hours >22.2+/-0.2 >19.6+/-0.2 These preliminary magnitudes were calibrated using several USNO-B1.0 stars in the I-band and several 2MASS standards in J. The afterglow decay index is constrained by our observations to be alpha >~ -0.8 from ~1 to 3 days post-burst. Because no optical source was detected with magnitude <20 at only minutes post-burst (see GCN 5033, Haislip et al. [z'>19.1 at 5.3 minutes] and GCN 5024, De Pasquale & Mangano [V=20 at 1 minute]), our I-band detection at 20.9 hours suggests that the early time decay rate of the afterglow must have been significantly more shallow than -0.8. This may partially account for the lack of detected variability of the UVOT source at early times. Possibly, a light curve break occurred between our two sets of observations.