TITLE: GCN CIRCULAR NUMBER: 6829 SUBJECT: GRB 070911, SMARTS optical/IR afterglow observations DATE: 07/09/27 20:39:59 GMT FROM: Bethany Cobb at Yale U B. E. Cobb, 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 070911 (Krimm et al. GCN 6777) with the following mid-exposure times: 2007-09-13 07:10:30 UT - 2.05 days post-burst 2007-09-15 06:42:00 UT - 4.03 days post-burst 2007-09-22 05:22:21 UT - 10.98 days post-burst For each observation, total summed exposure times amounted to 36 minutes in I and 30 minutes in J. A fading afterglow is detected in both I and J within the X-ray afterglow error region (Stratta et al. 6779). This afterglow has the following coordinates and magnitudes: RA/DEC: 01:43:14.4 -33:29:02.5 days post burst I-magnitude (or limit) J-magnitude (or limit) ---------------------------------------------------------------- 2.05 20.9 +/- 0.1 19.8 +/- 0.2 4.03 21.9 +/- 0.2 >19.5 +/- 0.1 10.98 >22.3 +/- 0.1 >19.7 +/- 0.1 The I-band photometry is calibrated to a number of secondary stars in the field of GRB 070911. The magnitude of these secondary stars was derived using Landolt standard star observations obtained during the first two epochs, which were photometric night at CTIO. The J-band images are calibrated based on several 2MASS stars in the field. The afterglow's behavior between the first two epochs is consistent with a decay rate of alpha = -1.4+/-0.3. Interestingly, no J-band afterglow was detected by Butler et al. (GCN 6780) to J > 16.5 only 3 hours after the burst. This indicates that a period of rebrightening might have occurred between 3 hours and 2 days post-burst, though steady decay is only ruled out at the ~1 sigma level.