TITLE: GCN CIRCULAR NUMBER: 9313 SUBJECT: GRB 090424, SMARTS optical/IR afterglow monitoring DATE: 09/05/01 03:00:53 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 090424 (GCN 9223, Cannizzo et al.) over nine epochs between ~0.5 and 5.6 days post burst. In our first epoch of imaging, total summed exposure times amounted to 15 minutes in I and V and 12 minutes in J and K. All later epochs had total summed exposure times of 36 minutes in I and 30 minutes in J. The optical afterglow of GRB 090424 (e.g. GCN 9223, Cannizzo et al. & GCN 9224, Yuan et al.) is observed to fade steadily in our imaging with no indication of a jet break between ~0.5 and 5.6 days post-burst. The combined light of the afterglow and the underlying galaxy fades with a decay rate of alpha = -0.72 (where afterglow flux is proportional to t^alpha). Assuming that the galaxy has an intrinsic magnitude of I=21.6 (based on the SDSS i and r values and the Lupton 2005 transformation equations), then the afterglow's true decay rate is alpha = -1.2 over these epochs. The afterglow + host galaxy photometry is measured as follows (no correction has been made for Galactic extinction): mid-exposure time (days post-burst) I mag J mag V mag K mag ------------------------------------------------------------------- 0.45031 19.42+/-0.04 18.31+/-0.06 20.61+/-0.04 16.38+/-0.05 0.50291 19.45+/-0.04 18.11+/-0.04 ... ... 0.57498 19.61+/-0.04 18.22+/-0.05 ... ... 0.62815 19.70+/-0.04 18.23+/-0.05 ... ... 1.53459 20.45+/-0.05 19.18+/-0.08 ... ... 2.49012 20.92+/-0.05 19.61+/-0.11 ... ... 3.51877 20.99+/-0.05 19.53+/-0.09 ... ... 4.56909 21.24+/-0.08 > 19.7 ... ... 5.56135 21.31+/-0.10 > 19.7 ... ... (Optical photometry is calibrated against Landolt standard stars and IR photometry is calibrated against 2MASS stars in the field.)