{
  "bibcode": "2009GCN..9313....1C",
  "body": "B. E. Cobb (UC Berkeley) reports:\n\nUsing the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained\noptical/IR imaging of the error region of GRB 090424 (GCN 9223, Cannizzo\net al.) over nine epochs between ~0.5 and 5.6 days post burst.  In our\nfirst epoch of imaging, total summed exposure times amounted to\n15 minutes in I and V and 12 minutes in J and K.  All later epochs had\ntotal summed exposure times of 36 minutes in I and 30 minutes in J.\n\nThe optical afterglow of GRB 090424 (e.g. GCN 9223, Cannizzo\net al. & GCN 9224, Yuan et al.) is observed to fade steadily in our\nimaging with no indication of a jet break between ~0.5 and 5.6 days \npost-burst.  The combined light of the afterglow and the underlying galaxy\nfades with a decay rate of alpha = -0.72 (where afterglow flux is \nproportional to t^alpha).  Assuming that the galaxy has an intrinsic \nmagnitude of I=21.6 (based on the SDSS i and r values and the Lupton 2005 \ntransformation equations), then the afterglow's true decay rate is\nalpha = -1.2 over these epochs.\n\nThe afterglow + host galaxy photometry is measured as follows (no\ncorrection has been made for Galactic extinction):\n\nmid-exposure\ntime (days\npost-burst) I mag       J mag          V mag         K mag\n-------------------------------------------------------------------\n0.45031     19.42+/-0.04  18.31+/-0.06  20.61+/-0.04  16.38+/-0.05\n0.50291     19.45+/-0.04  18.11+/-0.04  ...           ...\n0.57498     19.61+/-0.04  18.22+/-0.05  ...           ...\n0.62815     19.70+/-0.04  18.23+/-0.05  ...           ...\n1.53459     20.45+/-0.05  19.18+/-0.08  ...           ...\n2.49012     20.92+/-0.05  19.61+/-0.11  ...           ...\n3.51877     20.99+/-0.05  19.53+/-0.09  ...           ...\n4.56909     21.24+/-0.08   > 19.7       ...           ...\n5.56135     21.31+/-0.10   > 19.7       ...           ...\n\n(Optical photometry is calibrated against Landolt standard stars\nand IR photometry is calibrated against 2MASS stars in the field.)",
  "circularId": 9313,
  "createdOn": 1241146853000,
  "email": "bcobb@astro.berkeley.edu",
  "subject": "GRB 090424, SMARTS optical/IR afterglow monitoring",
  "submitter": "Bethany Cobb at UC Berkeley  <bcobb@astro.berkeley.edu>",
  "eventId": "GRB 090424"
}