GCN Circular 9313
Subject
GRB 090424, SMARTS optical/IR afterglow monitoring
Date
2009-05-01T03:00:53Z (15 years ago)
From
Bethany Cobb at UC Berkeley <bcobb@astro.berkeley.edu>
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.)