GCN Circular 10110
Subject
GRB 091018, SMARTS optical/IR afterglow observations
Event
Date
2009-10-29T21:47:42Z (16 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 091018 (GCN 10034,
Stamatikos et al.) over several epochs, starting ~3.9 hours post-burst.
For the first epoch, several dithered images were obtained in each filter,
with total summed exposure times of 180s in each of BRIJK and 120s in each
of H and V. For later epochs, total summed exposure times amounted to 15
minutes in I and V and 12 minutes in J and K.
At a mid-exposure time of 2009-10-19 00:57 UT (4.1 hrs post-burst), the
GRB afterglow (e.g. GCN 10034, Stamatikos et al., GCN 10036, Schaefer et
al., GCN 10039, Filgas et al.) is detected with the following magnitudes:
B = 19.59 +/- 0.04
V = 19.27 +/- 0.05
R = 18.88 +/- 0.04
I = 18.44 +/- 0.04
J = 18.0 +/- 0.2
H = 17.1 +/- 0.2
K = 16.1 +/- 0.2
Between 4.1 hrs and 33.0 hrs post-burst, the GRB afterglow fades with a
decay rate of approximately alpha = -1.3 (where afterglow flux is
proportional to t^alpha).
time
post-burst I-band magnitude
4.1 hrs 18.44 +/- 0.04
5.5 hrs 18.82 +/- 0.04
8.0 hrs 19.26 +/- 0.04
9.7 hrs 19.49 +/- 0.05
28.6 hrs 21.04 +/- 0.06
33.0 hrs 21.27 +/- 0.08
(Optical photometry is calibrated against Landolt standard stars
and IR photometry is calibrated against 2MASS stars in the field.)