GCN Circular 12458
Subject
GRB 110918A: Optical light curve for days 1.4 to 16.3
Event
Date
2011-10-19T12:14:28Z (14 years ago)
From
AAVSO GRB Network at AAVSO <matthewt@aavso.org>
Arto Oksanen (Hankasalmi Obs., Hankasalmi, Finland), Bradley Schaefer
(LSU), Caisey Harlingten (Harlingten Observatory, San Pedro de Atacama,
Chile), and Matthew Templeton (AAVSO) report the following observations of
GRB 110918A (Hurley et al., GCN Circ. #12357):
A. Oksanen (Hankasalmi Obs., Hankasalmi, Finland) reports observations of
the optical transient associated with the intense, long GRB 110918A at
z=0.982 (Hurley et al., GCNC #12357; Golonetskii, et al., GCNC #12362;
Mangano et al. GCN 12364; Levan et al. GCN 12368) using the Harlingten
Observatory 0.5-m Planewave telescope with Apogee Alta-U42D9 CCD located
in San Pedro de Atacama, Chile. The observations were made on 15 nights
from 1.417 to 16.325 days after the burst, all without a filter so that
the color sensitivity is like that of a broad R-band filter. We have
calibrated the optical transient magnitude with the five comparison stars
used by Perley et al. (GCN 12388) for which they quote the R-band
magnitudes from the USNO catalog. Our magnitudes (plus two taken from the
GCNs) are given in the following table:
JD R(GRB) T-T0 (days)
2455824.8105 19.18 � 0.04 1.417
2455825.5537 19.70 � 0.10 2.160 (Guidorzi et al. GCN 12382)
2455825.7625 19.97 � 0.05 2.369
2455826.6343 20.68 � 0.13 3.241 (Perley et al. GCN 12388)
2455826.7059 20.51 � 0.06 3.312
2455827.7137 20.85 � 0.07 4.320
2455828.7067 21.14 � 0.09 5.313
2455829.7193 21.26 � 0.09 6.326
2455830.7107 21.56 � 0.12 7.317
2455831.7025 21.34 � 0.09 8.309
2455832.7116 21.62 � 0.11 9.318
2455833.6930 21.90 � 0.12 10.299
2455834.7065 21.76 � 0.12 11.313
2455835.6894 21.58 � 0.10 12.296
2455836.7208 21.67 � 0.11 13.327
2455837.7159 21.94 � 0.12 14.322
2455839.7185 21.99 � 0.13 16.325
The flux up to 10 days after the burst is well fit by a power law with an
index of -1.24. After 10 days after the burst, the light curve appears to
flatten (i.e., the opposite of a jet break), for which we expect that the
underlying galaxy is appearing in the light curve.
There is certainly no jet break in the time interval from 1.417 to roughly
10-16 days after the burst. The index over this time interval is typical
for the interval before the jet break, which implies that the jet break is
at a time of greater than 10 days after the burst. The previous
suggestion of an early jet break was simply due to one group looking at
only two magnitudes with relatively large uncertainty (not counting
inconsistencies in the two calibrations) and closely spaced in time and
seeing an apparently steep slope. With our long interval with consistent
magnitudes with small statistical uncertainty, we can be certain that the
index is shallow and no jet break is present.
The light curve of this GRB may be viewed at the following URL:
http://www.aavso.org/sites/default/files/images/110918A_Oksanen.png
The AAVSO International High Energy Network was made possible through
grants from the Charles Curry Foundation and from NASA, and is supported
through the AAVSO Endowment.