GCN Circular 12458

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
Not found
 
 
; Levan et al. GCN 12368
Not found
 
 
) 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
Not found
 
 
) 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
Not found
 
 
)
2455825.7625    19.97 � 0.05 2.369
2455826.6343    20.68 � 0.13 3.241 (Perley et al. GCN 12388
Not found
 
 
)
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.