TITLE: GCN CIRCULAR NUMBER: 12458 SUBJECT: GRB 110918A: Optical light curve for days 1.4 to 16.3 DATE: 11/10/19 12:14:28 GMT FROM: AAVSO GRB Network at AAVSO 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.