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GCN Circular 18125

GRB 150811A: P60 Observations
2015-08-11T13:46:42Z (9 years ago)
S. Bradley Cenko at NASA/GSFC <>
D. A. Perley (DARK) and S. B. Cenko (GSFC) report:

The Palomar 60-inch automated telescope responded to the trigger for GRB 150811A (Evans et al., GRB 18119) and began observations at 04:10:54 UT, 4.75 minutes after the burst.  A sequence of four 60-second i-band images was acquired, followed by a repeating cycle of 60-second r, i, and z-band images, then two epochs of imaging in ri filters later in the night.

The optical afterglow (e.g. Butler et al, GCN 18120; Zheng et al., GCN 18121; Kuin et al., GCN 18122; Japelj et al., GCN 18123) shows a sharp rise in flux during the first few observations, from I ~ 17.5 mag at t_mid = 5.25 minutes to a peak of I ~ 16.2 mag at t = 12.42 minutes (calibrating to USNO B1 I-band standards).  Afterwards it levels out and then declines steadily, reaching I ~ 18.3 mag at t = 135.1 min and I ~ 19.0 mag at t = 303.85 min, corresponding to an average decay rate of approximately alpha = 0.8.

Further follow-up is planned.  Spectroscopy is highly encouraged.
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