[vsnet-grb-info 16943] GRB 150821A: Flattening in the UVOT light curve

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18224
SUBJECT: GRB 150821A: Flattening in the UVOT light curve
DATE:    15/09/01 15:33:59 GMT
FROM:    Paul Kuin at MSSL  <npkuin at gmail.com>

Paul Kuin, Massimiliano de Pasquale (UCL/MSSL), Daniele
Malesani (DARK/NBI) and Peter Brown (TAMU) report on
behalf of the Swift UVOT team:

The light curve from GRB 150821A as observed by Swift/UVOT has
been monitored continuously since its discovery, up to about
300 ks in all seven filters, and in u, v and white since.

The GRB brightness decreased as a straight power law with
index -1.32 (+0.06/-0.05) until about 10 ks. At 100 ks
there was a clear flattening in the light curve, and around
250 ks a slight brightening may have taken place, but the
brightness could also be flat within our observational errors.
Data taken at 835 ks are consistent with a flat or slightly
decreasing brightness.

The summed 5 ks image in the white band from 300 - 800 ks
shows a nearby extended object to the north-west with
white = 22.01 +/- 0.17 mag, slightly brighter than the GRB at
about 10" distance which currently is at white=23.2 +/- 0.7
magnitudes, and is located about 5" from the edge of the
nearby object.

At this time it is unclear whether the observed flattening is
due to the host galaxy (hosts at this distance have typically
a size of 1" only) or are the emergence of a supernova
contribution to the afterglow. At the redshift of z = 0.755
(D'Elia et al., GCN Circ. 18187), the observed brightness
would imply a SN significantly brighter than those typically
associated with GRBs.

Further ground observations are encouraged.