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GCN Circular 20222

Subject
GRB 160821B: HST detection of the optical and IR counterpart
Date
2016-12-01T02:36:37Z (7 years ago)
From
Eleonora Troja at GSFC <eleonora.troja@nasa.gov>
E. Troja (UMD/GSFC), N. Tanvir (U. Leicester), S. B. Cenko (NASA/GSFC),
A. Levan (U. Warwick), J. Barnes (U. Berkeley), A. Castro-Tirado (IAA-
CSIC), A. S. Fruchter (STScI), N. Gehrels (NASA/GSFC), J. Greiner (MPE),
N. Kawai (Tokyo Tech), R. Hounsell (UCSC), J. Hjorth (DARK/NBI), A. Lien
(NASA/GSFC), B. Metzger (Columbia),  D. Perley  (DARK/NBI), S. Rosswog
(U. Stockholm), T. Sakamoto (AGU), C. Thoene (IAA-CSIC), A. de Ugarte
Postigo (IAA-CSIC), and D. Watson (DARK/NBI) report:

We monitored the location of the short GRB 160821B (Siegel et al. GCN
19833; Xu et al. GCN 19834) with the Hubble Space Telescope under our
approved guest observer programs (GO14237 PI: Tanvir; GO14087 PI: Troja).
Observations were carried out with the Wide Field Camera (WFC3) in three
filters, F606W, F110W and F160W, at epochs 3.6, 10.4 and 23.2 days post-
burst. The GRB counterpart is clearly detected in all filters during the
first two epochs, and fades from a magnitude of F606W~25.8 (AB) in the
first epoch to become undetectable in the third epoch.

Assuming a redshift of z=0.162 from the nearby galaxy identified as the
likely host (Levan et al. GCN 19846), our observations rule out the
presence of an emerging supernova comparable to SN1998bw or to other SNe
associated to long GRBs. The observed fluxes constrain the contribution
of any r-process kilonova/macronova component to be at least a
factor ~5 fainter in the IR than that seen in GRB 130603B. The lack of
a bright supernova and the moderate-to-low ejecta mass implied by our
observations are consistent with this event being produced by the merger
of two neutron stars.

However, the current dataset cannot firmly exclude the presence of an
underlying, higher redshift host galaxy. Deeper HST observations aimed
at placing better constraints on the GRB redshift are on-going.

We thank the STScI staff, in particular Tricia Royle, for assistance
with rapidly scheduling our observations.
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