[vsnet-grb-info 18398] GRB 160821B: WHT afterglow confirmation and redshift of candidate host

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19846
SUBJECT: GRB 160821B: WHT afterglow confirmation and redshift of candidate host
DATE:    16/08/23 04:31:26 GMT
FROM:    Andrew Levan at U.of Leicester  <A.J.Levan at warwick.ac.uk>

A.J. Levan (U. Warwick), K. Wiersema (U. Leicester), N.R. Tanvir (U. Leicester), D. Malesani (DARK/NBI, DTU Space), D. Xu (NAOC/CAS), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI) report for a larger collaboration: 

We observed the location of GRB 160821B (Siegel et al. GCN 19833) with the William Herschel Telescope on La Palma, beginning at 22:57UT on 22 Aug 2016. Observations were obtained in the r and z bands. The candidate afterglow (Xu et al. GCN 19834) is present in our images, but has faded by approximately a factor of two between the two epochs taken approximately one day apart. This variability confirms the source as the afterglow of GRB 160821B. 

In addition we obtained a spectrum of the candidate host galaxy (Xu et al. GCN 19834). Several prominent emission lines are visible, including Hbeta, [OIII] (4959/5007) and H-alpha, at a provisional redshift of z=0.16. 

The offset of the afterglow from the candidate host galaxy is approximately 5.5”, corresponding to a physical offset of 15 kpc. While we cannot rule out a chance alignment of the afterglow with the host galaxy, we also note that the galaxy light does apparently extend to approximately the afterglow position. 

We thank the staff of WHT, in particular Cecelia Farina, for their help with these observations.