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GCN Circular 23168

Subject
GRB 180821A: VLT observations
Date
2018-08-23T16:22:15Z (6 years ago)
From
Paul Vreeswijk at Radboud U/Nijmegen <p.vreeswijk@astro.ru.nl>
Paul Vreeswijk (Radboud U. Nijmegen), Daniele Malesani, Jonatan
Selsing (DAWN/NBI, DARK/NBI), Nial Tanvir (U. Leicester), Giovanna
Pugliese (API/U. Amsterdam), Andrea Rossi (INAF-OAS) and Andrew Levan
(U. Warwick) report on behalf of the Stargate collaboration:

The Swift XRT error circle (Tohuvavohu et al., GCN 23158; Burrows et 
al., GCN 23164) of GRB 180821A (Krimm et al., GCN 23166) was observed
with the Very Large Telescope on the night of August 21/22, 2018,
using the telescope/instrument combinations Kueyen/X-shooter and
Antu/FORS2.

The FORS2 imaging observations started at 5:58 UT on August 22 (13.3
hours after the GRB trigger) and consisted of 8 exposures of 120
seconds in the z_Gunn filter. No object is detected in the enhanced
XRT error circle (Evans et al., GCN 23162), down to a 3-sigma limiting
magnitude of z(AB)=24.9 (see also Fernandez-Garcia, GCN 23159;
Guidorzi et al., GCN 23163; Kong, GCNs 23160 and 23165; Marshall et
al., GCN 23167). The z-band coverage of this field by the SkyMapper
Southern Sky Survey (see http://skymapper.anu.edu.au) was used in the
determination of the magnitude limit.

The X-shooter spectroscopic observations started at 6:55 UT on August
22 (14.3 hours after the GRB). A sequence of 4 spectra of 10 minutes
each was taken of the object at the Southwestern edge of the
preliminary XRT error circle, at coordinates R.A.: 00:31:09.0, decl.:
-38:37:39.5 (J2000). The reduced spectra show emission lines typical
of a star-forming galaxy, such as [OII] 3727, Hbeta, [OIII] 4959, 5007
and Halpha, at a redshift of z=0.58. Although the location of this
galaxy is consistent with the initial XRT afterglow localization, it
is about 6 arcseconds to the West and well outside of the enhanced XRT
error circle, and is therefore unrelated to the GRB.

We acknowledge the excellent assistance of the staff at Paranal
observatory, and in particular that of S. Brillant, C. Ledoux,
F. Gaete, E. Sedaghati and J. Velasquez.
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