GCN Circular 22180

L. Izzo (HETH/IAA-CSIC), J. Selsing (DARK), J. Japelj (API, Univ. Amsterdam), B.
Milvang-Jensen, J. P. U. Fynbo (both DARK), D. Xu (NAOC), D. A. Kann
(HETH/IAA-CSIC), N. R. Tanvir, R. Starling (both U. Leicester), A. J. Levan, K.
Wiersema (both U. Warwick), G. Pugliese (API, Univ. Amsterdam), V. D'Elia
(ASI-SSDC), and S. Campana (INAF-Brera) report on behalf of the Stargate
Consortium:

We observed the optical afterglow of GRB 171205A (V. D'Elia et al. GCN #22177)
with the ESO VLT/X-shooter spectrograph, covering the wavelength range
3500-20000 AA. Spectroscopy started at 08:56:18 UT on 2017-12-05 (i.e., 1.5 hr
after the GRB) and consisted of 1 exposure of 600 s taken in twilight.

From the acquisition image prior to the spectroscopy, a new optical transient is
clearly detected in the outskirts of the putative host galaxy (L. Izzo et al.
GCN #22178) at the following coordinates:

RA(J2000) = 11h 09m 39.573s Dec.(J2000) = -12d 35' 17.37"

This source is inside the error box reported by (J.P. Osborne et al. GCN
#22179). From the acquisition image we measure this source to have m(r)~16.0
mag, calibrated with a nearby Pan-STARRS star.

The source continuum is well detected across the entire spectral coverage and
clear emission lines are superposed on it identified as Halpha, [N II] 6584, and
[S II] 6717/32 located at z = 0.0368, suggesting an association with the nearby
galaxy. Additionally, a tentative detection of absorption lines identified as
NaI 5891/5897 in the continuum further supports z = 0.0368 as the redshift of
the GRB.

We acknowledge the excellent support from the ESO staff, particularly Jose
Velasquez and Zahed Wahhaj in obtaining these observations.