TITLE:   GCN CIRCULAR
NUMBER:  23488
SUBJECT: GRB 181201A: VLT/FORS2 tentative spectroscopic redshift
DATE:    18/12/02 16:53:48 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

L. Izzo, A. de Ugarte Postigo, D. A. Kann (all HETH/IAA-CSIC), D. B. 
Malesani (DAWN/NBI/DTU and DARK/NBI), K. E. Heintz (Univ. of Iceland), 
N. R. Tanvir (Univ. Leicester), V. D'Elia (SSDC), K. Wiersema (Univ. 
Warwick), C. Kouveliotou (GWU), and A. J. Levan (Univ. Warwick) report 
on behalf of the Stargate collaboration:

We observed the optical counterpart (Podesta et al., GCN #23470; Kong, 
GCN #23475; Lipunov et al., GCN #23476; Volnova et al., GCN #23477; 
Heintz et al., GCN #23478; Mazaeva et al., GCN #23479; Watson et al., 
GCN #23481; Belkin et al., GCN #23485; Bolmer & Schady, GCN #23486) of 
the long, very bright INTEGRAL GRB 181201A (Mereghetti et al., GCN 
#23469) with the ESO VLT UT1 equipped with the FORS2 spectrograph. We 
obtained a single 600 s spectrum in the low-resolution grism 300V. 
Observations started at 00:50:50 UT on December 2 (i.e., 22.16 hr after 
trigger).

In the spectrum, the continuum is detected at excellent signal-to-noise 
but is largely featureless. From a non-detection of any break in the 
continuum down to 3500 Å, we derive a redshift upper limit of z < 1.88. 
There are no strong lines visible, but we note the possible presence of 
three weak lines, which, if real, can be identified as the Mg II doublet 
and Mg I, implying a redshift of z = 0.450. No emission lines are 
visible. Pan-STARRS archive imaging (Chambers et al. 2016, 
arXiv:1612.05560) shows the presence of a possible host galaxy at i' ~ 
23.4 mag.

We acknowledge the ESO observing staff at Paranal, especially Cedric 
Ledoux, Joe Anderson, and Rodrigo Tomero. We are also indebted to the 
ESPRESSO commissioning team who kindly granted us permission to carry 
out this ToO observation during their time.