TITLE:   GCN CIRCULAR
NUMBER:  31800
SUBJECT: GRB 211024B: Host galaxy redshift from X-shooter/VLT
DATE:    22/03/28 15:57:26 GMT
FROM:    Antonio de Ugarte Postigo at OCA  <deugarte@oca.eu>

A. de Ugarte Postigo (Artemis/OCA), B. Schneider (CEA Paris-Saclay), 
D. B. Malesani (Radboud Univ. and DAWN/NBI), D. A. Kann (IAA-CSIC) 
report on behalf of the Stargate collaboration:

We observed the location of the ultra-long GRB 211024B (Gropp et al. 
GCN 30980, Zhu et al. GCN 30984) using the X-shooter spectrograph 
mounted on the UT3 of ESO’s Very Large Telescope. The observation 
was performed on 27 March 2022 (153 days after the GRB) and 
consisted of 4x1200 s in the UVB and VIS arms and 8x600 s in the NIR 
arm. We used the JH-slit that blocks part of the K-band and in return 
increases the efficiency of the J and H bands, resulting in a wavelength 
coverage between 3000 to 21000 AA. 

In a preliminary reduction we detect two weak emission lines that we 
identify as [OII]3727 and [OIII] 5008, as well as a marginal detection of 
[OII]4960 at a common redshift of z=1.1137+/-0.0002. The other line of 
the OII doublet, [OII]3729 is affected by a sky line and is consequently 
not detected. H-alpha falls within a telluric absorption and H-beta in a 
spectral region of low sensitivity, both resulting in non detections. Given 
the spatial coincidence of these emissions with the afterglow, we identify 
the underlying object as the host galaxy and propose this as the redshift 
of the GRB.

We acknowledge expert support from the ESO staff in Paranal, in 
particular Cedric Ledoux and Michael Abdul-Masih.