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GCN Circular 32595

Subject
GRB 220611A: Host galaxy redshift from VLT/X-shooter
Date
2022-09-27T15:22:02Z (2 years ago)
From
Benjamin Schneider at CEA <benjamin.schneider@cea.fr>
B. Schneider (CEA Paris-Saclay), D. B. Malesani (Radboud Univ. and DAWN/NBI),
J. P. U. Fynbo (DAWN/NBI), K. Wiersema (Univ. Lancaster), L. Izzo (DARK/NBI),
A. J. Levan (Radboud Univ.) report on behalf of the Stargate collaboration:

We observed the location of GRB 220611A (Cenko et al., GCN 32191;
Goad et al., GCN 32192; O���Connor et al., GCN 32203; Rastinejad et al.,
GCN 32208; Malesani et al., GCN 32222; Levan et al., GCN 32262) using the
X-shooter spectrograph mounted on the ESO VLT UT3 (Melipal). Our target was
the faint object visible in the archival optical images from the Legacy Survey,
coincident with the optical and X-ray afterglow, first noticed by
Rastinejad et al. (GCN 32208).

The observation was performed on 2022 September 23 (104 days after the GRB)
and consisted of 4x1200 s in the UVB and VIS arms and 8x600 s in the NIR arm.
A filter was adopted to block part of the K band which 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 several strong emission lines that
we identify as Lyman-alpha, [O II] 3729, [O III] 4959 and [O III] 5007 at a
common redshift of z = 2.3608 +/- 0.0002. The [O II] 3726 and H-beta lines
are affected by sky lines and are consequently not clearly detected. H-alpha
falls outside the covered wavelength range.

This result is supported by a VLT/MUSE spectrum obtained on 2022 August 24,
covering the wavelength range 4800 to 9300 AA, which shows no emission nor
absorption features, consistent with the measured redshift from X-shooter.

The positional coincidence between our target and the optical and X-ray
afterglow suggests a physical association. Furthermore, our spectra show
that the archival counterpart is a star-forming galaxy, akin to a typical
long GRB host. Our observations bring thus evidence that the GRB was not
physically associated with the local, nearby galaxy MCG-06-10-007 at z = 0.049
(Cenko et al., GCN 32191), and their proximity is a chance coincidence.

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