TITLE: GCN CIRCULAR NUMBER: 32595 SUBJECT: GRB 220611A: Host galaxy redshift from VLT/X-shooter DATE: 22/09/27 15:22:02 GMT 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.