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GCN Circular 35317

GRB 231210B: VLT/X-shooter redshift
2023-12-11T04:42:22Z (4 months ago)
Andrea Saccardi at Observatoire de Paris <>
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A. Saccardi (GEPI, Observatoire de Paris), B. Schneider (MIT), L. Izzo (INAF-Naples & DARK/NBI), D. B. Malesani (DAWN/NBI), C. Peña (Univ. of Utah) report on behalf of the Stargate collaboration:

We observed the field of the Swift GRB 231210B (Page et al., GCN 35314) using the ESO/VLT/UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600s each. The observation starting-time is 01:32:26 UT on Dec 11 2023 (~4 hr after the Swift trigger).

In images taken with the acquisition camera, we clearly detect the optical afterglow consistent with the position reported by Buckley et al., GCN 35313 and Pérez-Fournon et al., GCN 35316. We measure the following AB magnitudes (calibrated against SkyMapper stars): g' = 21.82 +/- 0.05 mag, r' = 21.23 +/- 0.04 mag, z' = 21.03 +/- 0.07 mag.

We detect a continuum over the entire wavelength range. From the detection of a broad Lya absorption at ~5000 AA and multiple absorption features, which we interpret as being due to NV, SiII, SII, OI, CII, ZnII, FeII, AlII, CIV, SiIV, FeII*, OI*, SiII*, CII*, we infer a common redshift of z = 3.13. We conclude this is the redshift of the burst. We also note the presence of additional absorption features likely due to multiple intervening systems.

We acknowledge the excellent support from the ESO staff in Paranal, in particular Maria Jose Rain, Zahed Wahhaj and Thomas Seifert.
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