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GCN Circular 33281

Subject
GRB 230204B: VLT/X-shooter redshift
Date
2023-02-06T11:18:58Z (a year ago)
From
Daniele B Malesani at Radboud U <d.malesani@astro.ru.nl>
A. Saccardi (GEPI, Paris obs.), D. A. Kann (Goethe Univ.), J. Palmerio 
(GEPI, Paris obs. and IAP), V. D���Elia (SSDC and INAF-OAR), B. Schneider 
(MIT), L. Izzo (DARK/NBI), A. de Ugarte Postigo (OCA), D. B. Malesani 
(Radboud Univ. and DAWN/NBI) report on behalf of the Stargate collaboration:

We observed the optical afterglow of GRB 230204B (Serino et al., GCN 
33265; Swain et al., GCN 33269), also known as AT 2023bic (Smartt et 
al., GCN 33278), using the ESO VLT UT3 (Melipal) equipped with the 
X-shooter spectrograph. Our spectra cover the wavelength range 
3000-25000 AA, and consist of 4 exposures by 600 s each. The observation 
mid time was 2023 Feb 6.22 UT (31.4 hr after the GRB).

In a 30 s image taken with the acquisition camera on Feb 6.19 UT, we 
detect the optical afterglow, for which we measure a magnitude r = 21.55 
+- 0.18 AB (calibrated against a single nearby star from the Pan-STARRS 
catalog).

A faint continuum is the detected in the visible and near-infrared arms. 
Several, weak absorption features can be identified, which we interpret 
as due to Mg II, Mg I and Fe II at a common redshift of z = 2.142. While 
individual lines have low S/N, the combined detection of multiple 
features provides a convincing measurement of the redshift of this 
absorption system.

The association with the PGC1 0045721 galaxy group is thus a chance 
superposition, as already suggested by Smartt et al. (GCN 33278).

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Claudia Paladini and Heidi Korhonen.
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