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.