GCN Circular 41022
Subject
GRB 250706B/C: VLT/X-shooter redshift z = 0.942
Event
Date
2025-07-08T12:04:56Z (a year ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
J. T. Palmerio (CEA/Irfu), B. Schneider (LAM), A. Saccardi (CEA/Irfu), M. Garnichey (LUX-Paris Obs.), G. Corcoran (UCD), V. D'Elia (ASI/SSDC), M. De Pasquale (Univ. Messina), L. Izzo (INAF/OACn and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), P. O'Brien (Leicester), N. R. Tanvir (Univ. Leicester), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration:
We observed the optical afterglow (Zhu et al., GCN 40991) of the SVOM GRB 250706B (Palmerio et al., GCN 40989), an extension of GRB 250706C detected by Konus-Wind and Fermi/LAT (Frederiks et al., GCN 41013; Longo et al., GCN 41019), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 2 exposures of 600 s each. The observation mid time was 2025 Jul 8.40 UT (1.70 days after the Konus-Wind trigger).
In a 30 s image taken with the acquisition camera on Jul 8.39 UT (1.69 days after the Konus-Wind trigger), the optical afterglow is clearly detected, and we measure a magnitude r = 20.37 +- 0.02 AB (calibrated against a single star from the SkyMapper catalog).
In a preliminary reduction of the spectra, continuum is visible over the entire covered wavelength range. From the detection a multitude of absorption lines, which we interpret as due to Al II, Ni II, Si II, Si II*, Al III, Cr II, Zn II, Ni II*, Fe II, Fe I, Mn II, Mg II, Mg I, Ca II, Ca I, we infer a redshift of z = 0.942. We also detect two emission lines which we identify as the [O III] 4959,5007 doublet at the same redshift. The detection of fine-structure lines, as well as of nebular emission, allows us to securely identify z = 0.942 as the redshift of GRB 250706B/C.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Diego Parraguez. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).