GCN Circular 42350
Subject
GRB 251017A: VLT/MUSE spectroscopic redshift z = 4.327
Event
Date
2025-10-18T10:50:11Z (3 days ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
L. Izzo (INAF/OACn & DARK/NBI), A. Saccardi (CEA/Irfu), M. Garnichey (LUX-Paris Obs.), M. Ferro (INAF-OABr), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), N. R. Tanvir (U. Leicester), A. L. Thakur (INAF-IAPS), G. Corcoran (UCD), report on behalf of the Stargate collaboration:
We observed the optical afterglow (Perez-Garcia et al., GCN 42323; Antier et al., GCN 42325; Goad et al., GCN 42326; Sasada et al., GCN 42329; Lipunov et al., GCN 42330; Fu et al., GCN 42331; Mo et al., GCN 42336; Adami et al., GCN 42339; Xin et al., GCN 42341) of GRB 251017A (Gupta et al., GCN 42322; Woolf et al., GCN 42335), using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph. The mid time of our observation was 2025 Oct 18 01:14:28 UT (16.56 hr after the GRB trigger), and consisted of 8 exposures of 700 s each.
We extracted a 1D spectrum centered on the afterglow emission, using an aperture radius of 0.6 arcsec. In the reduced spectrum, which covers the wavelength range 4750 - 9330 AA, we detect a clear continuum and a trough due to Lya absorption visible at ~6470 AA. From the identification of several absorption features as due to Si II, Si II*, O I, O I*, CII, C II*, Si IV, C IV, Fe II, Al II, we infer a common redshift of z=4.327 for the GRB.
We note that our spectroscopic redshift measurement is consistent with the photometric redshift value provided by Angulo et al. (GCN 42348).
We acknowledge expert support from the observing staff in Paranal, in particular Robert de Rosa, Miguel Lopez, Thallis Pessi and Julien Drevon.
The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).