GCN Circular 41811
Subject
EP250911a: GTC/OSIRIS+ spectroscopic redshift z = 3.84
Event
Date
2025-09-12T09:59:04Z (3 months ago)
Edited On
2025-09-12T13:30:07Z (3 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), A. J. Levan (Radboud and Warwick), G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. L. Cabrera Lavers (GTC), D. Reverte-Payá (GTC), A. Pérez-Romero (GTC) report for a larger collaboration:
We observed the afterglow (Quirola-Vasquez et al., GCN 41790; Angulo et al., GCN 41793; Lai et al., GCN 41801; Evans et al., GCN 41802; Moskvitin & Spiridonova, GCN 41805; Ma et al., GCN 41807) of EP250911a (Liang et al. GCN 41788; Liang et al., GCN 41809) using the Gran Telescopio Canairas (GTC) equipped with the OSIRIS+ instrument.
In a sequence of images taken in the r and z filters (mean times 13.97 and 14.12 hr after the trigger time, respectively), we measure magnitudes r = 22.0 +/- 0.1, z = 21.4 +/- 0.1 (all AB, calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction).
A sequence of 4 spectra with 1200 s exposure time each was obtained using the R1000B grism, covering the wavelength range 3700-7700 AA (mean time 15.4 hr after trigger). Inspection of the combined spectrum shows a weak continuum. A broad trough is observed around 5890 AA, which is interpreted as a DLA feature at a redshift z ~ 3.84. This is confirmed by the detection of a few metal absorption lines at a consistent redshift, among which we identify S II / Si II 1260, O I / Si II 1303, C II 1334, Si IV 1394, C IV 1548,1550. We also detect a continuum break around 4400 AA, which corresponds to the Lyman limit at this redshift.
Note that, despite some temporal and spatial proximity, this redshift is inconsistent with the distance (~1.1 Gpc) derived for the LVK GW source S250911ac (LVK collaboration, GCNs 41786, GCN 41795), excluding an association between the two events.