GCN Circular 44915
Subject
EP260610b: GTC likely spectroscopic redshift z = 1.10
Event
Date
2026-06-11T14:52:44Z (2 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
A. P. C. van Hoof (Radboud), D. Mata Sanchez (IAC and ULL), G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), David Garcia Alvarez (GTC), report on behalf of a larger collaboration:
We observed the optical counterpart (Levan et al., GCN 44897; Xin et al., GCN 44900; Zhu et al., GCN 44902; Burkhonov et al., GCN 44904; Moreno Mendez et al., GCN 44906) of the fast X-ray transient EP260610b (Wu et al., GCN 44895; Guo et al., GCN 44912) using the Gran Telescopio Canarias (GTC) located in the island of La Palma (Canary Islands, Spain), equipped with the OSIRIS+ spectrograph. A sequence of 3x1200 s observations was secured using grism R1000B, which covers the wavelength range 3600-7790 AA. The observation mid time was 2026 Jun 11.183 UT (15.58 hr after the EP/WXT trigger).
In the acquisition image (taken 14.96 hr after trigger), we measure an AB magnitude r = 23.38 +- 0.11, calibrated against the Pan-STARRS catalog, and not corrected for Galactic extinction. The delivered seeing was 0.8".
Weak continuum is detected across the entire wavelength range. From the lack of hydrogen absorption, we can set a firm upper limit to the redshift z < 2.1. Weak absorption features can also be identified matching Mg II and Fe II at z = 1.10. Line-stacking analysis confirms a significant absorption system at this redshift, which we suggest to be the likely redshift of EP260610b.