GCN Circular 43490
Subject
GRB 251122A / EP251122a: VLT/X-shooter host redshift z = 1.765
Event
Date
2026-01-21T17:54:30Z (9 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
B. Schneider (LAM), A. Saccardi (CEA/Irfu), J. T. Palmerio (CEA/Irfu), S. D. Vergani (LUX-Paris Obs.), D. B. Malesani (DAWN/NBI and Radboud), M. Ferro (INAF/OAB), G. Corcoran (UCD), C. Lachaud (APC), Z.-Y. Lin (APC), report on behalf of the Stargate collaboration:
At the position of the optical counterpart (Cheng et al., GCN 42803; Li et al., GCN 42806; Sun et al., GCN 42807; Ma et al., GCN 42811; Gupta et al., GCN 42813; Busman et al., GCN 42814; Volnova et al., GCN 42950) of the high-energy transient GRB 251122A / EP251122a (Wang et al., GCN 42799; Lin et al., GCN 42800), a galaxy is visible in the Legacy survey, with magnitude r ~ 24.6, which is also detected in archival images from the Hyper Suprime-Cam Legacy Archive (Tanaka et al. 2021, doi:10.1093/pasj/psab034)
A spectrum of this object was secured using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. The spectrum covers the wavelength range 3000-25000 AA, and consists of 4 exposures of 1200 s each. The observation mid time is 2025 Dec 23.08 UT (30.5 days after the GRB).
A single, very clear emission line is detected at 18140 Å. We discuss its interpretation below.
- At the X-shooter resolution we would expect to resolve it if it was the [O II] 3727/3729 doublet (at z = 3.87). While the line is indeed resolved in wavelength space, it does not show a doublet profile; there is also no hint of Lyalpha nor [O III] 5008 in emission at this redshift.
- The line cannot be [O III] 5008 (at z = 2.62), otherwise we would also detect [O III] 4959, which is expected from first principles to be ~1/3 as intense, and is excluded by our data. We also do not detect Halpha in the K band, a region clean of atmospheric absorption.
- The line is however consistent with Halpha at z = 1.765; at this redshift, both Hbeta and [O III] fall at observed wavelengths with poor atmospheric transmission, and their non-detection is thus not unexpected. The [O II] doublet is not detected at this redshift, although it falls in a noisy region of the spectrum (at the interface between the X-shooter VIS and NIR arms). Dust extinction could also contribute to the faintness of this line.
We conclude that z = 1.765 is most likely the redshift of GRB 251122A / EP251122a.
We acknowledge excellent support from the observing staff at Paranal, in particular Claudia Cid, Florian Rodler, Jonathan Smoker and Thomas Rivinius. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).