GCN Circular 45579
Subject
GRB 260914A: VLT/X-shooter spectroscopic redshift z = 0.778
Event
Date
2026-09-15T08:47:39Z (3 days ago)
From
Andrea Saccardi at CEA/Irfu <andrea.saccardi@cea.fr>
Via
Web form
A. Saccardi (CEA/Irfu), M. Garnichey (LUX-Paris Obs.), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), V. D’Elia (ASI/SSDC), M. De Pasquale (U. of Messina), L. Izzo (INAF/OAC), A. L. Thakur (INAF/IAPS), S. D. Vergani (LUX-Paris Obs.), report on behalf of the Stargate collaboration:
We observed the optical counterpart (Akl et al., GCN 45564; Palmerio et al., GCN 45565; Li et
al., GCN 45566; Shi et al., GCN 45567; Saccardi et al., GCN 45568) of the SVOM GRB 260914A (Wang et al., GCN 45562; see also Hamburg, GCN 45569) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
In a 30-s image taken with the acquisition camera (20.189 hr after the SVOM/ECLAIRs trigger), the counterpart is detected with a magnitude r = 22.19 +- 0.12 AB, calibrated against Pan-STARRS and not corrected for Galactic extinction. This is similar to the magnitude r ~ 22.1 of the underlying object seen in the Legacy survey, indicating that the transient is no longer dominating the total emission.
A spectrum covering the wavelength range 3000-21000 AA was secured, with a total exposure time of 4x600 s in each of the UVB, VIS, and NIR arms. Our observation started at 07:33 UT on 2026 September 15, i.e. about 20.34 hr after the SVOM trigger.
From a preliminary analysis, only faint continuum is present, but clear emission lines are detected in the visible and near-infrared arms, which we identify as H-alpha, the [O III] 4959, 5007 doublet, H-beta, and the [O II] 3726, 3729 doublet, all at a common redshift of z = 0.778.
The chance association probability between the underlying archival object at z = 0.778 and the afterglow is around 0.3% (Bloom et al. 2001), indicating that this is the likely host galaxy. We note though the presence of a bright galaxy (LEDA 1860670) some 10" away from the counterpart, with a spectroscopic redshift z = 0.112 from DESI (DESI collaboration et al., doi:10.3847/1538-3881/ae4c43) and a larger, but comparable, chance association probability. As long GRBs have mostly small host offsets, we favor the z = 0.778 interpretation, though we cannot conclusively rule out an association with the low-redshift galaxy.
We acknowledge excellent support from the ESO staff in Paranal.