GCN Circular 45582
Subject
GRB 260914A: VLT/X-shooter observations and redshift confirmation
Event
Date
2026-09-15T11:39:05Z (3 days ago)
From
Yu-Han Yang at University of Rome Tor Vergata <yyang@roma2.infn.it>
Via
Web form
Y.-H. Yang, M. Yadav, E. Troja, N. Monsalves (U Rome) report on behalf of the ERC BHianca team:
We observed the field of GRB 260914A (Wang et al., GCN 45562) with the X-shooter spectrograph on the ESO Very Large Telescope (VLT). Observations began at UT 06:03:21 on 2026-09-15 (18.8 hours after the GRB trigger) at an average airmass of ~1.8, and consisted of spectroscopy (4 × 600 s) and imaging with the acquisition and guiding (A&G) camera (360 s in each of the r and z filters).
We placed the 1 arcsec slit center at the position of 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). Based on a preliminary analysis, we identify the emission lines of H-alpha, H-beta, [O II] and [O III] in the VIS and NIR arms confirming the presence of an underlying star-forming galaxy at z ~0.778, as reported by Saccardi et al. (GCN 45579).
For this galaxy, the chance coincidence probability is Pcc~ 0.3% (Saccardi et al. GCN 45579). However, the burst is located ~10 arcsec from a bright (r~16.5 mag) and nearby (z~0.112) galaxy, LEDA 1860670, with Pcc~0.1% derived using the prescription of Becerra et al. (2023, MNRAS, 522, 5204) which is calibrated down to bright galaxies. Therefore, on probabilistic grounds, the host galaxy’s association remains uncertain.
Finally, we note that the UVB arm shows a weak continuum. As the underlying galaxy has catalogued magnitude g~23.4 AB (Legacy Survey DR10, Dey et al. 2019), the observed trace is likely contributed from the GRB afterglow. Over the faint continuum, we tentatively identify weak Mg II absorptions at a consistent redshift of z ~0.778. If confirmed, this would solidify the association between the GRB and the underlying, distant galaxy.
We thank the staff at the VLT for the rapid execution of these observations.