GCN Circular 41883
Subject
EP250919a/GRB 250919A: Gemini/GMOS-S spectroscopic redshift z = 1.145
Event
Date
2025-09-19T08:07:27Z (4 days ago)
From
Andrew Levan at Radboud University <a.levan@astro.ru.nl>
Via
Web form
A. J. Levan (Radboud and Warwick), J. N. D. van Dalen (Radboud), G. Corcoran (UCD), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), J. A. Quirola-Vasquez (Radboud), F. E. Bauer (SSI and UTA), A. Martin-Carrillo (UCD) report on behalf of a larger collaboration:
We obtained spectroscopic observations of the optical counterpart (Li et al., GCN 41877) of EP250919a (Liang et al., 41879), probably connected to GRB 250919A (Fermi GBM team, GCN 41874; Wang et al., GCN 41882; as also indicated by the consistency with the Fermi/LAT GRB localization; https://gcn.gsfc.nasa.gov/fermi_grbs.html). Observations were secured at the Gemini-South Telescope equipped with the GMOS spectrograph using grating B480, and began at 05:30 UT, approximately 3.25 hr (5 hr) after the EP (Fermi and SVOM) triggers. At this point we obtained 4x200 s observations covering the spectral range 3800-8000 AA.
In the acquisition image, the optical counterpart is clearly detected with an AB magnitude r = 17.52 +- 0.02 (calibrated against nearby objects from the SkyMapper catalog, not corrected for Galactic extinction).
The spectral trace is extremely bright and reveals multiple absorption features from Mg II, Mg I, Fe II, Al III, all at a common redshift of z = 1.145. We therefore suggest this is the redshift of EP250919a / GRB 250919A.
We thank the staff of Gemini, in particular Karleyne Silva for the rapid execution of these observations.