TITLE:   GCN CIRCULAR
NUMBER:  42398
SUBJECT: EP251023a: Keck/LRIS spectroscopic redshift z=2.232
DATE:    25/10/23 14:39:53 GMT
FROM:    Ryan Chornock at UC Berkeley <chornock@berkeley.edu>


R. Chornock (UC Berkeley) reports:

We observed the optical afterglow (Wortley et al., GCN 42387; Li et al., GCN 42389; Liu et al. GCN 42391) of the X-ray transient EP251023a (Wu et al., GCN 42388) using the Low-Resolution Imaging Spectrometer on the Keck-I telescope at a mean time of 13:26UT on 2025 October 23 (~10.9 hours after the EP trigger). Observations covered the range 3100-10300 Angstroms. 

The afterglow remains bright and the continuum is well detected across the full spectral range. Multiple absorption systems are present, but the highest redshift absorber has many absorption lines (including Lyman-alpha, Si II, Si II*, O I, O I*, C II, Si IV, C IV, Fe II, Mg I and Mg II) present at a common redshift of z=2.232+/-0.001.