GRB 240821A: Keck/LRIS spectroscopic observations

We observed the location of the GRB 240821A optical afterglow (Quirola-Vasquez et al., GCN 37319) with the Low Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) with the Keck I 10 m telescope by centering on the slit the host-galaxy counterpart from the Legacy Survey as noticed by Quirola-Vasquez et al. (GCN 37319). Observations were performed on Sep. 2, 2024, and consisted of 4 x 1200 s exposures with the 600/4000 grism and 400/8500 grating. The spectrum has low signal-to-noise ratio owing to the faintness of the host galaxy. We detect and confirm the emission line at 8123 Ang reported by Saccardi et al. (GCN 37369) and Schneider et al. (GCN 37731) from VLT/X-shooter data. The [N II] 6584 and possibly [O II] 3727 emission lines reported by Schneider et al. (GCN 37731) are marginally detected. These results confirm the host-galaxy redshift of z = 0.238. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.