TITLE: GCN CIRCULAR NUMBER: 38294 SUBJECT: EP241021a and EP241026b: Keck/LRIS spectroscopic observations DATE: 24/11/22 03:40:03 GMT FROM: Weikang Zheng at UC Berkeley <weikang@berkeley.edu> WeiKang Zheng, Thomas G. Brink, Alexei V. Filippenko (UC Berkeley), and Yi Yang (Tsinghua Univ., Beijing), report on behalf of the KAIT GRB team: We observed the field of both EP241021a (Hu et al., GCN 37834) and EP241026b (Lian et al., GCN 37902) with the Low Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) on the Keck I 10 m telescope. Observations were performed on Oct. 30, 2024 UTC, with the 600/4000 grism and 400/8500 grating. The optical counterpart of EP241021a (Fu et al., GCNs 37840, 37842; Li et al., GCNs 37844, 37846; Ror et al., GCN 37845; Zheng et al., GCN 37849; Moskvitin & Spiridonova, GCN 37850; Pugliese et al., GCN 37852; Pérez-Fournon et al., GCN 37858; Bochenek & Perley, GCN 37869; Kumar et al., GCN 37875; Busmann et al., GCN 37877; J-Jin et al., GCN 37892; Freeburn et al., GCNs 37911, 37942, Quirola-Vasquez et al., GCN 37930; Freeburn et al., GCN 37942; Moskvitin et al., GCN 37951; Pan et al., GCN 37968; Klingler et al., GCN 37990; Carotenuto et al., GCN 38014; Schneider et al., GCN 38022; Bochenek et al., GCN 38030; Svinkin et al., GCN 38034; Aryan et al., GCN 38042; Freeburn et al., GCN 38062; Schneider et al., GCN 38071) was reported to be rebrightening (Quirola-Vasquez et al., GCN 37930; Freeburn et al., GCN 37942; Moskvitin et al., GCN 37951; Pan et al., GCN 37968) before our observation. The spectrum of EP241021a shows a well-detected continuum throughout the complete range (3400-10,200 Ang) with 2 x 1200 s exposure time. We clearly detect emission lines of [O II] 3727 Ang, H_beta, and [O III] 4959, 5007 Ang at a redshift of 0.748. We also detect absorption lines of the Mg II 2796, 2803 Ang doublet at the same redshifts of 0.748. Two additional sets of Mg II 2796, 2803 Ang doublets are also detected with slightly lower signal-to-noise ratio (S/N) at redshifts of 0.48 and 0.62. Our results are consistent with those reported by Pugliese et al. (GCN 37852) and Pérez-Fournon et al. (GCN 37858). The reported possible LBT optical counterpart of EP241026b (Rossi et al., GCN 37938) was confirmed by Bochenek et al. (GCN 38018). Our spectrum shows weak traces with 3 x 1200 s exposure time, but still has low S/N owing to the faintness of the counterpart. We detect the continuum throughout the complete range (3400-10,200 Ang), but no obvious emission nor absorption lines are detected. The continuum is detected at ~3400 Ang, suggesting an upper limit of 1.8 for the redshift. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/38294. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...