{
  "circularId": 38294,
  "submitter": "Weikang Zheng at UC Berkeley <weikang@berkeley.edu>",
  "body": "WeiKang Zheng, Thomas G. Brink, Alexei V. Filippenko (UC Berkeley), and Yi\nYang (Tsinghua Univ., Beijing), report on behalf of the KAIT GRB team:\n\n\nWe observed the field of both EP241021a (Hu et al., GCN 37834) and\nEP241026b (Lian et al., GCN 37902) with the Low Resolution Imaging\nSpectrometer (LRIS; Oke et al. 1995) on the Keck I 10 m telescope.\nObservations were performed on Oct. 30, 2024 UTC, with the 600/4000 grism\nand 400/8500 grating.\n\n\nThe optical counterpart of EP241021a (Fu et al., GCNs 37840, 37842; Li et\nal., GCNs 37844, 37846; Ror et al., GCN 37845; Zheng et al., GCN 37849;\nMoskvitin & Spiridonova, GCN 37850; Pugliese et al., GCN 37852;\nPérez-Fournon et al., GCN 37858; Bochenek & Perley, GCN 37869; Kumar et\nal., GCN 37875; Busmann et al., GCN 37877; J-Jin et al., GCN 37892;\nFreeburn et al., GCNs 37911, 37942, Quirola-Vasquez et al., GCN 37930;\nFreeburn et al., GCN 37942; Moskvitin et al., GCN 37951; Pan et al., GCN\n37968; Klingler et al., GCN 37990; Carotenuto et al., GCN 38014; Schneider\net al., GCN 38022; Bochenek et al., GCN 38030; Svinkin et al., GCN 38034;\nAryan et al., GCN 38042; Freeburn et al., GCN 38062; Schneider et al., GCN\n38071) was reported to be rebrightening (Quirola-Vasquez et al., GCN 37930;\nFreeburn et al., GCN 37942; Moskvitin et al., GCN 37951; Pan et al., GCN\n37968) before our observation. The spectrum of EP241021a shows a\nwell-detected continuum throughout the complete range (3400-10,200 Ang)\nwith 2 x 1200 s exposure time. We clearly detect emission lines of [O II]\n3727 Ang, H_beta, and [O III] 4959, 5007 Ang at a redshift of 0.748. We\nalso detect absorption lines of the Mg II 2796, 2803 Ang doublet at the\nsame redshifts of 0.748. Two additional sets of Mg II 2796, 2803 Ang\ndoublets are also detected with slightly lower signal-to-noise ratio (S/N)\nat redshifts of 0.48 and 0.62. Our results are consistent with those\nreported by Pugliese et al. (GCN 37852) and Pérez-Fournon et al. (GCN\n37858).\n\n\nThe reported possible LBT optical counterpart of EP241026b (Rossi et al.,\nGCN 37938) was confirmed by Bochenek et al. (GCN 38018). Our spectrum shows\nweak traces with 3 x 1200 s exposure time, but still has low S/N owing to\nthe faintness of the counterpart. We detect the continuum throughout the\ncomplete range (3400-10,200 Ang), but no obvious emission nor absorption\nlines are detected. The continuum is detected at ~3400 Ang, suggesting an\nupper limit of 1.8 for the redshift.\n\n\nThe data presented herein were obtained at the W. M. Keck Observatory,\nwhich is operated as a scientific partnership among the California\nInstitute of Technology, the University of California, and the National\nAeronautics and Space Administration. The Observatory was made possible by\nthe generous financial support of the W. M. Keck Foundation. The authors\nwish to recognize and acknowledge the very significant cultural role and\nreverence that the summit of Maunakea has always had within the indigenous\nHawaiian community. We are most fortunate to have the opportunity to\nconduct observations from this mountain.\n",
  "bibcode": "2024GCN.38294....1Z",
  "subject": "EP241021a and EP241026b: Keck/LRIS spectroscopic observations",
  "eventId": "EP241021a",
  "submittedHow": "email",
  "createdOn": 1732246803199
}