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.