GCN Circular 38293

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 M-type star 1RXS J225817.2-110434, the optical counterpart
of an EP flare event (trigger ID 01709065118; Zhou et al., GCN 37715
Not found
 
 
; Zheng
et al., GCN 37733
Not found
 
 
), with the Low Resolution Imaging Spectrometer (LRIS; Oke
et al. 1995) on the Keck I 10 m telescope. Observations were performed on
Oct. 5, 2024 UTC, with the 600/4000 grism and 400/8500 grating. Two spectra
with exposure times of 60 s and 80 s were obtained at 07:26 and 09:46 UTC,
corresponding to 1.72 and 4.05 hr (respectively) after the EP/WXT trigger
time. The spectra exhibit a red continuum flux with strong superposed
hydrogen Balmer emission lines as well as Ca II H&K and TiO emission lines,
consistent with a typical dMe star spectrum. Moreover, the flux of the
Balmer emission lines dropped by nearly half between the two epochs. The
continuum below 3500 Ang also dropped significantly. This is consistent
with the light-curve behavior reported by Lick/KAIT (Zheng et al., GCN
37733
Not found
 
 
), where the star brightness decreased after the EP/WXT trigger.


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.