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GCN Circular 37866

Subject
GRB 241025A: NOT spectroscopic redshift of z = 4.20
Date
2024-10-25T06:17:59Z (8 days ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
D. Xu (NAOC), L. Izzo (INAF/OACn and DARK/NBI), Z.P. Zhu, S.Y. Fu (NAOC), N. Koivisto (NOT), T. Bäcker, M. Varding, R. Tuomainen, A. Willstedt Justel, T. Gobold (UH) report on behalf of a larger collaboration:

We have observed the optical counterpart (Ambrosi et al., GCN#37859; Jiang et al., GCN#37862) of GRB 241025A (Ambrosi et al., GCN#37859; Fermi GBM team, GCN#37860; Wang et al., GCN#37863; Li et al., GCN 37864), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC spectrograph. Grism #4 was used, covering the wavelength range 3650-9500 AA. Four spectra of 1200 s exposure time each have been obtained, with the first sequence starting at 02:07:32 UT on 2024-10-25, i.e., 0.51 hr after the Swift trigger.

Preliminary analysis of the data reveals a bright continuum in the red part of the spectrum, with a clear break around 6310 AA, which we interpret as due to Lyman alpha. We also identify the 
Lyman limit, the Lyman forest, and several metal absorption features, interpreted as due to Si II, Si IV, O I, C II, C IV, Fe II and Al II, all at a common redshift of z = 4.20.
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