TITLE:   GCN CIRCULAR
NUMBER:  34485
SUBJECT: GRB 230818A: NOT photometry and spectroscopy
DATE:    23/08/19 06:20:27 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

D.B. Malesani (Radboud and DAWN/NBI), D. Xu, S.Q. Jiang, T.H. Lu, Z.P. Zhu (NAOC),  J.P.U Fynbo (DAWN/NBI), L. Izzo, (INAF-OACN & DAWN/NBI), Z. Gray (NOT) report:

We observed the optical counterpart (Lipunov et al., GCN 34476; Gompertz et al., GCN 34480) of GRB 230818A detected by Fermi/GBM (Fermi/GBM team, GCN 34478) and Swift/BAT (Tohuvavohu et al., GCN 34479), using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera.

Observations started at 23:53:33 UT on 2023-08-18, i.e., 26 min post-burst, and 2x1200 s spectra were obtained, with a spectral coverage of 3600 - 9500 AA.

The spectrum shows a trace throughout the whole range, with a broad absorption feature at ~ 4150 AA, which can be interpreted as Lya at z~2.4. Redward several prominent absorption features are present, which can be interpreted as due to Si II, O I, C II, Si IV, C IV, Ni II, Fe II at a common redshift of z = 2.42, being consistent with the Lya feature. We thus think this is the redshift of the burst.

After the spectroscopy, an 150 s exposure in the Sloan i-band was obtained. We measured m(i) = 20.02 +/- 0.02 at 1.21 hr post-burst, calibrated with nearby SDSS stars.