TITLE:   GCN CIRCULAR
NUMBER:  34497
SUBJECT: GRB 230818A: SAO RAS 6-m telescope spectroscopy
DATE:    23/08/19 20:52:59 GMT
FROM:    Moskvitin Alexander at SAO RAS <mosk@sao.ru>

A. S. Moskvitin, D. V. Oparin (SAO RAS), A. S. Pozanenko (IKI),
report on behalf of the larger GRB follow-up collaboration.

We observed the GRB 230818A OT (Lipunov et al., GCNs 34476, 34477,
34481, 34482; Gompertz et al., GCN 34480; Moskvitin et al., GCN 34483,
34495; Malesani et al., GCN 34485; Siegel and Tohuvavohu, GCN 34486;
Pankov et al., GCN 34487; Quadri and Strabla, GCN 34488;
Klose et al., GCN 34491) with the BTA 6-meter telescope of SAO RAS
equipped with Scorpio-II. The observations were started since ~ 0.5 h
after the burst (Fermi: GBM team, GCN 34478; Swift: Tohuvavohu et al.,
GCN 34479; AstroSat: Pari et  al., GCN 34492).

In the 4 x 600 sec spectrum of (4200--8530 AA, FWHM resolution ~ 7A)
we identified the absorption lines (C IV, Si II) at a common redshift
of z = 2.42.  We also identified Si IV and Al II lines at a common
redshift of 2.38 which might be an intervening system. Thus we confirm
the redshift measurement obtained by Malezani et al. (GCN 34485).

Using acquisition observation we obtained preliminary photometry
of the afterglow as following (calibrated against Pan-STARRS nearby
stars)

Date        UT-start   Exptime   Filter  t-T0      OT        Err
                       sec               days
2023-08-19  00:00:28   2 x 20     r'     0.02339   19.50 +/- 0.02