TITLE: GCN CIRCULAR NUMBER: 31016 SUBJECT: GRB 211023B: NUTTelA-TAO / BSTI Early Optical Limits (Preliminary) DATE: 21/10/28 01:28:54 GMT FROM: Bruce Grossan at LBNL/UCB SSL Bruce_Grossan@lbl.gov

B. Grossan (UCB, NU), T. Komesh (NU), Z. Maksut (NU), M. Krugov (FAI), E. Linder (UCB, NU), E. Abdikamalov (NU), G. F. Smoot (HKUST, UCB, NU), report on behalf of the Energetic Cosmos Laboratory:

The Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO) observed the field of GRB 211023B soon after receipt an automated GCN / BAT position alert, observing in Sloan g', r' and i' bands, with the Burst Simultaneous Three-Channel Imager (BSTI; Grossan, Kumar & Smoot 2019, JHEA, 32, 14).

We started observations at 21:26:33 UT on 2021-10-23, 1241 s after the BAT trigger. Observations were made partly cloudy conditions starting at only about 18 deg. target altitude. No source consistent with the XRT (P. Evans et al., GCN Circ. 30966) or UVOT position (S. Dichiara; GCN Circ. 30960) was detected. Note that these observations provide essentially full-time coverage, simultaneous in all three bands. We report the following results:

start time t-t0(s) end time UL g' UL r' ULi' exposure_time (s) ------------ -------- ------------ ------ ------ ------ -------------------- 21:26:33 1241 21:27:33 17.6 18.7 17.8 60 21:27:56 1324 22:47:56 18.5 19.5 18.7 300

start time is in UT. t-t0(s) gives the time since trigger, in seconds. UL g', r', i', gives the 5 sigma upper limit sensitivity in magnitudes in the respective filter, for images co-added to the given exposure time. The first row in the table corresponds to co-adds of an initial short exposure image sequence of 7.5 s for g' and i', and 0.3 s for r' (these sub-second exposures are read-noise suppressed by our EMCCD cameras, with high gain electron multiplication active; other images are taken in conventional CCD operation mode). The second row corresponds to co-adds from a continuing series of 15 s exposures for g' and i', and 3 s for r', uninterrupted for the 80 minutes. Calibration was done with the 2-5 bright Pan-STARRS catalog stars on our images.

We caution the reader that these are preliminary results, without color or other corrections. Please also note that times are approximate. ---------------------------------- NU = Nazarbayev University, Nur-Sultan, Kazakhstan UCB = University of California, Berkeley, USA HKUST = Hong Kong University of Science and Technology FAI = Fesenkov Astrophysical Institute, Kazakhstan

The NUTTelA-TAO Team acknowledges the support of the staff of the Assy-Turgen Astrophysical Observatory, Almaty, Kazakhstan, and the Fesenkov Astrophysical Institute, Almaty, Kazkhstan.