TITLE: GCN CIRCULAR NUMBER: 33547 SUBJECT: GRB 230328B: 3.6m DOT multiband optical observation DATE: 23/03/31 07:49:48 GMT FROM: Amit Kumar Ror at ARIES mitturor77894@gmail.com

Amit K. Ror, Rahul Gupta, S. B. Pandey, A. Ghosh, A. Aryan, and K. Misra (ARIES) report:

GRB 230328B was triggered by Swift-BAT at 14:54:48 UT and Fermi-GBM at 14:54:47.43 UT on 28-03-2023 (Gropp et al. 2023 GCN 33527, Dalessi et al. 2023 GCN 33531). The prompt emission was also detected by other space-based facilities like AstroSat-CZTI (Waratkar et al. 2023, GCN 33532), GRBAlpha (Dafcikova et al. 2023, GCN 33543), and Konus-Wind (Svinkin et al. 2023, GCN 33544). We searched for extended GeV emission for a temporal window of 10 ks since the GBM trigger, based on available LAT data. However, we did not find the detection of any GeV photons, and an upper limit was obtained for an energy flux < 5.2 x 10^{-10} erg/cm2/s in the energy range 100 MeV - 100 GeV with TS < 20.

We observed the field of GRB 230328B using the 3.6m Devasthal Optical Telescope located at the Devasthal observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India. We have taken multiple frames with an exposure time of 300 sec each in several optical passbands. We clearly detect the optical afterglow discovered by Swift-UVOT in each individual image. The estimated preliminary magnitude is as follows:

Date Start_UT T_start-T0 (days) Filter Exp time (sec) magnitude

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2023-03-28 21:51:54.44 ~0.29 R 300 sec 20.30 +/- 0.08

The magnitude quoted is not corrected for the galactic and host extinctions in the direction of the burst. Photometric calibration is performed using the standard stars from the USNO-B1 catalog.

The detection of the optical afterglow is consistent with the observation of Gropp et al. 2023 (GCN 33527), Pankov et al. 2023 (GCN 33528), Belkin et al. 2023 (GCN 33530), Lu et al. 2023 (GCN 33534), Catapano et al. (33535), Suresh et al. (GCN 33536), Adami et al. (GCN 33537), Gompertz et al. (GCN 33538), Komesh et al. (GCN 33539), Lu et al. 2023 (GCN 33540)

By combining our R-band magnitude with the observations of Pankov et al. 2023 (GCN 33528) and Belkin et al. 2023 (GCN 33530), we have determined the preliminary flux decay index of 0.55 +/-0.06.

This circular may be cited. The 3.6m Devasthal Optical Telescope (DOT) is a recently commissioned facility in the Northern Himalayan region of India (long: 79 41 04E, lat: 29 21 40N, alt: 2540m) owned and operated by the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital ( https://www.aries.res.in). The authors of this GCN circular thankfully acknowledge the consistent support from the staff members to run and maintain the 3.6m DOT.