TITLE: GCN CIRCULAR NUMBER: 39902 SUBJECT: GRB 250327b: Liverpool Telescope optical observations DATE: 25/03/28 09:30:33 GMT FROM: A. Bochenek at Liverpool John Moores University a.m.bochenek@2023.ljmu.ac.uk

D. A. Perley and A.Bochenek (LJMU) report:

We observed the field of GRB250327b (Bouchet et al., GCN 39888) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 1x100s exposures in the SDSS g, r, i, and z filters, starting at 2025-03-28 00:17:25 UT, approximately 3.1 hours after the trigger.

We report a detection in all our images at the position of the optical counterpart (Moskvitin et al., GCN 39889), given in the table below. Our results agree with previous optical observations (Moskvitin et al. GCN 39889; Xin et al. GCN 39890; O’Neill et al. GCN 39891; Malesani et al. GCN 39893; Lian et al. GCN 39894; Shrestha et al. GCN 39896; Ducoin et al., GCN39897).

MJD (mid) T_mid - T_0 Filter Mag. (AB) 60762.01268 3.11 hours r 18.47 ± 0.02 60762.01412 3.14 hours i 17.98 ± 0.04 60762.01556 3.18 hours g 19.87 ± 0.05 60762.01706 3.21 hours z 17.82 ± 0.06

The photometry was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.

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