TITLE: GCN CIRCULAR NUMBER: 39646 SUBJECT: Liverpool Telecope observations of AT 2025dws / GRB 250309B DATE: 25/03/10 03:52:06 GMT FROM: Daniel Perley at Liverpool JMU d.a.perley@ljmu.ac.uk

D. A. Perley (LJMU), A. Bochenek (LJMU), and A. Y. Q. Ho (Cornell) report:

We obtained imaging observations of AT2025dws (Stein et al., GCN 39639), a possible optical counterpart of GRB 250309B (Preis & Greiner, GCN 39629; McDermott et al., GCN 39635) with IO:O on the Liverpool Telescope on UT 2025 March 10 between 01:46:53 and 02:01:08 UT. Two 100s exposures were acquired in each of the g, r, i, and z bands. The source was weakly detected in all four filters. We report the following photometry, calibrated relative to SDSS standards (dt is reported relative to the GRB 250309B GBM trigger time; McDermott et al., GCN 39642):

MJD dt filter mag err 60744.07423 0.7558 g 20.47 0.19 60744.07708 0.7587 r 20.30 0.14 60744.07991 0.7615 i 20.07 0.19 60744.08274 0.7643 z 19.44 0.17

The red colours and significant fading are consistent with the interpretation of this source as the afterglow (see also Pérez-Fournon et al., GCN 39643; Stein et al., GCN 39644; Alexander et al., 39645).

No host galaxy subtraction or extinction correction has been performed.

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