TITLE: GCN CIRCULAR NUMBER: 39773 SUBJECT: GRB 250309B/AT2025dws: further ATCA observations DATE: 25/03/18 17:15:37 GMT FROM: James Leung at U. Toronto / HUJI jamesk.leung@utoronto.ca

J. K. Leung (U. Toronto/HUJI), G. E. Anderson (Curtin University), A. J. van der Horst (GWU), L. Rhodes (TSI/McGill), Maria Drout (U. Toronto), Andrew Hughes (U. Oxford) report on behalf of a larger collaboration:

We report a second epoch of Australia Telescope Compact Array (ATCA) radio observations centred in the direction of the candidate optical counterpart AT2025dws (Stein et al., GCN 29639; Pérez-Fournon et al., GCN 29643; Alexander et al., GCN39645; Stein et al, GCN 39644; Perley et al. GCN 39646; Wang et al., GCN 39648; Ducoin et al., GCN 39650; Shin et al., GCN 39654) to GRB 250309B (Preis and Greiner, GCN 39629; McDermott et al., GCN 39642; Page and Evans, GCN 39649; Kozyrev et al., GCN 29652; and Frederiks et al., GCN 39655). These observations follow an earlier detection of the radio counterpart by the ATCA (An et al., GCN 39699).

The observations were taken at a mean epoch of UT12:57 on 2025-03-13T12:57 at central frequencies 5.5, 9, and 16.7 GHz. In our preliminary analysis, we detect a clear radio source at the position of the candidate optical and radio counterparts in both the 5.5 and 9 GHz images (0.24+/-0.04 mJy and 0.30+/-0.03 mJy, respectively) and attain a 5-sigma non-detection limit of <0.45 mJy/beam at 16.7 GHz.

We thank the CSIRO Space and Astronomy staff for supporting these observations. We acknowledge the Gomeroi people as the traditional owners of the Observatory site. The Australia Telescope Compact Array is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO.

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