TITLE: GCN CIRCULAR NUMBER: 39501 SUBJECT: GRB 250221A: ATCA Detections and Upper Limits DATE: 25/02/26 21:23:21 GMT FROM: agul8829@uni.sydney.edu.au
A. Gulati (USyd), G. E. Anderson (Curtin), Claire Morley (Curtin), S. Chastain (UNM), J. K. Leung (UofT/HUJI), A. J. van der Horst (GWU), and L. Rhodes (TSI/McGill) on behalf of the ATCA PanRadio GRB collaboration
We observed long GRB 250221A (Palmer et al., GCN 39396) as part of The Australia Telescope Compact Array (ATCA) "PanRadio GRB" Large Project C3542 (PI: G. Anderson) at 5.5 and 9 GHz on 2025-02-21 (starting 6 minutes post-burst, for 10 hours), and on 2025-02-23 (starting 2.2 days post burst for 4 hours).
No radio sources were detected near the Swift/XRT enhanced position (Beardmore et al., GCN 39404) in the first epoch, with a 3-sigma upper limit of 24 uJy at 9 GHz. We detect a radio counterpart in the second epoch at a position consistent with the Swift/XRT enhanced position, with a flux of 233 +/- 15 uJy at 9 GHz. This value significantly exceeds the 10 GHz VLA detection at 1.95 days (Ricci et al., GCN 39433), possibly due to strong interstellar scintillation near the typical 10 GHz transition frequency.
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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