TITLE: GCN CIRCULAR NUMBER: 42869 SUBJECT: GRB 250702B: detection of the compact radio counterpart 139 days after the burst DATE: 25/11/28 12:16:56 GMT FROM: ailing.wang.wal@gmail.com

Ailing Wang (IHEP), Tao An, Yuanqi Liu (SHAO), Jinjun Geng, Songbo Zhang, Xuefeng Wu (PMO), Xinwen Shu (ANU) et al. report:

We are conducting a multi-telescope, multi-epoch, multi-frequency radio campaign on GRB 250702BCD to trace its long-term afterglow evolution. Previous radio observations from other facilities have been reported by independent groups (e.g. GCNs 40983, 40985, 41053, 41059, 41061). Here we report results from Karl G. Jansky Very Large Array (VLA) observations on 2025 November 18 in B-array configuration. The data were calibrated and imaged with standard procedures in CASA. At C band (the central frequency of 6 GHz), we detect a radio source at the position of the known counterpart to GRB 250702B. The source is unresolved with a synthesized beam of 1.42 arcsec × 2.40 arcsec (PA = 28 deg) and has a peak flux density of ≈ 200 μJy beam⁻¹. The measured peak position is RA (J2000) = 18:58:45.5648 Dec (J2000) = -07:52:26.235 which is consistent with previously reported radio positions of the counterpart (GCN 41053). Complementary observations with eMERLIN and the European VLBI Network (EVN) in September yield flux densities consistent with the VLA measurement. This agreement indicates that the bulk of the radio emission, arising from long-lived synchrotron radiation (Levan et al. 2025, O'Connor et al. 2025), remains confined to a compact component on milliarcsecond to sub-arcsecond scales.

This late-time detection at approximately 139 days after the 2025 July 2 trigger provides an important additional point on the radio light curve and confirms the persistence of a compact radio source. The full multi-epoch, multi-resolution dataset will be used to characterize the long-lived synchrotron emission and continued long-term monitoring is scheduled. We thank the TACs and operations staff of the VLA, eMERLIN, EVN, and ATCA for approving and supporting these observations.

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