TITLE: GCN CIRCULAR NUMBER: 43228 SUBJECT: GRB 251129A / EP251129a: VLA detection of a radio counterpart DATE: 25/12/24 10:46:36 GMT FROM: Tao An at SHAO, CAS <antao@shao.ac.cn> Tao An (SHAO), Ailing Wang (IHEP), Zigao Dai (USTC), and collaborators report on VLA observations of GRB 251129A. We observed GRB 251129A, also reported as EP251129a (e.g., GCNs 42877, 42879, 42880, 42882, 42884, 42887, 42888, 42891, 42897, 42900, 42902, 42959, 42972, 42985), with the Karl G. Jansky Very Large Array (VLA) under program VLA/25B-403 (AA627) on 2025 December 15 UT, approximately 16.0 days after the burst (relative to the SVOM/ECLAIRs trigger at 2025 Nov 29 02:32:05 UT). Observations were obtained in C band (4–8 GHz). A significant radio source is detected at the GRB position. Using standard wideband imaging, we measure: - Position (J2000): RA = 14:57:18.867 ± 0.044 s Dec = +79:17:34.74 ± 0.09″ - Peak flux density: 75.1 ± 8.4 μJy beam⁻¹ at 5 GHz 62.2 ± 9.4 μJy beam⁻¹ at 7 GHz The implied in-band spectral index across 5–7 GHz is consistent with being flat within uncertainties. The VLA position is consistent with the reported optical afterglow position (e.g., GCN 42883) and is also consistent with the X-ray localizations reported by SVOM/MXT, Swift/XRT, and Einstein Probe (e.g., GCN 42877, 42884, 42891), supporting the association of this radio source with GRB 251129A. This VLA detection provides an important radio counterpart to the unusual multi-wavelength behaviour of GRB 251129A reported in earlier GCNs, including its ultra-long high-energy emission and peculiar X-ray temporal and spectral evolution. We note that our eMERLIN observations reported in GCN 43040 resulted in a non-detection at C band between 2025 December 3-5, placing a 3σ upper limit of 0.066 mJy beam⁻¹ on the flux density. The new VLA detection therefore indicates a rising or previously sub-threshold radio afterglow component emerging on week timescales. More radio observations are encouraged to measure the temporal decay and spectral evolution and to distinguish between internal-engine and external-shock interpretations of this event. We thank the VLA Time Allocation Committee for approving these observations and the NRAO VLA operations and scheduling teams for the rapid execution and support that made this time-critical observation possible. View this GCN Circular online at https://gcn.nasa.gov/circulars/43228. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...