TITLE: GCN CIRCULAR NUMBER: 43040 SUBJECT: GRB 251129A: e-MERLIN C-band observations DATE: 25/12/09 00:50:36 GMT FROM: Tao An at SHAO, CAS antao@shao.ac.cn

Tao An (SHAO), Ailing Wang (IHEP), Emmanuel Bempong-Manful (UoM), Zigao Dai (USTC), and collaborators report on e-MERLIN observations of GRB 251129A.

We observed GRB 251129A (GCN 42877, 42879, 42880, 42882, 42884, 42887, 42888, 42891, 42897, 42900, 42902, 42959, 42972, 42985) with the e-MERLIN array in the C band between 2025 December 3, 17:40 UT and December 5, 04:19 UT (project RR20009). Four antennas, Pickmere (Pi), Darnhall (Da), Knockin (Kn), and Cambridge (Cm), participated in the observation.

Observing setup: - Frequency coverage: 4.82–5.33 GHz (C band), total bandwidth 512 MHz - Antennas: Pi, Da, Kn, Cm - Polarizations: Right and left circular - Spectral setup: 4 × 128 MHz spectral windows, 128 × 1 MHz channels per window (post-averaging) - Observation span: 2025-12-03 17:40 UT to 2025-12-05 04:19 UT

The data were processed using the standard e-MERLIN pipeline followed by calibration and imaging in CASA.

No significant radio emission was detected at the position of GRB 251129A. We place a 3σ upper limit of 0.066 mJy beam⁻¹ on the flux density.

We encourage coordinated and continued radio follow-up to further constrain the flux density and probe potential variability at C band and complementary frequencies. We thank the e-MERLIN TAG and operations teams for their support.

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