Radio Follow-up of a Subthreshold Gamma-Ray Burst in the Sky Localization Area of GW241125

Since the Fermi satellite’s identification of a candidate γ -ray burst (GRB) temporally coincident with GW150914, several tentative, and often debated, associations between electromagnetic (EM) transients and gravitational-wave (GW) signals from binary black hole (BBH) mergers have been reported. One such event, GW241125, was identified during the fourth observing run (O4) of Advanced LIGO and found to be spatially (within the large GW localization uncertainty) and temporally coincident with a subthreshold GRB detected by the Neil Gehrels Swift Observatory (Swift) Burst Alert Telescope (BAT) Gamma-Ray Urgent Archiver for Novel Opportunities (GUANO). Here, we present results from a radio follow-up campaign targeting the BAT-GUANO localization region, carried out with the Karl G. Jansky Very Large Array (VLA). We also reanalyze Swift/X-Ray Telescope observations of the field, and combine these results with optical upper limits. Our analysis constrains the isotropic kinetic energy of a putative relativistic jet launched in the BBH merger to ≲3 × 10 50 erg for n ISM = 1.0 cm −3 . We also discuss both the challenges and diagnostic power of radio follow-up in assessing candidate BBH–GRB associations, and present projections for analogous studies in the LIGO–Virgo–KAGRA observing run 5 (O5), and in the era of next-generation ground-based instrumentation. The enhanced sensitivity and localization capabilities of detector networks such as Cosmic Explorer and the Einstein Telescope, paired with the enhanced sensitivity of next-generation radio interferometers such as the next-generation VLA and the Square Kilometre Array, will significantly strengthen coordinated multimessenger follow-up of BBHs. These next-generation facilities are likely to probe whether BBHs host relativistic ejecta.