Dual-jet radio monitoring of Mrk 501 yields candidate binary supermassive black hole with modeled 100-year merger

The Max Planck-led team analyzed VLBI and single-dish data spanning dozens of epochs to track a newly detected jet whose position angle changed by several degrees over weeks, repeating in a manner explained by a binary orbital plane. This supplies the first claimed direct imaging signature of a close pair, addressing the long-standing absence of resolved binaries despite frequent galaxy mergers. The 100-year coalescence estimate derives from fitting jet wobble to a precessing binary model rather than from measured orbital decay.

The result bears on the final-parsec problem: if the inferred separation is already sub-parsec, gas-driven or stellar-scattering mechanisms must have operated efficiently. Yet the same modeling that yields the short merger time also predicts detectable gravitational-wave background contributions at nanohertz frequencies, a prediction testable by pulsar-timing arrays within the next decade. Original coverage omitted quantitative uncertainties on the separation and ignored possible non-binary explanations such as jet instabilities or disk warping.

A 2022 Einstein-ring candidate event offers an independent geometric test but remains single-epoch and requires confirmation via continued monitoring. High-resolution space-VLBI or ngEHT observations at 230 GHz could resolve the two cores directly and measure relative proper motion, falsifying or strengthening the binary hypothesis within five years.