A delayed radio flare traces kinetic energy injection in the supermassive black hole binary candidate SDSS J143016.05+230344.4

Context . SDSS J143016.05+230344.4 ( z = 0.08105) has been proposed as a candidate pre-coalescence supermassive black hole binary and shows remarkable multiwavelength variability. Its radio evolution provides a direct probe of the compact emitting region and of the physical origin of the late-time activity. Aims . We aim to localize the variable radio emission, characterize its spectral evolution, and constrain whether the radio brightening is produced by a newly emerging compact component, external absorption, or dissipation in a structured circumnuclear environment. Methods . We analyzed 4.7–22.2 GHz very long baseline interferometry (VLBI) observations obtained between 2022 February and 2024 February, together with quasi-simultaneous connected-array spectra covering 0.7–16.5 GHz. We combined the VLBI brightnesstemperature and compactness constraints with spectral decomposition and equipartition-based estimates of source size and ambient density structure. Results . At all epochs, the radio emission is dominated by a single unresolved milliarcsecond core with T B ≳ 10 7 K, constraining the variable emission to ≲0.3 pc. The broadband spectra require two synchrotron self-absorbed components: a persistent low-frequency component with ν p,steady ≈ 0.74 GHz and S p,steady ≈ 1.22 mJy, and a flare component whose turnover evolves from (6.35 GHz, 0.18mJy) in February–May 2022 to (8.61 GHz, 0.38 mJy) in December 2022, and then to (5.83 GHz, 0.25 mJy) in March–April 2023. The flare contribution at 15 GHz reaches ~80% and matches the near-epoch VLBI recovery fraction, showing that the high-frequency brightening arises from a newly formed compact synchrotron component. A second brightening of the 15.2 GHz VLBI core is detected between September 2023 and February 2024, while the source remains unresolved. Equipartition scalings imply characteristic radii of ~5 × 10 −4 pc for the flare and ~9 × 10 −3 pc for the steady component, and indicate a steep inner circumnuclear density profile, n ∝ R −1.7 . Conclusions . The delayed radio flare is best explained by dissipation in an outflow or jet-base disturbance propagating through a structured circumnuclear medium.