A Delayed Radio Flare Traces Kinetic Energy Injection in the SMBHB Candidate SDSS~J143016.05+230344.4

Abstract

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. 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. At all epochs, the radio emission is dominated by a single unresolved milliarcsecond core with $T_{\mathrm{B}}\gtrsim 10^7$ K, constraining the variable emission to $\lesssim 0.3$ pc. The broadband spectra require two synchrotron self-absorbed components: a persistent low-frequency component with $\hat{I}{½}_{\mathrm{p},\mathrm{steady}}\approx 0.74$ GHz and $S_{\mathrm{p},\mathrm{steady}}\approx 1.22$ mJy, and a flare component whose turnover evolves from $(6.35\mathrm{GHz},0.18\mathrm{mJy})$ in 2022 February-May to $(8.61\mathrm{GHz},0.38\mathrm{mJy})$ in 2022 December, and then to $(5.83\mathrm{GHz},0.25\mathrm{mJy})$ in 2023 March-April. The flare contribution at 15 GHz reaches $\sim 80\%$ 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 2023 September and 2024 February, while the source remains unresolved. Equipartition scalings imply characteristic radii of $\sim 5\times 10^{-4}$ pc for the flare and $\sim 9\times 10^{-3}$ pc for the steady component, and indicate a steep inner circumnuclear density profile, $n\propto R^{-1.7}$. The delayed radio flare is best explained by dissipation in an outflow or jet-base disturbance propagating through a structured circumnuclear medium.

Under Review