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

Abstract

We present 4.7--22.2\,GHz Very Long Baseline Interferometry (VLBI) monitoring of the candidate pre-coalescence supermassive black hole binary SDSS~J143016.05+230344.4 ($z=0.08105$) from 2022 February to 2024 February, together with quasi-simultaneous 0.7--16.5\,GHz connected-array spectra. At all epochs, the radio emission is dominated by a single unresolved milliarcsecond core with $T_{\rm B}\gtrsim10^{7}$\,K, confining the variable emission to $\lesssim0.3$\,pc. The spectra require two self-absorbed synchrotron components: a persistent low-frequency component with $ν_{\rm p,steady}\approx0.74$\,GHz and $S_{\rm p,steady}\approx1.22$\,mJy, and a flare component whose turnover evolves from $(6.35\,{\rm GHz},0.18\,{\rm mJy})$ in 2022 February--May to $(8.61\,{\rm GHz},0.38\,{\rm mJy})$ in 2022 December and then to $(5.83\,{\rm GHz},0.25\,{\rm mJy})$ in 2023 March--April. The 15\,GHz flare fraction peaks at $\simeq80\%$ and matches the near-epoch VLBI recovery fraction, showing that the high-frequency brightening arises from a new compact synchrotron component. A second 15.2\,GHz VLBI-core brightening is detected from 2023 September to 2024 February while the source remains unresolved. Equipartition scalings imply characteristic radii of $R_{\rm eq}\sim5\times10^{-4}$\,pc for the flare and $\sim9\times10^{-3}$\,pc for the steady component, and a steep inner circumnuclear density profile, $n\propto R^{-1.7}$. The delayed radio peak is consistent with dissipation of an outflow or jet-base disturbance in a structured circumnuclear medium, while a uniform free--free absorber is disfavored.

Figures (5)

• Figure 1: Representative natural-weighted VLBI image of J1430$+$2303 at 15.2 GHz (2024 February 18; VLBA). The restoring beam ($1.2 \, \text{mas} \times 0.5 \, \text{mas}$) is shown in the lower-left corner. The rms noise is 0.027 mJy beam$^{-1}$. Contours start at $3\sigma$ and increase by factors of $\sqrt{2}$. Figure \ref{['fig:tar']} in Appendix presents the complete montage.
• Figure 2: Radio spectrum and component decomposition. Left: all available connected-array measurements (Epochs I/II/III; filled symbols), additional archival radio points (open symbols), and the best-fit total models for each epoch (solid curves). Right: decomposition into a time-independent steady SSA component (black dashed curve) plus a time-variable flare SSA component (colored dotted curves). Colored solid curves show the epoch-wise totals. VLBI-core flux densities are overplotted as stars for comparison but are not used in the fit.
• Figure 3: Natural-weighted VLBI images of J1430$+$2303. (a) - (c): VLBA images at 4.7, 6.2 and 7.6 GHz, respectively; (d) - (e): VLBA images at 15.2 GHz observed on 8 and 31 May 2022, respectively; (f) 22-GHz EVN image observed on 16 February 2023. The ellipse in the bottom-left corner is the shape of the restoring beam. The contours start at 3$\sigma$ and increase in a step of $2^{1/2}$. The observation information are listed in Table \ref{['tab:VLBIobs']} and the image parameters are presented in Table \ref{['tab:mod']}.
• Figure 4: Relative astrometric positions of the VLBI core across epochs. The origin (0,0) is the core position measured at the last epoch (14$^h$30$^m$16.0409$^s$, +23$^d$03$^m$44.53875$^s$). Error bars show the 1$\sigma$ uncertainties from the Difmap visibility-domain model fits. Measurements from different frequencies within the same epoch are plotted separately.
• Figure 5: The Corner plot shows all the one and two dimensional projections of the posterior probability distributions of the total flux density ($\rm S_{1}$) in mJy, source size ($\rm d_{1}$) in mas, relative RA ($\rm x_{1}$) in mas, and relative DEC ($\rm y_{1}$) in mas from the 15.2 GHz data (ba157a1; 2022 May 31). The red solid line indicates the best fit and the black dashed line indicates the confidence interval of $1\sigma$.