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Electromagnetic Signatures of Supermassive Binary Black Holes. I. Thermal Synchrotron, Self-Lensing Flares, and Jet Precession

Hong-Xuan Jiang, Xinyu Li, Yosuke Mizuno, Ziri Younsi, Christian M. Fromm

Abstract

The recent evidence for a nanohertz gravitational wave background from Pulsar Timing Arrays highlights the urgent need to identify electromagnetic counterparts to supermassive binary black holes. Here, we perform global 3D general relativistic magnetohydrodynamic (GRMHD) simulations of a secondary black hole (mass ratio $q=0.1$) interacting with a Magnetically Arrested Disk around a primary black hole using a time-dependent superposed Kerr-Schild metric and post-processed general relativistic radiation transfer calculations based on thermal electron distribution function (eDF). We explore three orbital configurations: a vertical impact orbit, a coplanar embedded orbit, and a high-spin, eccentric, inclined scenario. Despite clear orbital periodicity and recurrent shock formation, the thermal synchrotron light curves frequently lack expected shock-induced flares. In vertical impacts, shock brightenings are typically sub-dominant to the stochastic MAD variability of the primary black hole, unless viewed at specific alignment phases. Conversely, coplanar orbits produce distinctive, rapid flares driven by gravitational self-lensing. We identify a frequency-dependent emission hierarchy: the primary black hole dominates sub-millimeter flux, while the secondary dominates near-infrared emission due to higher electron temperatures in thermal eDF. Finally, spin-orbit coupling drives Lense-Thirring precession, yielding twisted, wobbling jets reminiscent of OJ~287. Crucially, we demonstrate that intrinsic MAD turbulence may easily mask shock-induced flares at radio frequencies. We strongly advocate coordinated sub-millimeter and near-infrared monitoring to robustly isolate supermassive binary black hole self-lensing signatures.

Electromagnetic Signatures of Supermassive Binary Black Holes. I. Thermal Synchrotron, Self-Lensing Flares, and Jet Precession

Abstract

The recent evidence for a nanohertz gravitational wave background from Pulsar Timing Arrays highlights the urgent need to identify electromagnetic counterparts to supermassive binary black holes. Here, we perform global 3D general relativistic magnetohydrodynamic (GRMHD) simulations of a secondary black hole (mass ratio ) interacting with a Magnetically Arrested Disk around a primary black hole using a time-dependent superposed Kerr-Schild metric and post-processed general relativistic radiation transfer calculations based on thermal electron distribution function (eDF). We explore three orbital configurations: a vertical impact orbit, a coplanar embedded orbit, and a high-spin, eccentric, inclined scenario. Despite clear orbital periodicity and recurrent shock formation, the thermal synchrotron light curves frequently lack expected shock-induced flares. In vertical impacts, shock brightenings are typically sub-dominant to the stochastic MAD variability of the primary black hole, unless viewed at specific alignment phases. Conversely, coplanar orbits produce distinctive, rapid flares driven by gravitational self-lensing. We identify a frequency-dependent emission hierarchy: the primary black hole dominates sub-millimeter flux, while the secondary dominates near-infrared emission due to higher electron temperatures in thermal eDF. Finally, spin-orbit coupling drives Lense-Thirring precession, yielding twisted, wobbling jets reminiscent of OJ~287. Crucially, we demonstrate that intrinsic MAD turbulence may easily mask shock-induced flares at radio frequencies. We strongly advocate coordinated sub-millimeter and near-infrared monitoring to robustly isolate supermassive binary black hole self-lensing signatures.
Paper Structure (18 sections, 14 equations, 10 figures, 1 table)

This paper contains 18 sections, 14 equations, 10 figures, 1 table.

Table of Contents

  1. Introduction
  2. Numerical Method
  3. Results
  4. MHD process during BH-disk interaction
  5. Observational signatures of runs $\tt{VT}$ and $\tt{CP}$
  6. Synthetic horizon scale images of the BBH system
  7. Morphology and Observational Signatures of the Precessing Binary
  8. Summary and Discussions
  9. Simulation setup and detailed prescriptions
  10. Binary orbit and spin evolution
  11. Superposed Kerr-Schild binary metric
  12. GRMHD setup on a time-dependent superposed Kerr-Schild spacetime
  13. GRRT equations, electron thermodynamics, and post-processing setup
  14. Appendix information
  15. Power spectrum of the secondary accretion rate
  16. ...and 3 more sections

Figures (10)

  • Figure 1: Mass accretion rates $\dot{M}$ and magnetic fluxes $\Phi_{\rm B}$ measured at the event horizons of the primary and secondary BHs. (a) Accretion rate of the primary BH, $\dot{M}_{\rm BH1}$, for all runs (black: run $\tt{VT}$; red: run $\tt{CP}$; green: run $\tt{EP}$; blue: run $\tt{BASE}$). (b) Accretion rate of the secondary BH, $\dot{M}_{\rm BH2}$, for runs $\tt{VT}$, $\tt{CP}$, and $\tt{EP}$ (colors as in panel a). (c) Horizon-threading magnetic flux of the primary BH, $\Phi_{\rm B,BH1}$, for all runs. (d) Horizon-threading magnetic flux of the secondary BH, $\Phi_{\rm B,BH2}$, for runs $\tt{VT}$, $\tt{CP}$, and $\tt{EP}$. In all runs, the secondary BH is ramped in gradually over $t=10{,}000$–$11{,}000\,\rm M$; consequently, runs $\tt{VT}$, $\tt{CP}$, and $\tt{BASE}$ are identical for $t<10{,}000\,\rm M$, and their divergences occur only after $t\gtrsim 11{,}000\,\rm M$. (e) Close-up view of the accretion rate evolution for run $\tt{VT}$ during $t \in [1.4, 1.6] \times 10^{4}\,\rm M$, comparing $\dot{M}_{\rm BH1}$ (black solid; left axis) and $\dot{M}_{\rm BH2}$ (red dashed; right axis).
  • Figure 2: Thermal synchrotron light curves from GRRT post-processing. Panels (a)--(d) show $t=26000$--$30000\,M$ time series, with the binary-run flux on the left axis and the single-BH control run $\tt{BASE}$ on the right axis (orange dotted). The top row shows run $\tt{VT}$, and the middle row shows run $\tt{CP}$. (a) run $\tt{VT}$ at $230\,\mathrm{GHz}$ for $\phi_{\rm obs}=90^\circ$ (blue) and $\phi_{\rm obs}=0^\circ$ (purple), and for $\phi_{\rm obs}=90^\circ$ with plasma within $r<10\,r_{\rm g}$ of the secondary excised (green, lower opacity). (b) Same as (a) at $138,\mathrm{THz}$; the inset ($t=27600$–$28000\,\rm M$) highlights a particularly strong shock-driven flare during which the full-domain flux is $\sim3\times$ higher than the excised case, whereas differences are otherwise modest. (c) run $\tt{CP}$ at $230\,\mathrm{GHz}$ compared to run $\tt{BASE}$. (d) Same as (c) at $138\,\mathrm{THz}$. (e,f) Orbit-folded run $\tt{CP}$ light curves versus orbital phase at $230\,\mathrm{GHz}$ and $138\,\mathrm{THz}$. Faint curves show individual orbits; the thick curve and shaded band give the mean and $\pm1\sigma$.
  • Figure 3: Synthetic horizon-scale total-intensity ($I_\nu$) images at $230\,\mathrm{GHz}$. Panels (a--c): run $\tt{VT}$ with $\phi_{\rm obs}=90^\circ$ at three representative times. Movie: https://youtu.be/Z201nzWcnss. Panels (d--f): run $\tt{VT}$ with $\phi_{\rm obs}=0^\circ$ at the corresponding epochs. Movie: https://youtu.be/R_2J4vxRqgg. Panels (g--i): run $\tt{CP}$ with $\phi_{\rm obs}=90^\circ$ at three representative times. Movie: https://youtu.be/1NRrb8LMbWI. The color scale shows $I_\nu$ in $\rm Jy\,pixel^{-1}$ (logarithmic), and each panel is labeled by its snapshot time and viewing geometry.
  • Figure 4: Synthetic total-intensity ($I_\nu$) images from run $\tt{EP}$ at $86\,\mathrm{GHz}$ at three representative times. Panel (a) $t=21,000\,\rm M$; Panel (b) $t=24,560\,\rm M$; Panel (c) $t=26,300\,\rm M$. The color scale shows $I_\nu$ in $\mathrm{Jy\,pixel^{-1}}$ (logarithmic), and each panel is annotated with the snapshot time and viewing geometry. Movie: https://youtu.be/dOQ8ppdLvWU.
  • Figure 5: Gas density slice from run $\tt{VT}$ at $t=26{,}410\,\rm M$ (edge-on slice), overlaid with the meshblock boundaries.
  • ...and 5 more figures