Monte Carlo Spectra of Accreting Supermassive Black Hole Binaries 233 views

We present a Monte Carlo radiative transfer (MCRT) post-processing analysis of radiation magneto-hydrodynamic (RMHD) simulations of accreting supermassive black hole binaries (SMBHBs), applied to both a circumbinary disk (CBD) and an isolated mini-disk configuration. The RMHD simulations, performed with Athena++, evolve the gas dynamics and thermodynamics but treat radiative transfer in the frequency-integrated (grey) approximation, producing no spectral information. We post-process the simulation fluid variables with an MCRT code to produce frequency-resolved spectral energy distributions in the EUV to soft X-ray band (0.01–30 keV). For the CBD, we find that the emergent spectrum peaks between 10^−2 and 10^−1 keV at ~0.4–1% of the Eddington luminosity, consistent with the sub-Eddington accretion rate of the RMHD simulation. Under the tabulated Rosseland-mean (“user”) opacity, Compton scattering has negligible effect on the spectrum, and the key spectral differences arise from whether inner-boundary photons are included; the screened and unscreened spectra diverge primarily in the hydrogen ionization regime and in energy ranges that are observationally challenging, with the spectra remaining similar in the practically observable bands. Under the analytic free-free opacity, by contrast, Compton scattering suppresses the luminosity by nearly two orders of magnitude. For the mini-disk, the spectrum peaks at higher energies (~10^−1 keV) than the CBD, consistent with the hotter, more compact inner disk. A dedicated camera-tracking light curve pipeline produces synthetic light curves at arbitrary observer viewing angles; the resulting light curve shows a sharp flux peak recurring once per orbital period at Fν ~ 1.6 × 10^35 erg s−1 Hz−1, and the emission is azimuthally symmetric at the inclination studied. These results demonstrate the importance of opacity model choice for spectral predictions of SMBHB accretion flows, and establish a pipeline for producing synthetic EM observables from RMHD simulations for comparison with current and future time-domain surveys.

BS (Bachelor of Science)

black hole; simulation; radiative transfer; monte carlo ; circumbinary disk; computational astrophysics

English

2026-05-13