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arXiv · 2510.21919

Suppressed "lump" EM signature in radiation pressure dominated accreting massive black hole binaries

Fabiola Cocchiararo·Alessia Franchini·Alessandro Lupi·Alberto Sesana
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Abstract

We investigate the impact of radiation pressure on electromagnetic signatures of accreting massive black hole binaries (MBHBs) at milli-parsec separations, using 3D hyper-Lagrangian resolution hydrodynamical simulations. We model binaries embedded in a self-gravitating circumbinary disc that evolves following an adiabatic equation of state, including viscous heating and black-body cooling. Focusing on binaries with a total mass of $10^6 \, M_{\odot}$, eccentricities $e=0,0.45,0.9$ and mass ratios $q=1, 0.7$, we find that radiation pressure significantly affects both the spectral energy distributions (SEDs) and the light curves (LCs). The emission from the mini-discs shifts from the optical towards UV frequencies and with a peak luminosity orders of magnitude higher, while the circumbinary disc becomes colder and dimmer as a result of its geometrically thinner configuration. Temporal variability is affected as well: near UV and soft-X ray fluxes are higher and more variable. Crucially, radiation pressure suppresses the characteristic "lump" formation in equal-mass circular systems, while a lump is formed for higher eccentricities without imprinting any modulation on the flux. In the circular case we still find a modulation on the cavity edge timescale at a frequency $0.36 \, f_{\rm K}$, while in eccentric binaries, only robust orbital period modulations ($f=1,2 \, f_{\rm K}$) are observed, with no modulation associated with the cavity orbital motion. Moreover, the enhanced emission from the mini-discs and streams due to radiation pressure, one redshifted, results in brighter flux in the optical G band, proving detectability of MBHBs signatures even at higher redshift ($z=0.6-1.0$). Our results reveal that radiation pressure plays a crucial role in shaping MBHBs spectral and time-domain features, with implications for their identification in time-domain surveys.

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Fabiola Cocchiararo, Alessia Franchini, Alessandro Lupi, Alberto Sesana. 2025-10-24. Suppressed "lump" EM signature in radiation pressure dominated accreting massive black hole binaries. https://arxiv.org/abs/2510.21919

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