M. Paola Vaccaro

M. Paola Vaccaro

she/her

PhD candidate

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Pages

Posts

Future Blog Post

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Blog Post number 4

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Blog Post number 3

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Blog Post number 2

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Blog Post number 1

less than 1 minute read

Published:

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portfolio

publications

Impact of gas hardening on the population properties of hierarchical black hole mergers in AGN discs

Published in A&A, 2024

Hierarchical black hole (BH) mergers in active galactic nuclei (AGNs) are unique among formation channels of binary black holes (BBHs) because they are likely associated with electromagnetic counterparts and can e ciently lead to the mass growth of BHs. Here, we explore the impact of gas accretion and migration traps on the evolution of BBHs in AGNs. We have developed a new fast semi-analytic model, that allows us to explore the parameter space while capturing the main physical processes involved. We find that an e ective exchange of energy and angular momentum between the BBH and the surrounding gas (i.e., gas hardening) during inspiral greatly enhances the e ciency of hierarchical mergers, leading to the formation of intermediate-mass BHs (up to 104 M) and triggering spin alignment. Moreover, our models with e cient gas hardening show both an anticorrelation between the BBH mass ratio and the e ective spin and a correlation between the primary BH mass and the e ective spin. In contrast, if gas hardening is ine cient, the hierarchical merger chain is already truncated after the first two or three generations. We compare the BBH population in AGNs with other dynamical channels as well as isolated binary evolution.

Recommended citation: Vaccaro, M.P. et al., A&A, 685, A51 (2024)
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The role of migration traps in the formation of binary black holes in AGN disks

Published in arxiv, 2025

Binary black holes (BBHs) forming in the accretion disks of active galactic nuclei (AGNs) represent a promising channel for gravitational-wave production. BBHs are typically expected to originate at migration traps, i.e. radial locations where the Type I migration of embedded stellar-mass black holes (BHs) transitions from outwards to inwards. In this work, we test this assumption by explicitly simulating the radial migration of BH pairs in AGN disks under different torque prescriptions, including thermal effects and the switch to Type II migration. We map where and when binaries form as a function of supermassive BH (SMBH) mass, disk viscosity, and migrating BH mass. We find that, for SMBH masses below $M_\bullet^{\rm thr} \simeq 3\times 10^7 M_\odot$, the majority of pair-up events occur near migration traps (roughly 80%). In contrast, for higher SMBH masses, differential migration dominates and off-trap pair-ups prevail. Most notably, disks without migration traps ($\alpha = 0.01$, $M_\bullet \gtrsim 10^8 M_\odot$) present a significant overdensity of pair-ups at $R\simeq 10^2 R_{\rm S}$ due to traffic-jam-like accumulations where the gamma profile changes slope sharply. We also investigate hierarchical BBH formation, showing that higher-generation pair-ups cluster more tightly around trap or traffic-jam radii. Our results provide realistic prescriptions for BBH pair-up locations and timescales, highlighting the limitations of assuming fixed BBH formation sites

Recommended citation: Vaccaro, M. P., Seif, Y., & Mapelli, M., 2025, arXiv:2508.03637.
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talks

teaching

Teaching experience 1

Undergraduate course, University 1, Department, 2014

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Teaching experience 2

Workshop, University 1, Department, 2015

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