Signatures of hierarchical mergers in black hole spin and mass distribution

Hiromichi Tagawa,Imre Bartos,Zoltán Haiman,Kazuyuki Omukai,Bence Kocsis

doi:10.1093/mnras/stab2315

Abstract

ABSTRACT Recent gravitational wave (GW) observations by LIGO/Virgo show evidence for hierarchical mergers, where the merging BHs are the remnants of previous BH merger events. These events may carry important clues about the astrophysical host environments of the GW sources. In this paper, we present the distributions of the effective spin parameter (χeff), the precession spin parameter (χp), and the chirp mass (mchirp) expected in hierarchical mergers. Under a wide range of assumptions, hierarchical mergers produce (i) a monotonic increase of the average of the typical total spin for merging binaries, which we characterize with $\scriptstyle{{\bar{\chi }}_\mathrm{typ}\equiv \overline{(\chi _\mathrm{eff}^2+\chi _\mathrm{p}^2)^{1/2}}}$, up to roughly the maximum mchirp among first-generation (1g) BHs, and (ii) a plateau at ${\bar{\chi }}_\mathrm{typ}\sim 0.6$ at higher mchirp. We suggest that the maximum mass and typical spin magnitudes for 1g BHs can be estimated from ${\bar{\chi }}_\mathrm{typ}$ as a function of mchirp. The GW data observed in LIGO/Virgo O1–O3a prefers an increase in ${\bar{\chi }}_\mathrm{typ}$ at low mchirp, which is consistent with the growth of the BH spin magnitude by hierarchical mergers at ∼2σ confidence. A Bayesian analysis using the χeff, χp, and mchirp distributions suggests that 1g BHs have the maximum mass of ∼15–$30\, {\rm M}_\odot$ if the majority of mergers are of high-generation BHs (not among 1g–1g BHs), which is consistent with mergers in active galactic nucleus discs and/or nuclear star clusters, while if mergers mainly originate from globular clusters, 1g BHs are favoured to have non-zero spin magnitudes of ∼0.3. We also forecast that signatures for hierarchical mergers in the ${\bar{\chi }}_\mathrm{typ}$ distribution can be confidently recovered once the number of GW events increases to ≳ O(100).

Highlights

Recent detections of gravitational waves (GWs) by LIGO (Aasi et al 2015) and Virgo (Acernese et al 2015) have shown evidence for a high rate of black hole (BH)–BH and neutron star (NS)–NS mergers in the Universe (Abbott et al 2019, 2021; Venumadhav et al 2020)
We focus on distributions of the effective and precession spin parameters (χ eff and χ p) and the chirp mass, and predict characteristic features in them expected from hierarchical mergers
We identify and characterize features expected in hierarchical mergers using mock GW data, and find that intrinsic properties of 1g BHs can be constrained by recovering the features, which enables us to distinguish astrophysical models

Summary

Introduction

Recent detections of gravitational waves (GWs) by LIGO (Aasi et al 2015) and Virgo (Acernese et al 2015) have shown evidence for a high rate of black hole (BH)–BH and neutron star (NS)–NS mergers in the Universe (Abbott et al 2019, 2021; Venumadhav et al 2020). A possible list of these currently includes isolated binary evolution (e.g. Dominik et al 2012; Kinugawa et al 2014; Belczynski et al 2016; Spera et al 2019) accompanied by mass transfer (Inayoshi et al 2017; Pavlovskii et al 2017; van den Heuvel, Portegies Zwart & de Mink 2017), common envelope ejection Several GW events were reported by LIGO and Virgo whose measured physical properties pose interesting constraints on their astrophysical origin. These include nine candidates for mergers in the upper-mass gap (∼50–130 M ), such as GW190521

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Conclusion