Formation and merging of Mass Gap Black Holes in Gravitational Wave Merger Events from Wide Hierarchical Quadruple Systems

TL;DR

This paper explores how hierarchical quadruple systems can produce mass-gap black holes through sequential mergers, potentially explaining certain gravitational wave observations.

Contribution

It introduces a new formation channel for mass-gap black holes via secular evolution in quadruple systems leading to observable gravitational wave events.

Findings

Mass-gap black holes can form from neutron star mergers in quadruple systems.

Sequential mergers in hierarchical quadruples produce high mass ratio, eccentric black hole mergers.

This channel may explain specific LIGO/Virgo gravitational wave detections.

Abstract

We investigate secular evolution in hierarchical quadruple systems as a formation channel of mass-gap black holes (with masses of about $3-5$ $M_{\odot}$) in systems that will eventually lead to binary black hole mergers detectable by ground-based gravitational wave detectors (LIGO/Virgo). We show that in a 3+1 hierarchical system, two episodes of induced mergers would first cause two neutron stars to merge and form a mass-gap black hole, which will subsequently merge with another (more massive) black hole through a second induced merger. We demonstrate that such systems are stable to flybys, and their formation would predict a high mass ratio and eccentric merger of a mass-gap black hole with a more massive black hole companion. Such a formation channel may explain observed gravitational wave events such as the recently-discovered LIGO/Virgo events S190814bv and S190924h.