Black holes in the low mass gap: Implications for gravitational wave observations

TL;DR

This paper explores the existence and detectability of low-mass black holes in the 3-4 solar mass range, formed from neutron star mergers, and discusses how future gravitational-wave detectors could identify them and clarify their origins.

Contribution

It introduces the concept of low-mass black holes in the low mass gap and analyzes their potential signatures in gravitational-wave data, emphasizing future detector capabilities.

Findings

Third-generation detectors can uncover low-mass black hole mergers.

Joint measurements of chirp mass and spin can reveal formation scenarios.

GW190425 supports a double Gaussian neutron star mass model.

Abstract

Binary neutron-star mergers will predominantly produce black-hole remnants of mass $\sim 3-4\,M_{\odot}$, thus populating the putative \emph{low mass gap} between neutron stars and stellar-mass black holes. If these low-mass black holes are in dense astrophysical environments, mass segregation could lead to "second-generation" compact binaries merging within a Hubble time. In this paper, we investigate possible signatures of such low-mass compact binary mergers in gravitational-wave observations. We show that this unique population of objects, if present, will be uncovered by the third-generation gravitational-wave detectors, such as Cosmic Explorer and Einstein Telescope. Future joint measurements of chirp mass ${\cal M}$ and effective spin $\chi_{\rm eff}$ could clarify the formation scenario of compact objects in the low mass gap. As a case study, we show that the recent detection of GW190425 (along with GW170817) favors a double Gaussian mass model for neutron stars, under the assumption that the primary in GW190425 is a black hole formed from a previous binary neutron star merger.