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

TL;DR

The study investigates a potential population of low-mass black holes in the mass gap, formed when neutron-star binaries merge in dense stellar environments and produce second-generation remnants. It develops a population synthesis framework that combines three neutron-star mass distributions, a dynamical pairing prescription with a tunable index $\beta$, and summation of progenitor masses to yield 2g black holes, then predicts distinct gravitational-wave signatures in chirp mass ${\cal M}$ and effective spin $χ_{\rm eff}$ detectable by current and, critically, third-generation detectors. The results show three characteristic peaks in the ${\cal M}$ distribution corresponding to 1g+1g, 1g+2g, and 2g+2g mergers, with the relative heights sensitive to the 2g abundance $\kappa$ and the pairing index $\beta$; a joint ${\cal M}$-$χ_{\rm eff}$ analysis indicates clear separation of channels, suggesting that CE/ET could uncover or constrain this population. GW190425 is used as a case study, and the analysis indicates that a double-Gaussian NS mass model is favored if the primary is a BH born from a prior NS–NS merger, highlighting how future observations can quantify the role of dynamical channels in populating the mass gap and determine the fraction of NS mergers that yield 2g BHs.

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 $χ_{\rm eff}$ 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.

Figures (4)

• Figure 1: Mass distribution of 1g NSs (black) and 2g BHs (cyan, red, green, blue) for the three mass distributions used in this work and various choices of the pairing-probability spectral index $\beta$.
• Figure 2: Detection rate $(r/f_{\rm dyn})$ per chirp mass bins for 1g+1g, 1g+2g and 2g+2g mergers as observed by LIGO (left) and Cosmic Explorer (right). Different colors correspond to different pairing probabilities $p_{\rm pair}\propto (m_2/m_1)^{\beta}$. Upper, middle and lower panels show results from the three different mass distributions for 1g NSs: single-Gaussian, double-Gaussian and uniform, respectively. Also shown are the $90\%$ credible bounds on the chirp mass of GW170817 (magenta) and GW190425 (yellow).
• Figure 3: Joint chirp-mass effective-spin distribution as observed by Cosmic Explorer. The 1g NS mass distribution is modeled by a single Gaussian and we assume $\beta=0$. The color bar indicates the detection rate $(r/f_{\rm dyn})$ per bin.
• Figure 4: The posterior probability of $\alpha$ computed using Eq. (\ref{['eq:p_alpha2']}).