Black holes in the low-mass gap: Implications for gravitational-wave observations
Anuradha Gupta, Davide Gerosa, K. G. Arun, Emanuele Berti, Will M. Farr, B. S. Sathyaprakash
Abstract
Binary neutron-star mergers will predominantly produce black-hole remnants of mass $\ensuremath{\sim}3--4\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$, thus populating the putative 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 $\mathcal{M}$ and effective spin ${\ensuremath{\chi}}_{\mathrm{eff}}$ 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.