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

Abstract

Binary neutron-star mergers will predominantly produce black-hole remnants of mass $\sim 3-4M_{\odot }$, 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 $\mathcal{M}$ and effective spin $\ddot{I}{‡}_{\mathrm{eff}}$ 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.

8 pages, 4 figures, 1 table. v4: matches the version accepted for publication in Phys. Rev. D