Formation of mass gap objects in highly asymmetric mergers

TL;DR

This paper proposes a model where highly asymmetric black hole-neutron star mergers can produce mass-gap objects through accretion of bound supernova ejecta, explaining GW190814's unusual secondary mass.

Contribution

It introduces a new formation scenario for mass-gap objects involving accretion of supernova ejecta in asymmetric mergers, which can be tested with future gravitational wave data.

Findings

Predicts a correlation between primary and secondary masses in asymmetric mergers.

Suggests mass-gap objects are detectable only in extreme mass ratio binaries.

Provides a testable model for future LIGO/Virgo observations.

Abstract

The LIGO/Virgo Collaboration (LVC) recently reported the detection of GW190814, a merger of a $23^{+1.0}_{-0.9}~M_{\odot}$ primary black hole (BH), and a $2.6^{+0.08}_{-0.08}~M_{\odot}$ secondary. The secondary's mass falls into the mass-gap regime, which refers to the scarcity of compact objects in the mass range of 2-5 $M_{\odot}$. The first clue to the formation of the GW190814 lies in the fact that the primary is a very massive BH. We suggest that the secondary was born as a neutron star (NS) where a significant amount of the supernova ejecta mass from its formation remained bound to the binary due to the presence of the massive BH companion. The bound mass forms a circumbinary accretion disk, and its accretion onto the NS created a mass-gap object. In this scenario, LIGO/Virgo will only detect mass-gap objects in binary mergers with an extreme mass ratio. We also predict a correlation between the mass of the secondary and the mass of the primary in such asymmetric mergers. Our model can be tested with future data from the LVC's third-observing run.