Mass-gap extreme mass ratio inspirals

TL;DR

This paper introduces a new class of extreme-mass-ratio inspirals involving objects in the lower mass gap, analyzes their formation rates in different channels, and discusses their potential detectability and implications for astrophysics.

Contribution

It proposes the concept of mass-gap EMRIs, calculates their formation rates in dry and wet channels, and explores their observability and astrophysical significance.

Findings

Wet channel produces more observable mass-gap EMRIs.

Mass segregation suppresses dry channel EMRI rates.

Detection or non-detection constrains supernova mechanisms and primordial black hole abundance.

Abstract

In this work, we propose a new subclass of extreme-mass-ratio-inspirals (EMRIs): mass-gap EMRIs, consisting of a compact object in the lower mass gap $\sim (2.5-5) M_\odot$ and a massive black hole (MBH). The mass-gap object (MGO) may be a primordial black hole or produced from a delayed supernova explosion. We calculate the formation rate of mass-gap EMRIs in both the (dry) loss-cone channel and the (wet) active galactic nucleus disk channel by solving Fokker-Planck-type equations for the phase-space distribution. In the dry channel, the mass-gap EMRI rate is strongly suppressed compared to the EMRI rate of stellar-mass black holes (sBHs) as a result of mass segregation effect. In the wet channel, the suppression is roughly equal to the mass ratio of sBHs over MGOs, because the migration speed of a compact object in an active galactic nucleus disk is proportional to its mass. We find that the wet channel is much more promising to produce mass-gap EMRIs observable by spaceborne gravitation wave detectors. (Non-)detection of mass-gap EMRIs may be used to distinguish different supernova explosion mechanisms and constrainthe abundance of primordial black holes around MBHs.