Eccentricity as a signature of hierarchical subsolar-mass mergers in collapsar disks

TL;DR

This paper explores gravitational-wave signatures of hierarchical subsolar-mass mergers in collapsar disks, highlighting eccentricity as a key indicator of such merger scenarios.

Contribution

It demonstrates through simulations that hierarchical mergers can produce significant eccentricity, which may be detectable in gravitational-wave signals.

Findings

Hierarchical mergers can result in eccentricities up to 0.6 initially.

A substantial eccentricity (up to 0.1) can survive until merger.

Detection of eccentricity in gravitational waves could indicate hierarchical formation.

Abstract

In this work, we investigate gravitational-wave signatures of a proposed subsolar-mass merger scenario resulting from fragmentation inside a collapsar accretion disk. This scenario has gained recent interest with the electromagnetic transient AT2025ulz, a possible superkilonova counterpart candidate to the sub-threshold gravitational wave event S250818k. One prediction of fragmentation is the formation of multiple smaller neutron-star fragments, some of which might merge hierarchically. Such mergers are expected not only to produce individual electromagnetic counterparts, but also, because of their repeated capture and merger dynamics, to impart kicks to the system and thereby drive orbital eccentricity. By performing numerical relativity simulations of hierarchical compact object mergers modeled as black holes in a disk-like geometry consistent with this scenario, we demonstrate the build-up of potentially large eccentricity for the final merger, of order $e \simeq 0.6$ initially, and show that, because of the short lifetime of the system, a substantial part of this eccentricity , up to $e\simeq 0.1$, can survive until merger in the general case. As a result, future detections of eccentricities in potential subsolar-mass gravitational-wave candidate events would be a strong indicator for a hierarchical formation scenario.