Opening
The discovery of AT2025ulz—a potential electromagnetic counterpart to the sub-threshold gravitational wave event S250818k—has opened a new window into how compact objects might form and merge in extreme environments. A new study from Wu, Most, Vu and colleagues investigates whether orbital eccentricity could serve as a smoking gun for a specific formation pathway: hierarchical mergers of subsolar-mass objects born from fragmentation inside collapsar accretion disks. Using numerical relativity simulations, the team demonstrates that this scenario naturally produces measurable eccentricity signatures that could distinguish hierarchical mergers from other formation channels.
What they found
The authors modeled hierarchical mergers of compact objects in a disk-like geometry consistent with collapsar fragmentation. Their key finding is that repeated capture and merger dynamics—characteristic of hierarchical assembly—impart gravitational kicks to the system that drive orbital eccentricity. In their simulations, the final merger begins with potentially large eccentricity of order e ≈ 0.6. Crucially, despite the short dynamical lifetime of these systems, a substantial fraction of this eccentricity survives until the final coalescence: up to e ≈ 0.1 can persist in the general case.
This result is significant because it provides a testable prediction. The authors note that the survival of measurable eccentricity depends on the system's short lifetime—a key feature that distinguishes this scenario from other formation pathways where longer dynamical timescales would allow eccentricity to dissipate.
Why it matters
Eccentricity is a powerful diagnostic tool in gravitational wave astronomy. Most compact object mergers detected to date have been nearly circular, making eccentricity a rare and informative signature. If future subsolar-mass gravitational wave candidates show evidence of non-negligible eccentricity, this would provide strong support for a hierarchical formation scenario within collapsar disks rather than isolated binary mergers or other assembly channels. This connects directly to multi-messenger astronomy: hierarchical mergers are expected to produce multiple electromagnetic counterparts as successive mergers occur, potentially explaining the unusual transient properties of events like AT2025ulz.
What's next
The authors' work establishes eccentricity as a key observable to monitor in upcoming detections. Future gravitational wave observations with improved sensitivity and parameter estimation will be essential for measuring eccentricity in subsolar-mass events. The connection between gravitational wave signatures and electromagnetic counterparts also motivates coordinated observations across the electromagnetic spectrum during transient alerts.
Starithm continuously monitors real-time gravitational wave alerts and electromagnetic transients, enabling researchers to test predictions like these as new events are discovered.