Constraining the Progenitor Orbits of Sequential Black-Hole Mergers in Hierarchical Triples: Peters-Based Estimates

Abstract

Two black-hole merger events may originate from sequential mergers in a pre-existing hierarchical black-hole triple if their spatial localizations overlap and the remnant mass of the first merger is compatible with a component mass of the second. In this scenario, the inner binary merges first, leaving a remnant that remains bound to the tertiary and subsequently undergoes a second merger. We present a method to constrain the progenitor triple's outer orbit by modeling the gravitational-wave-driven evolution of the remnant-tertiary binary using Peters' equations. Given an assumed upper bound on the recoil velocity of the first merger, the method requires only the black-hole masses, the time delay between the mergers, and the eccentricity at a reference frequency - quantities that can be inferred from the merger events themselves. We investigate the potential constraining power by constructing a constraint atlas spanning a range of system properties and identify the conditions under which the method places tight and reliable bounds on the progenitor triple's outer orbital parameters. By imposing an upper bound on the detector-band eccentricity, we also obtain meaningful constraints for candidate hierarchical mergers proposed in the literature, including GW190514-GW190521. We provide numerical tools for applying the method to user-specified systems and evaluating the reliability of the resulting Peters-based constraints. This framework offers a computationally inexpensive first step toward reconstructing hierarchical-merger progenitors and provides a foundation for extensions incorporating more sophisticated relativistic treatments.