The Role of Natal Kicks in Forming Asymmetric Compact Binary Mergers

Abstract

In their most recent observing run, the LIGO-Virgo-KAGRA Collaboration observed gravitational waves from compact binary mergers with highly asymmetric mass ratios, including both binary black holes (BBHs) and neutron star-black holes (NSBHs). It appears that NSBHs with mass ratios q ≃ 0.2 are more common than equally asymmetric BBHs, but the reason for this remains unclear. We use the binary population synthesis code cosmic to investigate the evolutionary pathways leading to the formation and merger of asymmetric compact binaries. We find that within the context of isolated binary stellar evolution, most asymmetric mergers start off as asymmetric stellar binaries. Because of the initial asymmetry, these systems tend to first undergo a dynamically unstable mass transfer phase. However, after the first star collapses into a compact object, the mass ratio is close to unity and the second phase of mass transfer is usually stable. According to our simulations, this stable mass transfer fails to shrink the orbit enough on its own for the system to merge. Instead, the natal kick received by the second-born compact object during its collapse is key in determining how many of these systems can merge. For the most asymmetric systems with mass ratios of q ≤ 0.1, the merging systems in our models receive an average kick magnitude of 255 km s−1 during the second collapse, while the average kick for non-merging systems is 59 km s−1. Because lower mass compact objects, like neutron stars, are expected to receive larger natal kicks than higher mass BHs, this may explain why asymmetric NSBH systems merge more frequently than asymmetric BBH systems.

Black Hole Formation Accompanied by the Supernova Explosion of a 40 M ⊙ Progenitor Star

“…The absolute value of the neutrino kick is subdominant but reaches ∼400 km s −1 . The aggregate kick speed is quite unlike the birth kicks inferred for the stellarmass black holes in X-ray binaries (Willems et al 2005;Atri et al 2019;Oh et al 2023) and suggests that the black holes of this birth channel are unlikely to have companions. This would make observing the population of such black holes exceedingly difficult.…”

“…The anisotropic blast hydrodynamics, aspherical infall, asymmetric neutrino emissions, and integrated anisotropic gravitational attraction between the core and clumpy ejecta together result in a net recoil kick of the PNS core. Pulsars as a group manifest some of the fastest speeds seen in the galaxy (Lyne & Lorimer 1994;Burrows & Hayes 1996;Faucher-Giguère & Kaspi 2006;Scheck et al 2006Scheck et al , 2008Ng & Romani 2007;Nordhaus et al 2012;Wongwathanarat et al 2013;Janka 2017;Yang et al 2021;), but black holes have generally been thought to receive a significantly smaller recoil kick (Willems et al 2005;Atri et al 2019;Oh et al 2023). However, the grossly asymmetrical explosion dynamics of our 40 M e model results in a kick speed (absolute value of the vector sum of the various components) just before black hole formation of ∼1150 km s −1 (Janka 2013), using the formalism of .…”

A Channel to Form Fast-spinning Black Hole–Neutron Star Binary Mergers as Multimessenger Sources. II. Accretion-induced Spin-up

“…The MT stability is also affected by the star's rotation. For instance, when an accretor (i.e., BH in this work) rotates, the accretion rate of the rotating accretor might be reduced by a factor of (1 − ω/ω crit ) (e.g., Stancliffe & Eldridge 2009;Ablimit 2021;Ablimit et al 2022;Oh et al 2023).…”

Two of a Kind: Comparing Big and Small Black Holes in Binaries with Gravitational Waves

“…Supernova kicks may also be different for firstborn versus secondborn components in merging binaries, because kicks determine whether or not the binary can merge within the age of the Universe (Kalogera 1996;Gallegos-Garcia et al 2022). Because natal kicks are related to the remnant mass via the supernova prescription (Fryer et al 2012;Mandel et al 2021), different preferences for the natal kick magnitudes between firstborn and secondborn BHs may cause the primary and secondary mass distributions to differ in merging binaries formed through isolated binary evolution (e.g., Oh et al 2023).…”