Unraveling the Origin of Unequal Mass Gravitational Wave Events: Insights from a Galactic High Mass X-ray Binary

TL;DR

This paper links the formation of asymmetric mass-ratio gravitational wave events like GW190814 to the evolutionary pathways of Galactic High Mass X-ray Binaries, emphasizing the importance of local analogs for understanding GW source origins.

Contribution

It introduces a unified evolutionary model connecting HMXBs and asymmetric GW events, highlighting the role of conservative mass transfer and natal kicks in their formation.

Findings

Galactic HMXB 4U 1700-37/HD 153919 shares evolutionary traits with GW190814-like events.

Conservative mass transfer and natal kicks are crucial for forming asymmetric GW sources.

Estimated GW event rates align with observed data.

Abstract

The catalog of Gravitational Wave (GW) events is rapidly growing, providing key insights into the evolution of massive binaries and compact object formation. However, a key challenge is to explain the origin of exceptional events such as GW190814, among the most asymmetric mass-ratio mergers to date ($q\approx 0.1$). We show that it shares an evolutionary pathway with the most unequal mass Galactic High Mass X-ray Binary (HMXB) 4U 1700-37/ HD 153919. We demonstrate this unique connection by utilizing a rich set of existing observational constraints for the HMXB and compute detailed binary evolution models to explain its formation history. We find that conservative mass transfer, along with a directed natal kick are essential to explain its current state. We show that this system is unlikely to form a GW source due to a failed Common Envelope (CE) phase in the future, in agreement with previous work. With additional models, we show that a similar pathway naturally forms GW190814-like events, provided the first phase of mass transfer remains conservative, and the first-born (lower mass) compact object receives a large natal kick ($\gtrsim 100\,\mathrm{km/s}$) for the subsequent CE phase to be successful and form a asymmetric mass-ratio GW source. Anchored by the number of analogous Galactic HMXBs, we estimate rates for such GW events, which broadly agree with their observed rate. Our work demonstrates a unified formation pathway for highly asymmetric mass-ratio HMXBs and GW events. Moreover, it highlights the critical role of finding and characterizing local analogs in different evolutionary phases, and using them as a bridge to understand the origin of GW sources, especially the outliers like GW190814.