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

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.

Figures (8)

• Figure 1: GW190814 and the HMXB 4U 1700-37/ HD 153919 represent extremes in their respective populations. GW190814 has one of the most asymmetric mass-ratio among GW events, while the HMXB 4U 1700-37 contains the most massive companion star among known HMXBs. Left panel: The mass of the primary (heavier) compact object and the mass-ratio of the system for all the confident GW events in the cumulative GWTC 4.0 catalog 2025arXiv250818079T. Right panel: The mass of the companion star and the mass-ratio of the system for all the HMXBs in the catalog of 2023AA...671A.149F. Blue circles denote Supergiant XRBs, while red circles represent Be XRBs. Open circles correspond to systems that are cataloged as having a NS in them, but do not have a mass estimate, for which we set their mass to be $1.4\,\mathrm{M}_{\odot}$. Due to the heterogeneous datasets and methods used to estimate masses, and for illustration purposes, we only report the central values here and not their measurement errors. GW190814 and 4U 1700-37 are highlighted with larger circles in both panels. The goal of this work is to demonstrate an evolutionary link between the formation of both these systems.
• Figure 2: Reconstruction of the properties of the HMXB 4U 1700-37 pre-SN. The corner plot shows distributions of the pre-SN mass, pre-SN period and natal kick strength of the binary population that satisfy the observational constraints of the HMXB. Diagonal panels show their 1D marginal distributions (with median and $1\sigma$ uncertainties reported on the top), while other panels show correlations between different parameters, with darker colors representing regions of higher density. The goal of our binary evolution models is to match these pre-SN constraints.
• Figure 3: The evolution of the donor and accretor in our fiducial model for forming the HMXB 4U 1700-37/ HD 153919 at $Z = Z_\odot = 0.0146$. Their initial masses are $40\,\mathrm{M}_{\odot}$ and $28\,\mathrm{M}_{\odot}$, respectively, and the initial orbital period is $3\,\mathrm{days}$. Top panel: Evolution of the donor on the HR diagram. Blue and red curves highlight fast and slow Case A mass transfer, respectively, while green shows Case AB mass transfer. The diamond, plus, and star markers correspond to the Terminal Age Main Sequence (TAMS), core-helium ignition, and core-helium depletion, respectively. The inset panel zooms into the evolution of the donor during Case A mass transfer. Middle Panel: Evolution of the accretor on the HR diagram, representing our model for HD 153919. The purple and blue markers with errorbars show the observed location of HD 153919 from 2020AA...634A..49H and 2021AA...655A..31V, respectively. Bottom panel: The evolution of the mass of the donor (dash-dot) and accretor (solid) over time.
• Figure 4: The evolution of the accretor in our fiducial model for forming a GW190814-like binary at a $Z = Z_\odot/10 = 0.00146$. The initial masses of the donor and accretor are $27\,\mathrm{M}_{\odot}$ and $25\,\mathrm{M}_{\odot}$, and the initial orbital period is $3.5\,\mathrm{days}$. Top panel: Evolution of the accretor on the HR diagram. The colors and marker symbols follow the same style as in Fig. \ref{['fig:xrb_fiducial_hr']}. The inset panel zooms into the late stages of the core-helium burning phase, where it ascends the RSG branch, and developes a convective envelope (shown in purple). Bottom panel: The evolution of the mass of the donor (dash-dot) and accretor (solid) over time. The former forms the lighter compact object, while the latter has a massive helium core (shown in golden), and is the progenitor of the massive BH in an asymmetric mass-ratio GW190814-like event.
• Figure 5: The need for stellar radii $\gtrsim 1250\,\mathrm{R}_{\odot}$ for a successful Common Envelope phase ($\alpha_{\text{CE}} < 1$). Top panel: The evolution of the binding energy of the envelope as a function of the total radius of the star. Blue curves include the contribution of internal energy, while golden curves only include the gravitational binding energy. The solid lines correspond to the $X = 0.1$ definition for the core-envelope boundary, while the shaded bands encompass the $X = 0.01$ and $X = 0.3$ limits. The vertical dash-dot lines denote the stellar radius when it reaches TAMS, ignites helium in its core (He ZAMS), and finishes core-helium burning (He TAMS), respectively. Bottom panel: The minimum $\alpha_{\text{CE}}$ required to eject the envelope as a function of the stellar radius, with the same color and shading conventions as the top panel.