Subsolar-mass binary mergers of strange stars and neutron stars: gravitational waves and ejecta

Numerical relativity simulations reveal how subsolar strange star mergers differ from neutron stars

Scientists use gravitational waves to listen to collisions of massive, dense objects. However, a mystery remains: what happens when these objects are significantly smaller than our Sun? Standard stellar evolution suggests a minimum mass for neutron stars. New candidates for subsolar-mass objects have emerged, prompting a search for new physics.

Current models struggle to distinguish between different types of ultra-dense matter in this low-mass regime. Researchers aim to determine if these objects are traditional neutron stars (NS)—held together by gravity—or "strange stars" (SS)—hypothetical objects held together by the strong nuclear force. Because these two classes behave differently during a collision, identifying them requires understanding their unique signatures.

A new study using high-performance numerical relativity (simulations of gravity and fluid dynamics) reveals that these two classes leave different fingerprints on the gravitational-wave spectrum. The researchers find that strange star mergers are more violent. They create a distinct pattern of frequencies that could act as a "smoking gun" for exotic matter.

The degeneracy of subsolar mass signatures

As detectors like LIGO and Virgo improve, they scan for "subsolar" events—collisions involving objects with less than the mass of our Sun. Standard neutron stars should not exist at these masses. If we detect a subsolar merger, it implies something unconventional, such as a primordial black hole or a strange star.

The difficulty for observers lies in "degeneracy." During the early inspiral (the approach phase), both neutron stars and strange stars imprint a signal related to their tidal deformability (how much they stretch under gravity). The authors report that a "soft" (easily compressed) neutron star can look almost identical to a strange star during this stage. Much like two different types of sponges might look identical while being squeezed slowly, the true difference only appears when they are slammed together.

Violent bounces and shock heating

To break this degeneracy, the authors conducted the first numerical-relativity simulations of subsolar-mass binary strange star mergers. They used the SACRA-K code, a GPU-accelerated framework, to evolve the complex fluid dynamics of these collisions.

The simulation highlights a fundamental mechanical divergence. Neutron stars are gravitationally bound, meaning their outer layers are loosely held. As they approach each other, they undergo significant tidal deformation. They develop long "spiral arms" that shed mass before the actual impact occurs .

In contrast, strange stars are self-bound by the strong interaction. This makes them much more compact and resistant to stretching. Instead of shedding mass early, strange stars maintain their integrity until contact. The authors report that this leads to a more violent collision. The compact cores slam together at high velocities. This drives a powerful shock wave and a massive "radial bounce"—a sudden expansion of the merged core . This bounce is much weaker in the neutron star case.

Breaking the degeneracy with frequency ratios

The most significant result is the identification of a specific spectral signature. The researchers tracked three key gravitational-wave frequencies: the cutoff frequency ($f_{\text{cut}}$), marking where tidal effects dominate; the merger frequency ($f_{\text{merger}}$); and the post-merger peak frequency ($f_2$), representing the oscillation of the new remnant.

The paper finds that the compact strange star pushes the cutoff frequency higher. However, the violent radial bounce actually lowers the post-merger frequency $f_2$. This happens because the bounce reduces the average density of the remnant .

While individual frequencies depend on the star's mass and internal composition, the ratio $f_2/f_{\text{cut}}$ remains consistent within each class.

Specifically, the study shows that the ratio $f_2/f_{\text{cut}}$ creates a clean gap. Strange stars cluster between 2.11 and 2.34. Neutron stars occupy a higher range between 2.65 and 2.97 .

This ratio acts as a robust discriminant. It allows astronomers to identify the nature of the matter by looking at the crash and the aftermath.

Limits of the classification

The authors note specific edge cases where the distinction blurs. In highly asymmetric mergers—where one star is much heavier than the other—tidal forces can disrupt the smaller star prematurely.

The paper identifies a scenario with a mass ratio of $q = 1.5$ for strange stars. In this case, the tidal field disrupts the companion so effectively that the radial bounce is weakened. Here, the post-merger frequency $f_2$ actually rises toward the neutron star range . Furthermore, the study does not explore the long-term evolution of the remnants or the exact chemical composition of the ejecta.

A roadmap for exotic matter detection

Is this ready for the observatory? The answer depends on detector sensitivity.

The authors suggest these signatures are within reach of upcoming facilities like the Einstein Telescope. Additionally, the merger process ejects roughly $10^{-2} M_\odot$ of material. For neutron stars, this debris is neutron-rich and should power a visible "kilonova" (a bright electromagnetic flash). For strange stars, the ejecta consists of decompressed quark matter. This might appear as electromagnetically dark quark nuggets.

If we observe a subsolar merger that lacks a standard kilonova but shows the $f_2/f_{\text{cut}}$ ratio of a strange star, we may have evidence for strange quark matter. For researchers building search templates, the takeaway is vital. Ignoring these matter-driven frequency shifts could lead to missing these exotic events entirely.

Figures from the paper