Black Hole Meets Neutron Star. Nothing Happens. Everything Changes

Introduction: When Gravity Speaks and Light Doesn’t

Astronomy entered a new era in 2017 when scientists witnessed the first ever multi-messenger event: GW170817. It was a neutron star collision that didn’t just ripple space-time but also burst forth in light—gamma rays, optical waves, X-rays, and more. Since then, the race has been on to catch more of these cosmic spectacles. But what happens when nature offers only silence?

That’s exactly what happened with GW200115, a merger of what scientists believe was a black hole and a neutron star. While gravitational waves were clearly detected, telescopes across the globe turned their eyes and found nothing—no flash, no burst, no flicker. This absence, however, is far from a failure. In a striking twist, the paper Multi-Messenger Constraints on Magnetic Fields in Merging Black Hole–Neutron Star Binaries by Coughlin et al. flips this silence into a scientific advantage.

Instead of asking “Why didn’t we see anything?”, the researchers posed a more intriguing question: “What does this lack of light tell us about the magnetic fields in such systems?” The answer, as it turns out, offers rare insight into the hidden magnetism of neutron stars, and it begins to place firm astrophysical limits on just how strong their fields can be—when paired with black holes.

Background: Why Magnetic Fields Are Crucial in BH–NS Mergers

Neutron stars are the collapsed cores of massive stars—dense, spinning, and often cloaked in magnetic fields that put Earth’s to shame. Some neutron stars, known as magnetars, boast magnetic field strengths on the order of 10¹⁴–10¹⁵ gauss. These colossal fields actively shape the dynamics of a merger, influencing everything from plasma acceleration to jet formation.

When a neutron star spirals into a black hole, whether or not the neutron star is torn apart before being consumed—the so-called tidal disruption—plays a huge role in whether electromagnetic (EM) signals are emitted. A disrupted neutron star can leave behind debris, form an accretion disk, and under the right conditions, launch a relativistic jet.

But all of this is contingent on the presence and strength of magnetic fields. Strong fields can help drive outflows and ignite the jet-launching mechanisms needed for EM visibility. Weak fields, on the other hand, might result in a merger that remains quiet in light. The magnetic field of the neutron star is a gatekeeper to visibility—and that makes it a critical parameter to understand.

The Case of GW200115: What Happened?

On January 15, 2020, the LIGO and Virgo observatories captured gravitational waves from GW200115. The signal showed a ~5.7 M☉ black hole merging with a ~1.5 M☉ compact object—most likely a neutron star.

Key features stood out: the black hole had little to no spin, and the mass ratio (~3.7) suggested a prompt plunge—the neutron star likely fell directly into the black hole without tidal disruption.

What followed was silence. Despite rapid follow-up from X-ray, optical, infrared, and radio telescopes, no electromagnetic counterpart was detected. This null result became the focal point of the study: if strong magnetic fields had been present, they could’ve powered visible emission. So how strong were they really?

The Core Idea: Inference from Absence

Coughlin et al. proposed a new approach: instead of looking for what was seen, use what wasn’t seen to constrain astrophysical properties. If a highly magnetized neutron star had merged with a black hole and formed a disk, it should have launched a jet and possibly triggered a gamma-ray burst or kilonova.

The absence of such signals suggested the magnetic field was not strong enough to do so. By simulating a wide range of merger configurations—including various black hole masses, spins, and neutron star radii—and combining them with MHD physics, the team asked: for each case, would the resulting EM signal have been detectable?

If yes, but no signal was seen, then that field strength could be ruled out.

Modeling Assumptions

The simulations assumed dipolar magnetic fields embedded within the neutron star. During merger, instabilities such as Kelvin-Helmholtz and magnetorotational effects could amplify these fields, especially in the presence of a disrupted disk.

Jet luminosity was modeled as proportional to B², and the team modeled how the energy output would vary under different field strengths. They also considered baryon pollution, which could choke a jet, and the viewing angle effects, which can hide even powerful emissions if misaligned with Earth.

The brightness thresholds for kilonovae and gamma-ray bursts were compared to real observational limits from follow-up campaigns.

Results: The Magnetic Field Cap

The conclusion was clear: if the neutron star had magnetic fields exceeding ~10¹⁴ gauss, the event should have produced visible light under most reasonable assumptions. The fact that no EM counterpart was observed places an upper limit on the field strength.

The most consistent range, given the data, is between 10¹² and 10¹³.5 gauss—typical of ordinary pulsars, not magnetars.

Even under extreme edge-case scenarios (e.g., no disk formation, highly unfavorable viewing angles), the magnetic field still likely remained below the magnetar threshold. This sets one of the first multi-messenger constraints on NS magnetic fields at merger.

Implications for Future Events

This study opens a new front in multi-messenger astrophysics: using silence as a probe. As detectors grow more sensitive and EM follow-up improves, even “invisible” mergers can yield scientific dividends.

It also suggests that not all neutron stars are magnetars, and that highly magnetized NSs might be rarer in BH–NS binaries than previously thought. Future population studies could determine whether this is due to evolution, selection bias, or formation channels.

Most importantly, it shows that multi-messenger astronomy is not just about detecting more—but about interpreting the full ensemble of signals and silences alike.

Final Thoughts

GW200115 did not produce a light show. Yet in its quiet aftermath, it has told us something profound about neutron star interiors. Through simulations, modeling, and clever statistical inference, Coughlin et al. have turned a null result into a constraint on one of the most elusive properties in astrophysics: the strength of magnetic fields in dying neutron stars.

As our eyes on the cosmos grow sharper, even the universe’s silences will echo with discovery.

Reference

Coughlin et al. (2021), Multi-Messenger Constraints on Magnetic Fields in Merging Black Hole–Neutron Star Binaries
arXiv:2112.01979