For decades, astronomers predicted that black holes and neutron stars should collide. We knew they existed in binary systems, we knew their orbits decay over time, and we knew that eventually — inevitably — they would merge. Yet no one had ever directly observed such an event. Until now.
LIGO and Virgo detectors have reported the first confirmed detections of gravitational waves from black hole–neutron star mergers. Two such events, detected within ten days of each other, finally confirmed what stellar evolution models had long anticipated. Prof. Krzysztof Belczyński from the Nicolaus Copernicus Astronomical Center in Warsaw is among those who predicted these collisions over a decade ago — and his models appear to be holding up.
The events themselves, however, delivered a somewhat sobering message for astronomers hoping to see a full light show. In the most exciting scenario, the neutron star would be torn apart outside the black hole’s event horizon, flinging neutron-rich matter into space and producing a brilliant burst of electromagnetic radiation — a kilonova visible across the spectrum. Instead, both neutron stars appear to have been swallowed whole, crossing the event horizon before being disrupted. No electromagnetic counterpart was detected. Gravitational waves only.
This outcome is consistent with Belczyński’s earlier predictions: the black holes involved rotate too slowly, giving them large event horizons that engulf the neutron star before it can be shredded. The models suggest only a few to perhaps twenty percent of such mergers might produce detectable light.
With LIGO preparing for its next observing run, the number of detected neutron star–black hole mergers is expected to grow rapidly. More events mean sharper tests of our models — and perhaps, eventually, the electromagnetic fireworks astronomers are still waiting for.
Link to video: https://www.youtube.com/watch?v=Yi3f8hRYB7o