When a black hole consumes a neutron star, it’s typically thought to do so without an electromagnetic belch. New research explores the conditions under which a black hole’s neutron-star feast produces an observable electromagnetic signal.
Signaling a Collision
In the past few years, gravitational-wave detectors across the globe have detected a handful of neutron star–black hole mergers. What sorts of electromagnetic signals might accompany these outbursts of gravitational waves is a matter of intense interest.
In rare cases, if the black hole and neutron star are relatively close in mass, their gravitational tussle will rip the neutron star apart. As the shredded stellar material collects in a searingly hot disk around the black hole, it is expected to power a bright transient like a kilonova or a gamma-ray burst.
More commonly, the black hole is much more massive than the neutron star. In this case, the black hole should swallow the neutron star whole and without an electromagnetic trace — unless, as recent research shows, a strong magnetic field surrounds the neutron star.
Detailed Simulations
The simulated merger of a neutron star and a black hole, in which the neutron star is swallowed whole. Click to enlarge. [Kim et al. 2025]
As the simulated neutron star plunges toward the black hole, the neutron star’s magnetosphere — the region of space in which particles bend to the will of the neutron star’s magnetic field — begins to ripple with waves. The waves expand outward at nearly the speed of light, launching an exceptionally massive shock. This monster shock drives the creation of an electromagnetic transient: a flash of radio waves called a fast radio burst.
The “striped” pulsar-like wind, shown at 7 milliseconds post-merger. The color scale shows the toroidal magnetic field component. [Adapted from Kim et al. 2025]
Striped Winds and a Fleeting Pulsar Phase
A second type of transient arises after the neutron star merges with the black hole. After the neutron star vanishes within the black hole’s waiting maw, its leftover magnetic field rearranges and begins to rotate, dragged along by the spinning black hole. Briefly, the black hole enters a pulsar-like state, surrounded by a “striped” wind created by spiraling magnetic field lines in alternating directions. As this magnetic field dissipates, a “fireball” of magnetized electron–positron plasma is ejected. When these electron–positron pairs annihilate, they briefly release another electromagnetic signal: a burst of X-rays and gamma rays.
Kim and coauthors noted that their simulated neutron star–black hole pair is similar in mass to the colliding pairs spotted by gravitational-wave detectors. This suggests that detectable gravitational-wave events may be accompanied by brief bursts at radio, X-ray, and gamma-ray wavelengths, providing another avenue to learn about these cosmic collisions.
Citation
“Black Hole Pulsars and Monster Shocks as Outcomes of Black Hole–Neutron Star Mergers,” Yoonsoo Kim et al 2025 ApJL 982 L54. doi:10.3847/2041-8213/adbff9