In November 2024, a sudden burst of gamma and X-ray light illuminated Earth telescopes, potentially originating from an unexpected source. Moments earlier, the LIGO-Virgo-KAGRA observatories recorded gravitational waves from a black hole collision in the same area of the sky. While such intense cosmic events are not typically associated with visible light, this instance was different.
Astronomer Shu-Rui Zhang and his team from the University of Science and Technology of China have proposed an unusual explanation for this occurrence. They suggest the collision might have happened within the turbulent disk of dust and gas surrounding a third supermassive black hole, known as the active galactic nucleus (AGN) of the host galaxy.
Though confirming this hypothesis is challenging from a distance of over 4.2 billion light-years, the combined observations imply that, under specific conditions, colliding black holes can indeed emit a flash of light.
The researchers emphasize the predictive nature of their model and stress the need for more precise measurements of the merger’s orbital eccentricity and extensive observations of the host galaxy to validate their theory. Since gravitational waves were first detected in 2015, their catalog has expanded into the hundreds. Most of these signals are linked to black hole collisions, although many remain unconfirmed or unanalyzed.
Traditionally, these cosmic unions are devoid of light. Scientists have searched for visual counterparts, but the evidence indicates that when smaller black holes merge into larger ones, any potential light is obscured by the event horizon.
However, the gravitational wave event on 25 November 2024, labeled S241125n, was an exception. It sent ripples detected by LIGO-Virgo-KAGRA detectors worldwide, signaling a black hole merger 4.2 billion light-years away that resulted in an object approximately 150 times the mass of the Sun.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>Within 11 seconds, X-ray observatories detected a burst of X-ray light and a gamma-ray burst from the same sky region as the gravitational waves. The likelihood of this being a mere coincidence is low, with only one such chance event expected in 30 years of observations. Since gravity and light travel at the same speed, the sequence of events indicates the black holes merged first, followed by a luminous outburst.
Black holes are notorious for not emitting detectable light, leading researchers to suspect additional factors were at play. One bright phenomenon black holes can exhibit is accretion, where surrounding material forms a disk heated by gravity and friction, emitting light in the process.
This disk is a potential light source, as are astrophysical jets formed by material accelerated along magnetic field lines near the event horizon, launched from the black hole’s poles at high speeds.
The gamma-ray burst following S241125n displayed unusual characteristics compared to typical bursts from supernovae or neutron star mergers. Zhang and his team proposed that rapid accretion might explain it. For such a process to occur, the collision would need to happen in an environment with ample material for the black hole to consume.
They conducted simulations of two stellar-mass black holes colliding within the accretion disk of a much larger supermassive black hole, itself actively accreting material at the galaxy’s core. When two black holes with uneven masses merge, the new black hole can receive a “natal kick” due to the uneven mass distribution, propelling it forward.
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The simulations suggest such a kick within an AGN accretion disk would drive the newly formed black hole into dense dust and gas, triggering accretion and launching jets that resemble the observed gamma-ray burst.
This scenario is plausible, given the activity in galactic centers, which often contain smaller black holes and black hole binaries moving towards the center. Further evidence is needed to support the team’s hypothesis, but it offers an intriguing perspective on galactic centers and potential black hole collisions within them.
“Future studies of S241125n and similar events could provide deeper insights into the fundamental physics of black hole mergers and their role in the broader cosmic landscape,” the researchers write, potentially revealing new connections between gravitational waves, electromagnetic signals, and the environments hosting these remarkable phenomena.
The research has been published in The Astrophysical Journal Letters.
