On September 14, 2015, a monumental event occurred when the twin LIGO detectors recorded the first gravitational waves generated by the merger of two black holes, each with masses of 36 and 29 solar masses, located over a billion light-years away. Notably, just 0.4 seconds after this event, NASA’s Fermi satellite detected a faint gamma-ray signal from an unknown source.

Since then, LIGO has undergone enhancements and has partnered with Virgo, leading to the observation of approximately 50 additional black hole mergers. However, none of these events have shown any signs of gamma-rays, X-rays, or other electromagnetic signals—until May 21, 2019. On that date, the Zwicky Transient Facility recorded an electromagnetic flare that coincided with one of these mergers, prompting a reevaluation of our understanding of merging black holes and their potential to emit light.

When we consider what black holes are, it becomes evident why they shouldn’t emit light during a collision. Unlike other matter in the universe, black holes do not consist of tangible particles and do not interact with their surroundings in the same manner. They are characterized as regions of space where gravitational forces are so intense that nothing, not even light, can escape.

In a binary system of two black holes, gravitational radiation causes their orbits to decay over time. As they merge, their event horizons unite, yet light should still remain trapped. This scenario contrasts sharply with other astrophysical mergers. For instance, when two stars collide, they create bright phenomena like luminous red novae. Similarly, merging neutron stars can result in kilonovae, producing gamma-ray bursts and heavy elements.

In the case of black holes, however, the situation is different. Above a certain critical mass threshold—between 2.5 and 2.75 solar masses—objects like white dwarfs and neutron stars cannot exist; they must collapse into black holes instead. White dwarfs are supported by electron degeneracy pressure, while neutron stars rely on neutron degeneracy pressure. In both cases, when density reaches critical levels, nuclear reactions can occur, releasing electromagnetic radiation that we can observe.

No such reactions can happen when two black holes collide because their internal structures—believed to be singularities—are concealed within the event horizons. Consequently, any activity occurring during their merger remains inaccessible to outside observers. Thus, the only potential for observing electromagnetic signals must stem from interactions in the external environment.

The mechanism for light emission during black hole mergers must involve matter interacting outside their event horizons. Numerous astronomical scenarios show how matter can generate light by interacting with black holes, such as during tidal disruption events or in X-ray binaries where a star loses mass to an orbiting black hole.

As for the recent observations, they suggest that light emission during black hole mergers may be contingent on the presence of external matter. While most models predict minimal energy transfer during mergers, extreme conditions could lead to detectable light emissions.

The initial black hole merger observed by LIGO yielded a weak signal detected by NASA’s Fermi telescope, which was uncertain and potentially a false positive. Since then, subsequent mergers have not shown any signs of gamma-ray emissions, leading to doubts about the earlier event.

However, on May 21, 2019, LIGO identified three potential merger events, with one showing a 97% likelihood of being a black hole merger. This event was particularly notable due to the simultaneous detection of an electromagnetic flare by the Zwicky Transient Facility, indicating a possible correlation between the two phenomena.

This exciting development hints that the black hole merger may have transpired in a gas-rich region of a galaxy, with the resulting flare likely fueled by an accretion tail. If validated, this would mark the first time optical light has been associated with a black hole merger, suggesting that merging black holes might indeed produce observable light under specific circumstances.

Dr. Eric Burns, part of the NASA Fermi team, encapsulated this sentiment by stating that if confirmed, this discovery could pave the way for further joint gravitational wave and electromagnetic detections, allowing for a broader understanding of the cosmos. The future of merging black holes, it seems, could be bright after all.