The Gamma-ray Burst Monitor on NASA’s Fermi Gamma-ray Space Telescope picked up gamma-rays from a pair of merging black holes less than half a second after producing gravitational waves on Sept. 14, 2015. For the first time, this disproves that black holes merge without producing any light.

Light from short gamma-ray bursts (GRBs), usually last for less than two seconds. Experts say that these normally occur when neuron stars and black holes spiral inward and crash together. However, the same is not thought to happen in merging black holes.

Scientists believe that black hole mergers are not supposed to emit gamma-ray signals because any gas, which is supposed to produce light, is cleared away before plummeting into the black hole. Hence, some have concluded that the gamma-rays detected by the telescope, which lasted for only one second, was purely a coincidence.

Nonetheless, the team that monitored the occurrence asserts that there is only less than a 0.2 percent chance that the two black holes, each with 30 times the mass of the sun, emitted the gamma-rays randomly. Nevertheless, lead researcher Valerie Connaughton says that more studies are still needed to understand this and answer remaining questions.

“This is a tantalising discovery with a low chance of being a false alarm, but before we can start rewriting the textbooks we’ll need to see more bursts associated with gravitational waves from black hole mergers,” states Connaughton, whose study is under review by The Astrophysical Journal.

The scientists say that the light from a gravitational wave source will provide more insight about the event. Gamma-rays can even be used to locate the sources of gravitational waves.

NASA states that gamma-rays are the most energetic form of light and are emitted by the hottest parts of the universe. Gamma-rays were not studied until the Explorer XI, the first gamma-ray telescope, detected less than 100 gamma-ray photons in 1961.