Closest black hole binary system
The combined mass of these 2 black holes is 40 million times the mass of the Sun -- Representational ImageREUTERS/NASA/Handout

Scientists from the National Centre for Radio Astrophysics (NCRA) and the Rochester Institute of Technology have discovered the closest-ever supermassive black hole binary system.

The pair of gigantic orbiting black holes has been spotted in the spiral galaxy NGC 7674, located 400 million light-years away from the Earth. This is dozens of times the distance to neighbouring galaxy Andromeda but close enough to justify the rarity of supermassive black hole binaries.

The combined mass of these black holes is 40 million times the mass of the Sun and they're orbiting remarkably close around each other, with a separation of less than a light year – the smallest gap between black holes of a binary system. But, it's worth noting that the orbital period of the gigantic black holes is about 100,000 years.

Researchers found the existence of these two voids with the help of Very Long Baseline Interferometry (VLBI), a technique in which separate radio telescopes work as one to achieve an angular resolution of milliarcseconds or microarcseconds, roughly equalling ten million times the angular resolution of the human eye.

Using this method, they detected two compact sources of radio emission at the centre of NGC 7674. "The two radio sources have properties that are known to be associated with massive black holes that are accreting gas, implying the presence of two black holes," Dr Preeti Kharb from NCRA, the co-author of the paper said about the newly discovered system.

Black holes are dark and mysterious but are known to exist at the centres of most galaxies. Now, the discovery of this binary system builds on the evidence that suggests gravitationally bound supermassive black holes collide in galaxies and are the potential source of gravitational waves.

Before this, a group of scientists from Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves in 2015, confirming the existence of supermassive black hole binaries.