Powerful blasts of radiation from distant galaxies have revealed signs of an elusive medium-sized black hole, which may be a link between the small black holes that we have observed and the supermassive black holes that sit at the centres of galaxies. These medium-sized objects are notoriously hard to find because they don’t shine brightly like supermassive black holes and are too big for current gravitational-wave detectors to spot.

Rachel Webster at the University of Melbourne in Australia and her colleagues spotted this cosmic middleweight by examining archival data on about 2700 gamma-ray bursts, extraordinarily bright flashes of radiation thought to come from massive explosions in other galaxies. They searched this catalogue for evidence of gravitational lensing, which occurs when a massive, dense object stretches light around it.

The researchers found only one example of gravitational lensing, in which the light from the gamma-ray burst was warped by an object about 55,000 times as massive as the sun. An object that massive yet dim is probably a black hole – and if that is the case, it is far smaller than the smallest supermassive black hole we have ever seen and far larger than the largest “normal” one.

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“We can’t be 100 per cent sure [that this is a black hole], but the other likely objects are either not compact enough or not common enough,” says Webster. She and her colleagues estimated that if this object is a black hole, there is probably about one of a similar mass per 15 million billion cubic light years.

This object and others like it may be a link between small black holes and extremely large ones. “We’ve got these small black holes and we’ve got these supermassive black holes, but in the middle was nothing,” says James Paynter at the University of Melbourne, part of the research team. Studying these objects could help us figure out how supermassive black holes form, which is a major mystery in modern astrophysics.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-021-01307-1