Black holes seem to fall into two main categories: relatively small and supermassive. That leaves a huge empty middle ground, and astronomers have long proposed that there must be intermediate-mass black holes (IMBHs) somewhere in between. Now, scientists from the University of New Hampshire have captured the best evidence so far of the existence of these mysterious middleweights, as one emerges from the dark to snack on an unsuspecting star.

The baby of the black hole family are the stellar mass black holes, formed when a star collapses at the end of its life and creates an object with a mass a few times that of the Sun. Right up the other end of the scale are the supermassive black holes that can have the mass of millions or even billions of Suns and are usually found lurking at the center of galaxies.

But there are other classes proposed to exist. While some scientists are calling for the biggest of the biggest to be reclassified as "ultramassive" black holes, others are looking for evidence of those with intermediate masses.

Of course, it's not easy to find invisible objects in pitch-black space, but astronomers have their ways. A previous detection of a possible IMBH was found by watching for how its gravity stirs up the stars around it like a "cosmic spoon," but the best way is to look for when they make themselves known by ripping apart stars, putting on a stellar light show known as a tidal disruption event (TDE). As material from the hapless star falls into the black hole, it heats up to millions of degrees, generating an incredibly bright X-ray flare that's fairly easy to see.

And that's exactly what the New Hampshire researchers have spotted, using data from NASA's Chandra X-ray Observatory, the Swift Satellite and ESA's XMM-Newton. As the object began devouring a star in October 2003, the team was able to catch it in the act, detecting a huge multiwavelength radiation flare.

As that signature faded over the following decade, the mass of the black hole could be measured through the distribution of emitted photons over the energy. And in doing so, the team was able to determine it fit the bill for an IMBH.

"We feel very lucky to have spotted this object with a significant amount of high quality data, which helps pinpoint the mass of the black hole and understand the nature of this spectacular event," says Dacheng Lin, lead author of the study. "Earlier research, including our own work, saw similar events, but they were either caught too late or were too far away."

This method is a good indicator of the mass of a black hole, and this specific case provides good evidence for the existence of IMBHs. On top of that, it might also help explain why astronomers have had so much trouble spotting these middle-ground objects, despite their presumed commonality. Star-destroying events are relatively rare, so the intermediate black holes don't often come out of hiding.

"From the theory of galaxy formation, we expect a lot of wandering intermediate-mass black holes in star clusters," says Lin. "But there are very, very few that we know of, because they are normally unbelievably quiet and very hard to detect and energy bursts from encountering stars being shredded happen so rarely."

The research was published in the journal Nature Astronomy.

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