In a galaxy some 450 million light-years away, a brilliant flare of light revealed a probable black hole that suddenly woke from its slumber to snack on a passing star. This is not, in itself, a hugely unusual occurrence.
The sting in this particular tail is the identity of the black hole in question: an elusive middleweight, or intermediate-mass black hole. This is a mass range so rarely spotted that its absence has posed a huge challenge to our understanding of supermassive black hole formation.
Black holes, you see, usually come in two distinct mass regimes. There are the stellar-mass black holes, which fall in the same sort of mass range as stars, up to about 100 or so Suns’ worth of mass. These form when a massive star goes supernova, leaving the core behind to collapse under gravity, becoming a black hole.
A supermassive black hole is millions to billions of times the mass of the Sun. These behemoths lurk in the centers of galaxies, forming a gravitational hub. How we get supermassive black holes is a huge mystery, although one astronomers are closing in on.
Related: This Could Be The First Witnessed Birth of a Supermassive Black Hole
One possible pathway is growth from a stellar mass black hole seed – but the big problem with this is that astronomers have found shockingly few black holes between around 100 and 1 million solar masses. If black holes grow from stellar mass to supermassive, you’d expect to see the Universe peppered with objects partway through that journey.
This is where an X-ray source called HLX-1 comes in. This object, located in a not-too-distant galaxy, has been caught emitting very bright X-rays. It appeared in X-ray observations in 2009; by 2012, it was 100 times brighter. By 2023, it was dimmer again.
The wonderful thing about the light emitted by an object as it is devoured by a black hole is that it can tell us about the black hole’s size. The light emitted by HLX-1 was too bright to be a stellar-mass black hole; but not at the level of a supermassive black hole.
The best fit for the observed light and the changes it underwent, according to a team led by astronomer Yi-Chi Chang of the National Tsing Hua University in Taiwan, is an intermediate-mass object, between around 1,000 and 10,000 solar masses, waking up and having a snack. Whether it was a one-off snack or an orbiting star from which the black hole takes periodic bites is yet to be determined.
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“Now we need to wait and see if it’s flaring multiple times, or there was a beginning, there was peak, and now it’s just going to go down all the way until it disappears,” says astronomer Roberto Soria of the Italian National Institute for Astrophysics.
Whichever way it goes, the discovery is an exciting one, advancing what we know about the most gravitationally extreme objects in the Universe.
The research has been published in The Astrophysical Journal.
