The pantheon of black holes includes little ones not much bigger than our sun and supermassive ones at the centers of galaxies. But where are all the medium-sized black holes? That question has long troubled astronomers, but now they may have finally spotted one. The leading candidate for a just-right black hole—an object called X-1 in the constellation M82—has just received its most accurate mass estimate so far: about 400 times the mass of the sun.

Black holes, by their very nature, can’t be seen directly and reveal their presence only by the effect their gravity has on nearby stars and clouds of dust and gas. From such clues, astronomers have identified many stellar-sized black holes, created by the collapse of a giant star and other material it may have gobbled up afterward. These usually weigh up to tens of times the mass of the sun. Meanwhile, most and perhaps all galaxies seem to have a huge black hole at their hearts, with masses that range from hundreds of thousands to billions of solar masses.

In their search for the missing intermediate-mass black holes (IMBHs), astronomers have zoomed in on rare objects known as ultraluminous x-ray sources (ULXs). Such x-ray sources are thought to be binary systems made up of a black hole and another star orbiting each other, with the black hole sucking in material from its partner. That material first settles into a sort of gravitational vortex called an accretion disk, and before vanishing forever into the black hole the accreting material becomes so hot from friction that it emits x-rays. ULXs are extreme examples of this phenomenon, emitting an x-ray signal that is too bright to be produced by a stellar-sized black hole but still not bright enough to be from a supermassive black hole. X-1, which is 12 million light-years from Earth, is one such ULX, and several studies over the past decade suggested that it has a mass of hundreds of suns, based on its luminosity and temperature.

Still, astronomers couldn’t rule out that it was simply a very luminous but small black hole, says astronomer Dheeraj Pasham of the University of Maryland (UMD), College Park. So along with colleagues from UMD and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, Pasham set out to more accurately weigh X-1 using an obscure x-ray emission pattern detected in smaller black holes.

Over the past decade, astronomers studying the x-ray emissions from stellar black holes noticed that they seem to beat like a drum, with a consistent 3-2 ratio. One black hole’s x-rays might be beating at 150 times per second as well as 100 times, while another could be beating 300 and 200 times. Astronomers don’t know what is causing these syncopated emissions but assume that some sort of resonance must develop in the superheated, x-ray-emitting gas just before it falls into the black hole. Crucially, they also found that the frequency of the beats was inversely proportional to the mass of the black hole, so a 150-100 rhythm would signal a more massive black hole than a 300-200 rhythm.

The team wondered if this relationship could be extrapolated to IMBHs. For medium-sized black holes, the researchers predicted, the beats would have single-figure frequencies. The team gathered data on X-1 from NASA’s Rossi X-ray Timing Explorer, which operated from 1995 to 2012, looking for beat frequencies between 1 and 16 times per second. “We stacked up all the archival data and the [3-2 rhythms] just appeared,” Pasham says. As they reported online yesterday in Nature, they found that X-1 was beating out rhythms at 5.07 and 3.32 times per second; following the relation between frequency and mass, that corresponds to a black hole with a mass of 428 suns.

“Confirmation of a mass around 400 solar masses for M82 X-1 is very exciting because it puts the black hole clearly in the intermediate mass range—too heavy to be made in the collapse of a single star and much lighter than the supermassive black hole found in galactic nuclei. Thus, X-1 would represent a new class of object requiring a novel formation mechanism,” says astronomer Philip Kaaret of the University of Iowa in Iowa City, who was not involved in the research.

Others, however, are not yet sure this is the final proof of the existence of medium-sized black holes. “It’s not conclusive, but this is a good extra clue,” says astronomer Alessandro Patruno of Leiden Observatory in the Netherlands. He cautions it won’t be considered proof until researchers have a better handle on how these rhythms are created in accretion disks and whether it is justified to extrapolate from small black holes to much larger ones. “We don’t understand how accretion disks behave around ULXs,” he says.