Astronomers have used the death of a star to uncover details about a hidden intermediate-mass black hole.

The largest black holes in the cosmos did not spring forth fully grown. They must have started smaller, as seeds, then through collisions and gas-slurping quickly grown larger. The question astronomers face is, what were the seeds like? How did they form, how big were they at birth, and how did they grow?

To answer these questions, astronomers need to find seed-like objects. Called intermediate-mass black holes, or IMBHs, these objects should weigh in between stellar-mass black holes like Cygnus X-1 and the leviathans that lurk in massive galaxies’ centers, like the Milky Way’s own Sagittarius A*. Observers have found a few dozen candidates, with masses equivalent to tens to hundreds of thousands of Suns. But we know next to nothing about them.

Sixiang Wen (University of Arizona) and others have now taken a closer look at one of those candidates and extracted information we’ve never had before about an IMBH. The middling black hole appears to sit in a star cluster near a galaxy about 740 million light-years away in the constellation Aquarius. Normally the black hole is invisible, but astronomers spotted it when it tore up and swallowed a star, skirting itself in the glowing debris and lighting up in an event dubbed 3XMM J215022.4−055108 (hereafter J2150, because I’m verbose but not with my nomenclature).

This animation shows another tidal-disruption event, called ASASSN-14li. In an earlier paper, Wen and his colleagues used their method to determine that the black hole involved in ASASSN-14li has a mass of about 10 million Suns and a spin at least 30% the maximum permitted.
NASA’s Goddard Space Flight Center / CI Lab

As this tutu of hot gas swirled around and fell into the black hole, it heated up, emitting X-rays. The team used observations spanning 12 years from the XMM-Newton and Chandra X-ray space telescopes to watch the cataclysm unfold. The brightness at different X-ray energies and how that spectrum changes with time depend on the black hole’s mass and spin, because they determine the spacetime landscape that the gas is traveling through.

Using an approach originally designed for stellar-mass black holes, the team was able to calculate the IMBH’s approximate mass and spin: less than 22,000 Suns and about 80% the maximum, respectively. The study appears in the September 10th Astrophysical Journal.

Astronomers have used star-shredding calamities, called tidal-disruption events (TDEs), to measure supermassive black holes’ spins before — but they’ve never done it for an IMBH. “The added potential of constraining the spin, in addition to the mass of the black hole, is certainly exciting,” says TDE expert Suvi Gezari (Space Telescope Science Institute) of the new method.

But what’s truly curious about this result is the value of the spin. Black holes can have spins of 0 to 1, where 1 is the maximum permitted for the black hole’s mass. The spin’s value can tell us how a black hole grew. But there’s no good explanation for a value of 0.8. It’s slightly too high to match black holes made by merging smaller ones, which gravitational-wave measurements show often have spins clustered around 0.7. It’s also far, far too high to have grown by munching intermittent gas snacks from many directions, yet too low to have grown by eating a steady stream of gas — those black holes should spin close to the maximum.

How J2150’s IMBH formed and grew is thus a mystery. Wen personally favors either a runaway collisions of stars or even direct collapse, in which a large, pristine gas cloud crumpled in on itself and made a black hole from scratch. The black hole has the right mass to fit in the direct-collapse scenario, which vies with a couple of others as a favored origin for seeds in the early universe.

The next step, of course, will be to gather more TDE-revealed IMBHs. The eROSITA X-ray space telescope will be a key player: it has already turned up a baker’s dozen of TDEs since its launch in 2019, and astronomers expect it to find many more. Wen says that he and his colleagues cannot yet use eROSITA’s discoveries with their method, however, because there isn’t enough information: the telescope has been scanning the sky in survey mode, and they need longer exposures from targeted pointing, which will come later.  


S. Wen et al. “Mass, Spin, and Ultralight Boson Constraints from the Intermediate-mass Black Hole in the Tidal Disruption Event 3XMM J215022.4–055108.” Astrophysical Journal. September 10, 2021.


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