The cosmos is riddled with black holes, both big and small. But the middle ground — medium-sized black holes — is a little trickier. While most big galaxies harbor at least one supermassive black hole, and low-mass black holes like Cygnus X-1 are numerous (if harder to detect from a distance), but we only know about a few black holes in between.
But what if there was a trick to identifying them? Light emitted from matter close to a low-mass black hole flares in a predictable pattern that depends on the black hole’s mass. A trio of astronomers think they’ve identified that same pattern for a black hole in the “Cigar Galaxy”, more formally known as M82. If they’re right, the frequency of flares means the black hole has a mass between 300 and 500 times that of the Sun, squarely in the range for intermediate-mass black holes.
For low-mass black holes, the pattern is the juxtaposition of two evenly-timed bursts of light with three flares in the same time interval. Music aficionados recognize this rhythm as a triplet or “hemiola”: the playing of two different musical patterns simultaneously. Another musical metaphor: if we interpret the frequencies as notes instead of rhythms, the ratio of 3 to 2 is known as a “perfect fifth” in harmony, the foundation of any number of chords. Low-mass black holes “sing” in harmony with themselves, though with flashes of light instead of sound.
Do middleweight black holes do the same?
Stellar-mass black holes — which is the proper name for what I’ve been calling “low-mass” — form when a star much bigger than the Sun explodes and its core collapses. As a result, their size is constrained by how massive the original star was, so stellar-mass black holes are roughly between 3 and 50 times the mass of the Sun. Supermassive black holes, by contrast, formed early in the universe’s history by some mechanism we don’t fully understand yet, and they weigh in at more than 100,000 times the mass of our Sun.
That leaves a huge gap in the middle, where intermediate black holes (IMBHs) could exist. However, astronomers have only identified a half-dozen or so IMBH candidates, and some of those claims are controversial. Black holes don’t carry little signs telling us how massive they are, so without some other evidence — a star orbiting it, for example — researchers are stuck with more indirect methods to measure their masses.
We just need to listen for the song of the black hole.
For stellar-mass black holes, a reliable technique involves watching fluctuations in their X-ray emissions. Black holes themselves are invisible by nature, but by feeding on matter (electrons, protons, and a few heavier atomic nuclei) they can become some of the brightest objects in the universe. As that matter orbits the black hole, it heats up and emits a lot of light.
Those orbits aren’t perfect circles, though, meaning matter moves closer and farther away from the black hole in a regular pattern. Near the black hole’s “surface” (the event horizon, which is the point of no return for anything crossing it), that motion is exaggerated, resulting in larger amounts of emitted light. That’s the source of the 3-to-2 ratio of light bursts for stellar-mass black holes.
Which brings us back to the possible IMBH, known Star Trekishly as M82 X-1, the brightest X-ray source in M82. Because it’s so bright, some astronomers proposed it must be too big to be stellar-mass. Other researchers raised reasonable doubts to that idea, instead arguing the data are consistent with a black hole 20 times the mass of the Sun that happens to be feeding more aggressively than usual.
For that reason, Dheeraj Pasham, Tod Strohmayer, and Richard Mushotzsky looked at observations of M82 by the Rossi X-ray Timing Explorer (RXTE) satellite stretching back over many years. While the light from M82 X-1 fluctuates dramatically and (mostly) randomly, by averaging the emissions over the entire time period, the astronomers found two frequencies stood out from the noise. Those frequencies were about 5 Hz and 3.3 Hz, a ratio of 3-to-2.
The bigger the black hole, the lower the frequency, much as it is with musical instruments. A stellar-mass black hole would produce pulses in the 100 to 450 Hz range, though still with that 3-to-2 ratio between the flares. A 5 Hz pulse indicates a much bigger black hole, and a strong argument in favor of M82 X-1 having somewhere between 300 and 500 times the mass of the Sun. If a stellar-mass black hole is a violin, an IMBH is a double bass.
The story isn’t over, though. The three astronomers argued persuasively that the rhythm of X-ray bursts they measured is most likely due to the M82 X-1. But it’s still possible other sources — pulsars, other black holes — could be contaminating the data, tricking us into thinking the harmony is there when it’s not.
However, if they’re right and intermediate-mass black holes can be identified by their tell-tale harmony, then astronomers should be able to raise the number of observations from a half-dozen to many more. If we can learn how many IMBHs are out there and where they live, we’ll have another way to study the structure and evolution of galaxies. We just need to listen for the song of the black hole.