Astronomers have measured the spin of one of the universe's most massive black holes — and provided evidence that the behemoth has a companion.
How many astronomers does it take to measure the spin of a black hole?
If the black hole is the one of the most massive black holes known in the universe, 3.5 billion light-years away, and part of a binary system, it takes close to 100 astronomers using two dozen telescopes around the world.
Mauri Valtonen (University of Turku, Finland) and collaborators announced in the March 10th Astrophysical Journal Letters that the heavyweight black hole at the heart of a faraway galaxy, OJ 287, rotates at one-third the maximum spin rate that general relativity allows for black holes. That translates to a speed of 236 million mph (100 million meters per second) at the equator of its event horizon, which has a radius of roughly 30 billion miles.
“Black hole spin is a very tricky measurement to make,” says Eileen Meyer (University of Maryland). This particular measurement, she adds, is notable not only for its precision but also because it’s ‘slam-dunk evidence’ for what was long suspected: that a duo of supermassive black holes lurk at the center of this galaxy.
Black Hole Tango
Since 1891 astronomers have been observing flashes of radiation from OJ 287 that recur roughly every 12 years. A close inspection of newer data revealed the presence of double peaks in the flares.
Valtonen and colleagues suggested that a binary black hole would explain this odd behavior. In their scenario, a black hole with the mass of 18 billion Suns sits at the center of the galaxy, fed by glowing gas that swirls in a disk around it. A smaller black hole, one with a mere 150 million solar masses, orbits the larger black hole at an angle to its accretion disk. When the smaller black hole punches through the disk, the gas heats up and emits a weeks-long flash of radiation. The elongated orbit of the smaller black hole causes it to crash through the disk twice in rapid succession. Then many years go by before it returns.
The key to Valtonen’s setup is that the orbit of the smaller black hole should precess around its big brother, the orbital plane wobbling around like a spinning top. The rate of that precession depends directly on the more massive black hole’s spin rate.
Valtonen and colleagues examined eight historical bursts from OJ 287. They first measured what the precession rate would have to be to explain the data, then they calculated the behemoth black hole’s spin. And based on their model, they predicted that OJ 287’s next flare should occur on Nov. 25, 2015. (That’s near the 100th anniversary of the publication of Einstein’s general theory of relativity, which the astronomers used to predict the orbit’s precession).
Staszek Zola (Mount Suhora Observatory) led an international observing campaign to catch this predicted burst. The effort involved optical telescopes in Japan, South Korea, India, Turkey, Greece, Finland, Poland, Germany, the U.K., Spain, U.S.A., and Mexico. Close to 100 astronomers participated, including a number of amateur astronomers.
And their hard work paid off. They caught the burst as it began on November 18, 2015. It reached maximum brightness on Dec. 4. Those observations confirmed the team’s incredibly precise estimate of the more massive black hole’s spin, as well as their estimate for the smaller black hole’s mass.
Novel Method Sows Doubt
Not everyone is won over by the results. “This is a somewhat complicated model,” says Daniel Holz (University of Chicago). “I confess I’m skeptical of the claims in this paper that they have measured the spin of the black hole to such high precision.”
Marco Chiaberge (Space Telescope Science Institute) agrees the method is unconventional, but he adds, “It’s potentially very interesting.”
That’s because inconsistencies have hobbled the more usual method for measuring black hole spin.
Typically, astronomers look at X-rays reflected off the inner edge of the accretion disk, deep within the gravity well of the black hole. The accretion disk is only stable to a certain point — when material gets too close to the black hole, it falls in — and the position of that inner edge depends directly on the black hole’s spin.
But even though this method for measuring spin is well established, different research groups get widely different answers for the same black holes. “So far this method [of measuring the accretion disk’s inner edge] hasn’t been totally reliable,” says Meyer. “It is a very open question why this is happening, and it probably involves the fact that nature is more complicated than our simple models.”
A Slow Spin
Oddly enough, while previous measurements have generally found high spin rates (near the maximum rate) for most supermassive black holes, Valtonen’s team found that OJ 287’s behemoth spins at a relatively slow rate, a third of what’s theoretically possible.
Black holes are expected to spin fast if they grow from the gas that feeds in through the accretion disk, because it only feeds in from one direction. But if random mergers played an important role in this black hole’s growth, they would have slowed its spin. It's also possible that OJ 287’s jets might be tapping its spin energy and slowing the black hole's rotation.
It's clear that, even after more than 100 years of observations, OJ 287's black holes still have some stories to tell.