Astronomers have forecast a flare from a supermassive black hole duo 3.5 billion light-years away.
Scientists have forecast the behavior of light coming from the vicinity of one of the biggest supermassive black holes known. The light, which traveled to us over 3.5 billion years, flared in brightness just as they said it would — within four hours of the time they had predicted.
OJ 287: From 1913 to Now
The black hole in question is OJ 287, and it’s more than a heavyweight champion — it’s actually two black holes with 18 billion and 150 million Suns’ worth of mass, respectively. The bigger black hole is feeding from the disk of gas surrounding it, while the smaller of the two swings around in a highly elliptical orbit, punching through the disk twice every 12 years.
We know all this because astronomers have been watching OJ 287 since the 1890s, before they knew what it was. In the intervening century, the system has shot off two outbursts roughly every 12 years, almost like clockwork.
Yet this pattern took time to decipher, as the bigger black hole in OJ 287 is also a blazar. Its black hole, or the disk that feeds it, powers twin plasma jets shooting out along opposite directions, and one of these jets is pointed almost right at Earth. The volatility of this plasma-and-photon stream makes OJ 287 a highly variable visible-light source. It wasn’t until a century after its discovery that astronomers realized that there was a periodic signal hidden within the noise — and that dual dancing black holes could cause it.
Observations in 2005 confirmed those ideas, and astronomers made increasingly precise predictions for subsequent flares in 2007 and 2015. Now, Seppo Laine (Spitzer Science Center, Caltech), Lankeswar Dey (Tata Institute of Fundamental Research, India), and colleagues are publishing observations of the latest flare in the Astrophysical Journal Letters.
Forecasting a Flare
The authors predicted, and then watched for, a flare expected to arrive in the early hours of July 31, 2019. The only problem was that the event was due to happen at an inauspicious time for Earthbound observers, as OJ 287 was hidden in the glare of the Sun at the time.
The Spitzer Space Telescope, still operating at the time in a trailing-Earth orbit, came to the rescue. It observed OJ 287 from its vantage point 250 million kilometers (160 million miles) from Earth. And just as Spitzer set to watching, it spotted the flare — right on time.
The newest prediction was so detailed that its precision depended on a peculiar quality of black holes: These behemoths pack their mass in so tightly, we can’t observe the mass itself. The closest thing we can ever hope to see is the event horizon, the point of no return for anything hoping to escape the black hole’s gravitational pull. The researchers’ prediction assumed the event horizon’s surface is smooth, not bumpy or asymmetrical, and the right-on-time flare confirms this to be true.
The newest prediction also took into account the emission of gravitational waves. As the two massive black holes orbit each other, they release some energy in the form of these waves, altering their orbits and the predicted flare times.
These waves are not a frequency that the LIGO or Virgo detectors would be able to hear, nor are they within reach of the future-generation LISA space gravitational-wave detector. Instead, the frequencies are so low-pitched that they can only be caught in pulsar timing arrays, in which astronomers measure the slight variance in neutron stars’ pulses to indirectly detect gravitational waves.
Diving Deeper in OJ 287
While in the past not everyone has agreed on the dual black hole scenario for OJ 287, the latest flare prediction adds to a growing mound of evidence.
“I personally think the results are pretty convincing,” says Marco Chiaberge (Space Telescope Science Institute). While the scenario of having a black hole punch through an accretion disk around a bigger black hole is admittedly complicated, he says he doesn’t know of better alternatives.
Laine, Dey, and others are now using the flares to dig further into the properties of the black hole system. For example, they’ve found that the flare is produced by the interaction of charged particles, known as bremsstrahlung, a distinct type of emission that’s not typically found coming from blazars. They also found that their current measurements are consistent with previous predictions of the more massive black hole’s spin rate, which is surprisingly low at 38% of the maximum rate.
This all sets the stage for future observations by the Event Horizon Telescope (EHT), which famously imaged the black hole in nearby M87. EHT images radio waves using very long baseline interferometry (VLBI), but it would need even longer baselines to see a system like OJ 287 — astronomers would have to send at least one radio dish to space.
“As the authors state, the final confirmation [of OJ 287’s nature] may come when space VLBI and EHT will eventually be able to spatially resolve the binary system,” Chiaberge says. “That would be super exciting!!”