One of the most defining characteristics of a black hole is that nothing can escape once it passes the event horizon—not even light. So one would expect the same to be true when two black holes collide and merge. But some astronomers have posited that there could be unusual conditions in which such a merger could produce an accompanying explosion of light. By combining gravitational wave data with data collected during a robotic sky survey, one team thinks it has found the first evidence of such a phenomenon, according to a new paper in Physical Review Letters.
LIGO detects gravitational waves via laser interferometry, using high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. (LIGO has detectors in Hanford, Washington, and in Livingston, Louisiana. A third detector in Italy, Advanced VIRGO, came online in 2016.) On September 14, 2015, at 5:51am EST, both detectors picked up signals within milliseconds of each other for the very first time—direct evidence for two black holes spiraling inward toward each other and merging in a massive collision event that sent powerful shockwaves across spacetime.
The collaboration picked up two more black-hole mergers from that first run. The second run, from November 30, 2016, to August 25, 2017, produced seven more binary black-hole mergers (including the four just announced) and a binary neutron-star merger, supported by a simultaneous gamma-ray burst and signals in the rest of the electromagnetic spectrum, dubbed a "kilonova." It was an unprecedented recording of a major celestial event, combining light and sound, and officially marked the dawn of a new era in so-called "multi-messenger astronomy."
In December 2018, the collaboration reported four previously unannounced second-run detections of gravitational waves from merging black holes, including the biggest-known black-hole collision to date, roughly 5 billion years ago. LIGO/VIRGO kicked off its third run April 1, 2019, and within a month had detected five more gravitational wave events: three from merging black holes, one from a neutron star merger, and another may have been the first instance of a neutron star-black hole merger. (For hardcore LIGO buffs, there's now an iPhone app that lets you follow the event announcements, with an Android version in the works).
Ever since the 2017 "kilonova," astronomers have scrambled to look for a corresponding optical signature whenever LIGO/VIRGO picks up a gravitational wave signal for neutron star mergers or possible neutron star-black hole mergers. But the assumption has been that black hole-black hole mergers would not produce any optical signature, so there was no point even looking for one. Matthew Graham, an astronomer at Caltech and co-author of the new paper, is among the astronomers who proposed an alternate model last year, predicting that under certain conditions, and in a particular environment, such a merger would give off an optical signature in the form of an intense flare.
That's the significance of the current paper: Graham et al. have found the first possible evidence that their model may be correct. In this case, it relates to a binary black hole merger that LIGO spotted on May 21, 2019 (designated S190521g). That binary system may have formed in the accretion disk surrounding a supermassive black hole at the center of a galaxy.
Co-author K.E. Saavik Ford of the City University of New York Graduate Center likened the accretion disk to a swarm of stars and dead stars—including black holes. “These objects swarm like angry bees around the monstrous queen bee at the center," she said. "They can briefly find gravitational partners and pair up but usually lose their partners quickly to the mad dance. But in a supermassive black hole’s disk, the flowing gas converts the mosh pit of the swarm to a classical minuet, organizing the black holes so they can pair up.” When that binary pair finally merges, the new, larger black hole they form gets a powerful kick and plows through the gas in the accretion disk, which reacts by producing a bright flare astronomers can pick up with their telescopes.
While astronomers hunting for optical signatures for other types of mergers typically do so as quickly as possible, the moment LIGO/VIRGO reports an event for a black hole-black hole merger, "it takes time for the visual ball flare to build up," Graham told Ars. "We can wait a few days or a few weeks before we might actually see this, so it's a very different follow-up strategy."
Graham and his co-authors began to scour the night sky for evidence of just such an optical signature using the Zwicky Transient Facility (ZTF), a robotic camera attached to the 70-year-old Samuel Oschin telescope at the Palomar Observatory in San Diego County, California. ZTF performs robotic surveys of the night sky, looking for objects that erupt or vary in brightness: supernovas, stars being munched on by black holes, and asteroids and comets, for example. It scans the entire sky over three nights and the visible plane of the galaxy twice every night—even during the coronavirus shutdown.
Lurking within the data collected in the days and weeks after LIGO's May 21 detection was just such a signal, slowing fading over the ensuing month. The supermassive black hole is about 100 million solar masses, according to Graham—about the size of the Earth's orbit. The binary black holes that merged were about 50 solar masses, akin to the size of Manhattan or Long Island. As for the kick delivered to the new, larger black hole, it clocks in at about 500,000 miles per hour. "So these are significant energetic systems," said Graham.
"The flare was on the right timescale, and in the right location, to be coincident with the gravitational-wave event," said Graham, although he acknowledges that there is still a possibility that the flare was produced by something other than a binary black hole merger. There are also good reasons to rule out the most obvious alternative candidates.
It could be a tidal disruption event, for instance, except "the supermassive black hole is too massive [for that] and it doesn't look right," said Graham. And while the energies are about right for a supernova, "the time evolution is wrong and the shape doesn't look particularly right," he said. A less likely possibility is that the flash originates with a supernova embedded in the accretion disk of a quasar, which has never been observed before—and therefore would be highly intriguing to astronomers in its own right.
Of course, conditions need to be just right for this phenomenon to occur. According to Graham, even though the merging binary black holes were small in comparison to the supermassive black hole nearby, they were still quite large. Black holes in the 50 stellar mass regime typically are not created from a supernova explosion. It's far more likely each started out as a smaller black hole that merged with another before pairing with each other. "That sort of hierarchical merger model can only happen in a couple of preferred places in the universe," said Graham. "You need a deep gravitational well, and that's what a supermassive black hole gives you. It's the perfect environment for hierarchical merging to occur."
Graham thinks there could be more such binary black hole mergers occurring in the accretion disk of a supermassive black hole out there. That means there could be more powerful flares associated with those events, and he estimates astronomers might be able to spot between 25 to 50 percent of those, depending on their orientation. If this new model is correct, hunting for this type of optical signature could become a new observing strategy in the broader context of multi-messenger astronomy.
Graham et al. are currently conducting a systemic search of their data that coincides with all the LIGO events detected thus far from the collaboration's third run, hoping to find more. And this newly formed black hole should produce another energetic flare sometime in the next few years, when it enters the supermassive black hole's accretion disk again,
DOI: Physical Review Letters, 2020. 10.1103/PhysRevLett.124.251102 (About DOIs).
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