Black holes don’t emit light — that goes without saying. Furthermore, in “normal” situations, stellar-mass black holes should have rid themselves of their tutus of hot gas long before they collide, removing that source of light, too. But for the event S190521g, the normal rules may not apply.
As the number of gravitational-wave detections grows, astronomers have been working hard to figure out where the black holes that produce these waves are coming together. A key clue is the black holes’ spins. Although scientists don’t have precise spin measurements for most of the black holes the LIGO and Virgo detectors have caught colliding, they see hints that these spins’ orientations are all over the place, says LIGO astrophysicist Vicky Kalogera (Northwestern University).
Wonkily aligned spins suggest that these black holes likely didn’t begin as isolated binary stars, paired up together since birth and largely unaffected by their surroundings. Rather, the black holes may have come together later, perhaps in the hearts of ancient globular clusters. Globular clusters are a particularly good way for black holes of similar masses to hook up, because they would sink to similar spots in the cluster over time.
Another possibility is the fat disks of gas feeding some supermassive black holes. Stellar-mass black holes swarm around the supermassive one, “like angry bees around the monstrous queen bee at the center” study coauthor K. E. Saavik Ford (City University of New York) said in the group’s press release. Normally, the little black holes would only manage to pair up momentarily before being torn apart again. But if caught in the disk’s gas, they would migrate in more orderly ways, facilitating pair-ups.
If one of these pairs were to merge, the black hole created would bulldoze its way through the gas, causing a flare days to weeks after the merger and lasting roughly a month.
Now, Matthew Graham (Caltech), Ford, and their colleagues say they’ve found evidence for exactly this kind of flare.
The team has been looking for flares by following up on candidate events found during the LIGO-Virgo team’s third observing run. Of the 21 events they investigated, S190521g matched up with a peculiar flare from the active galactic nucleus (AGN) J1249+3449. (The putative merger doesn’t have a GW in its designation because it’s not officially confirmed yet; the LIGO-Virgo team is working on the paper now.)
The flare lasted about a month, and it showed no signs of changing color, as you’d expect from a supernova blast expanding and cooling. Nor does it seem to be from the supermassive black hole itself: Based on the AGN’s past flickering, there’s only a 0.002% chance the flare is from run-of-the-mill activity, the team reports June 25th in Physical Review Letters. After a careful rundown of the alternatives, the astronomers conclude a merger-induced flare is the most likely explanation.
The flare would come from the gas near the collision. Unless two black holes merging are exactly matched in mass, the larger object they create recoils from the energy of the crash, flying off like the proverbial bat out of hell. If this merger happens in an AGN disk, then the gas immediately around the black hole at first tries to travel with it, only to collide with the surrounding disk. This collision shocks and heats the gas. The black hole eventually shoots away and continues ramming through the disk.
Given how short the flare was, the team thinks that the black hole was actually kicked up and out of the disk. But the speed the researchers estimate it took from the merger isn’t enough to escape the supermassive black hole. Instead, it should loop back through the disk and create a second flare in the next few years, the team says — a specific prediction that will be stunning if confirmed.
Ryan Chornock (also Northwestern) is excited by the team’s result and agrees the researchers did a good job ruling out normal supermassive black hole activity. “However,” he warns, “AGN have a long history of surprising astronomers.” If astronomers can find more AGN in the regions of sky where they track gravitational-wave events back to, and if these AGN also flare at the right times, then that will give a huge boost to explaining gravitational-wave events with this scenario.
Reference: M. J. Graham et al. “Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g.” Physical Review Letters. Published June 25, 2020.