Black holes aren’t supposed to leak any light when they smash into each other — or at least that’s what physicists thought.
On May 21, 2019, astronomers using the Virgo interferometer and the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) detected a gravitational wave signature consistent with a black hole merger. This event, called S190521g, at first seemed to produce no visible light.
A subsequent review of data collected at a separate observatory, Caltech University’s Zwicky Transient Facility (ZTF), has yielded evidence of light coming from this same event, which, if confirmed, would be a first for astronomy. This research, led by astronomer Matthew Graham from Caltech, now appears in Physical Review Letters.
Colliding neutron stars, the super-dense remnants of exploded stars, produce all sorts of emission spectra, including infrared, ultraviolet, visible light, x-rays, gamma rays, and radio waves. Merging black holes, on the other hand, emit detectable radiation in the form of gravitational waves, which are ripples in spacetime itself. For merging black holes to produce light, something rather extraordinary must have happened, and as the new research suggests, something very much did.
The S190521g event happened near the supermassive black hole at the centre of our galaxy. This black hole is surrounded by a gigantic disk filled with all sorts of stuff, from gas, dust, and asteroids through to stars, neutron stars, and smaller black holes.
“These objects swarm like angry bees around the monstrous queen bee at the centre. They can briefly find gravitational partners and pair up but usually lose their partners quickly to the mad dance,” explained K. E. Saavik Ford, an astronomer at the City University of New York (CUNY) and a co-author of the new paper, in a press release. “But in a supermassive black hole’s disk, the flowing gas converts the mosh pit of the swarm to a classical minuet, organising the black holes so they can pair up.”
In the case of S190521g, the newly merged black hole was sent careening off, in an astrophysical event known as a “kick.” This kick caused the black hole to hurtle through the disk at breakneck speeds, triggering a reaction with the surrounding gas that produced an exceptionally bright and relatively long lived flare.
“This supermassive black hole was burbling along for years before this more abrupt flare,” said Graham. “The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event. In our study, we conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities.”
Those other possibilities include a supernova or a tidal disruption event, in which a star smashes into a black hole.
That said, the timing, duration, size, and location of the flaring event is not a typical occurrence near the supermassive black hole and within its surrounding disk, according to the review of data gathered over the past 15 years. Moreover, and as previously predicted by the same team, the flare should only make an appearance in the days or weeks following the black hole merger. And indeed, this is precisely what was observed with S190521g, as the flare appeared a few days after scientists detected the gravitational wave event. Interestingly, the flare faded slowly over the course of one month.
A next important step will be for researchers to document the same thing happening in other black hole mergers. That shouldn’t be too hard, as more of these events are expected to be spotted in the future given increasingly sophisticated detection equipment. And as a matter of fact, one of these future sightings could involve the same black hole, which is expected to return back into the disk and possibly meet up with another unsuspecting black hole.
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Scientists recently documented a collision involving a black hole and a much smaller object, either an unusually hefty neutron star or an unusually petite black hole. No light signature was detected from this event, so either these sorts of collisions don’t produce any light, or these two objects weren’t in an environment conducive to producing light. Another possibility is that this collision did in fact produce a flare, but scientists just haven’t seen it in the data. Excitingly, several observatories recorded the aftermath of this collision, so it’s possible more clues will be discovered.