Here at Astronomy For Change, we’ve discussed at length, the recent discoveries, observations and scientific breakthroughs regarding these enigmatic objects whose light was emitted billions of years before the sun burned bright in space:
A Discovery that Shook the World!
In a galaxy far, far away, KiloNova Explosion From Neutron Star Merger Observed – Gravity Waves Produced
Cosmic First #KiloNova #Explosion From #NeutronStar Merger – #GravityWave Event Visual Confirmation (video)
European Southern Observatory To Announce “Unprecedented Astronomical Discovery” Monday, 16 October
Now it appears, the first direct, visible-light observation of a Black Hole Merger has been made!
Astronomers have long believed – and in recent years observed – that two black holes orbiting each other, spiraling closer and closer, often merge. Until now, an optical counterpart to such a merger has never been observed. What has been observed are gravitational waves, ripples in spacetime, elusive signals from such mergers, the possibility of which was predicted over 105 years ago by Einstein with his General Theory of Relativity.
Since, by definition, a “Black Hole” precludes the escape of any electromagnetic signal, theorists have proposed scenarios where such black hole mergers might explode and thus produce visible light. Last month, for the first time, astronomers at the California Institute of Technology (CalTech) announced that they have evidence that is consistent with one of these light-producing scenarios.
More than a year later, after careful analysis and having conducted the coordination and review necessary for such an announcement, astronomers are prepared to state that, yes, these two events – one in visible light and one by gravity-wave detection – likely originate from the same black hole merger.
When two black holes spiral around each other and ultimately collide, they send out ripples in space and time, gravitational waves, a prediction of Einstein’s General Theory of Relativity and now confirmed. Because black holes do not emit light of any kind (Visible, UV, InfraRed, etc), these events are not expected to be visible and thus observed in the conventional sense. CUNY Graduate Center astrophysicists K. E. Saavik Ford and Barry McKernan have posited scenarios in which a black hole merger might explode, and thus be observable.
It now appears that astronomers have observed evidence of one of these light-producing scenarios.
Collaborators at Caltech’s Zwicky Transient Facility (ZTF), funded by the National Science Foundation (NSF) and located atop Palomar Mountain, home to the famous 5.1 Meter Hale Reflector and Palomar Observatory, the scientists have spotted what quite possibly is a light flare from a pair of coalescing black holes. The black hole merger was first witnessed by the NSF’s Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector on 21 May, 2019, in an event called S190521g. As the black holes merged, jiggling space and time, they sent out gravitational waves.
This supermassive black hole was burbling along for years before this more abrupt flare.
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.
So, how do merging black holes emit electromagnetic energy of any kind since, by definition, no light can escape from a black hole?
Graham and the study’s collaborators postulate that two small black holes resided within an accretion disk surrounding a much-larger supermassive black hole. Kathleen E. Saavik Ford of CUNY’s Graduate Center and professor at BMCC (Borough of Manhattan Community College) explains:
At the center of most galaxies lurks a supermassive black hole. It’s surrounded by a swarm of stars and dead stars, including [other] black holes. These objects swarm like angry bees around the monstrous queen bee at the center. 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.
CUNY’s Barry McKernan, and co-author of the study also explains:
It is the reaction of the gas to this speeding bullet [the newly formed black hole] that creates a bright flare, visible with telescopes.
Such a flare is predicted to ignite days to weeks following the initial splash of gravitational waves produced during the black hole merger. The CalTech statement goes on to explain:
ZTF did not catch the event immediately; when the team went back and looked through archival ZTF images months later, they found a signal that started days after the May 2019 gravitational-wave event. ZTF observed the flare slowly fade over the period of a month.
They attempted to get a more detailed look at the accompanying spectra of the supermassive black hole but, by that time, the flare had already faded. A spectrum would have offered more support for the idea that the flare came from merging black holes within the disk of the supermassive black hole. However, the researchers say they were able to largely rule out other possible causes for the observed flare, including a supernova or a tidal disruption event, which occurs when a black hole essentially eats a star.
What’s more, the team says it is not likely that the flare came from the usual disruptions associated with a supermassive black hole which regularly feeds off its surrounding disk. Using the Catalina Real-Time Transient Survey, led by Caltech, they were able to assess the behavior of the black hole over the past 15 years, and found that its activity was relatively normal until May of 2019, when it suddenly intensified.
It is believed that the newly formed black hole should cause another flare in the next few years. The same process that sent the merged, newly-formed black hole careening through the central black hole’s accretion disk – after the two smaller black holes merged and coalesced into one – should cause the new black hole to enter the supermassive black hole’s disk again
and thus produce another flash of light that ZTF should be able to observe.
Mansi Kasliwal of CalTech and co-author of the June 25, 2020 study went on to explain:
Supermassive black holes like this one have flares all the time. They are not quiet objects, but the timing, size, and location of this flare was spectacular
The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions. If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins.
In recent years, black hole mergers [in our universe] have been detected via ripples in spacetime (gravitational waves). Now, for the first time, astronomers believe they’ve observed the visible light emission from a black hole merger in a peculiar 3-black-hole system.
In summary, the preponderance of the evidence is consistent with the following scenario:
- Two lesser black holes are in orbit about each other within the larger accretion disk of the much more massive central black hole;
- Over time, the two lesser black holes, in a gravitational death-spiral, merge and the larger, merged black hole is sent careening through the accretion disk, super-heating the gas as it plows through it, causing a bright, observable flare.
As the universe continues to age, the number and frequency of such events as these will increase as mid-to-high mass stars continue to age and evolve. It should be pointed out that the galaxy in which this merger occurred is 12.8 billion light years distant and thus, the event occurred less than one (1) billion years after the Big Bang at a time when the universe was much younger and quite different; stars were generally more massive and thus shorter lived than today, an aspect of the early universe that allowed for the rapid synthesis of heavier elements necessary for the formation of life-supporting planets and life itself.
Over eons of time hence, the universe will slowly and inexorably evolve away from the warm, bright star-forming environment it is today. As more and more of the remaining hydrogen forms stars and as these newly-formed and existing stars age through their respective life-cycles, some of the higher-mass stars evolving towards black holes, the number and frequency of such bizarre events will increase until the universe is unrecognizable as compared to current form.
James Daly, Ph.D, author and curator, Astronomy for Change
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