If two black holes tango in space but astronomers cannot see them, can we still admire their flashy dance moves?
Because black holes have such a strong gravitational pull that not even light can escape, they can't be observed directly and are therefore difficult to study. But one pair of black holes has enamored astronomers with a complicated celestial dance that periodically produces extremely bright flashes of light — outbursts that are brighter than a trillion stars and even the entire Milky Way galaxy.
By studying the timing of these bright flashes of light, researchers have attempted to map out the complex choreography of the black holes' movements and predict exactly when the system will flare up again. After more than 120 years of observations and decades of building computer models, astronomers have finally figured out what these black holes are up to, thanks to data from NASA's now-retired Spitzer Space Telescope.
Video: Black Hole pair produce flares 'brighter than 1 trillion stars'
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The two "dancing" black holes are located 3.5 billion light-years from Earth at the center of a galaxy called OJ 287. The larger of the two is one of the biggest black holes ever found, weighing in at more than 18 billion times the mass of the sun. Orbiting around this big black hole is a much smaller black hole that's about 150 million times the mass of the sun. Twice every 12 years, the smaller black hole passes through the larger one's accretion disk, or the flat band of dust and gas falling into the black hole, creating brilliant flares of light.
Because the small black hole's orbit is irregular — its position shifts with each 12-year loop around its partner — these flashes don't occur on a regular schedule. Sometimes they might occur just one year apart, while other times up to a decade passes between flares. The seemingly random timing of the flares has made it difficult for astronomers to figure out exactly what kind of "dance" these black holes are doing.
One computer simulation in 2010 was able to predict the flares within one to three weeks. In 2018, another group of researchers led by Lankeswar Dey, a graduate student at the Tata Institute of Fundamental Research in Mumbai, published a new model that they claimed could predict the flares' occurrence within four hours. In a new study, published Tuesday (April 28) in The Astrophysical Journal Letters, Dey's group reports that Spitzer's observations of a flare on July 31, 2019, confirm that their model is correct.
The Spitzer Space Telescope, which NASA decommissioned in January, just happened to be in the right place at the right time to observe the flare on that day, when no other telescopes on Earth or in space were able to see it. At the time, OJ 287 was on the opposite side of the sun from Earth's perspective.
Spitzer was 158 million miles (254 million kilometers) away from Earth at the time, and from its vantage point the telescope had a clear view of OJ 287 for a little over a month, from July 31 to early September, NASA officials said in a statement.
"When I first checked the visibility of OJ 287, I was shocked to find that it became visible to Spitzer right on the day when the next flare was predicted to occur," Seppo Laine, a scientist at Caltech/IPAC in Pasadena, California, who oversaw Spitzer's observations of the system, said in the statement. "It was extremely fortunate that we would be able to capture the peak of this flare with Spitzer, because no other human-made instruments were capable of achieving this feat at that specific point in time."
To come up with this accurate prediction, the researchers didn't just look at the orbital mechanics of the system. They also had to account for gravitational waves, or ripples in space-time created when massive objects move through space, warping their surroundings. Astronomers expect the black hole system in OJ 287 to generate gravitational waves that are strong enough to alter the smaller black hole's orbit, according to the statement.
By incorporating gravitational waves into their calculations, the researchers were able to predict a 1.5-day time frame in which the system will produce a flare. But they narrowed that down even further, to just four hours, by taking into account the "no-hair theorem" of black holes — an idea that Stephen Hawking famously doubted. This theorem posits that black hole surfaces are featureless and symmetric, rather than bumpy and irregular. (Black holes don't literally have a "surface," but rather an invisible boundary known as the event horizon, where not even light can escape the black hole's gravitational pull.)
If the large black hole at the center of OJ 287 were bumpy, with its mass unevenly distributed, its gravitational pull on the smaller black hole would be inconsistent, which would affect the smaller black hole's orbit and the timing of the flares. But the smaller object's symmetrical, spirograph-shaped orbit supports the no-hair theorem, the new study claims.
"It is important to black hole scientists that we prove or disprove the no-hair theorem," Mauri Valtonen, an astrophysicist at University of Turku in Finland and a coauthor of the study, said in the statement. "Without it, we cannot trust that black holes as envisaged by Hawking and others exist at all."
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Email Hanneke Weitering at firstname.lastname@example.org or follow her @hannekescience. Follow us on Twitter @Spacedotcom and on Facebook.
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First off, it is important to appreciate that these are NOT "potential" sources of GWs. They are on-going real sources (although from quite some time ago). Their detection is where issues of "potential" should be addressed.
Interestingly, one of these is a neutron star binary, discovered in 1974 by Hulse and Baylor. They deduced from their observations that the binary was losing mass as a result of their intense orbital interactions, and is releasing about 7.35 x 10E24 watts. The ratio of observed orbital decay predicted by GR vs. their observations of this binary was 0.997 +/- 0.002. It should not be too surprising that the folks in Stockholm decided that Hulse and Baylor were smart enough to win one those highly coveted medals.
As a reference, our solar system is estimated to put out about 5,000 watts of GWs (i.e. 5 x 10E3 watts)!
(That also is real, and not potentially.)
But this monster GW source (subject of this thread) must be highly variable based on the difference in masses of these BHs, and their orbits, etc. This will likely cause major problems for interpretations, especially at high sensitives as there seems a risk of being swamped by GWs from various other sources. Comments, please?!
Perhaps the new LISA** (the Laser Interferometer Space Antenna) instrument will pick up some of these continuous sources. Sadly, it will not be launched for another decade+. There remains a distinct potential for their observations by Gravitational Wave Astronomy***.
(Incidentally, none of these articles are click-bait, since that is defined as false advertisement - see comments to first * reference. And there is still no option B.)
* and https://en.wikipedia.org/wiki/Hulse–Taylor_binary
The serendipity is in 1) being able to use Spitzer's last weeks when no ground based observatory could do it, as well as 2) having the smaller black hole orbit through the disk with two pass close together allowing ground based optical calibration of near-IR Spitzer data after the flare peak ; and 3) having the disk pass flare being a flat bremsstrahlung spectrum as no other flare (usually power-law spectra).
The work is in preparing Spitzer and the complicated GR analysis.
The importance is many-fold.
- This - the "no hair" membrane effects on the gravitational wave spectra and the flat bremssstrahlung - are the first observations of a unique black hole property and its unique disk pass property respectively, thus rejecting all other objects.
- It is also the first observation of general relativity up to the very event horizon of a black hole, confirming general relativity for all scales of cosmology. The earlier, implicit evidence has been from the EHT image of M87* black hole, where the innermost stable circular orbit was successfully predicted from linearized GR and will - in extremal black holes spinning at universal speed limit - overlap with the event horizon.
- Apart from the complicated retardation effects of the gravitational near field coupling to the far field gravitational waves (and the orbit precession, say) which contribute small nonlinearities, apparently all the general relativity description we need up to the event horizon is linearized. E.g. based on Minkowski flat space and small deviations thereof https://authors.l...p80a.pdf ].
The paper seem to think these systems and the pulsar observatory network that will try to see nHz range GW will augment other GW observations (e.g. Ligo/Virgo, the future LISA, ...).
On another tack, GW is doubly helpful. If they didn't do the precession trick on all planets but mostly Mercury - same as on this system - the solar system would likely not have been stable for 4.5 billion years http://www.scholarpedia.org/article/Stability_of_the_solar_system ].
"In most of the solutions, the trajectories continue to evolve as in the current few millions of years: the planetary orbits are deformed and precess under the influence of the mutual perturbations of the planets but without the possibility of collisions or ejections of planets outside the Solar System. Nevertheless, as predicted by the secular equations, in 1% of the cases, the eccentricity of Mercury increases considerably. In many cases, this deformation of the orbit of Mercury then leads to a collision with Venus, or with the Sun in less than 5 Ga, while the orbit of the Earth remained little affected. However, for one of these orbits, the increase in the eccentricity of Mercury is followed by an increase in the eccentricity of Mars, .... if we consider a pure Newtonian world, starting with the present initial conditions, the probability of collisions within 5 Gyr grows to 60%, which can thus be considered as an additional indirect confirmation of general relativity."
The three GW sources mentioned above offer the opportunity for Multi-Messenger Astronomy (MMA) of a very new type - GWs with optical+ data stream. Curiously, I already knew about MMA, but never really connected on how important it is until now. These GW sources are major observations relative to MMA. But they must be radiating significant GWs above the background. Does anyone even have a clue what that might be for LISA?
And at least one GW detection was fixed to a gamma ray burst, if I remember that correctly, the first instance of MMA with GWs?
Also, it seems that the intensity of a GW source can be highly directional. Illustrations of GWs swirling out from an orbiting pair of neutron stars suggest there is a plane of propagation for these waves related to the plane of the orbit. I recall reading something about a need to be aligned well enough with GW propagation to detect it, depending on its overall intensity. Is this accurate, or do the wave forms broaden over time, and/or is there any scenario for the spherical propagation of high energy GWs?