What Can We Learn From the Spin of a Rotating Black Hole?
Everything in the universe spins. Amazing, isn’t it? Though you think you are standing still right now, you are on a planet that spins around its axis. Earth also happens to spin around the Sun. Subsequently, the Sun happens to spin around in our galaxy, and the galaxy spins around with other galaxies in our super cluster. You are spinning in so many ways. And one of the most mysterious objects in the universe also spins: black holes. So what can we learn from this quality of the otherwise mysterious singularity?
Evidence of the Spin
A black hole is formed from a supernova of a massive star. As that star collapses down, the momentum it carried is conserved, and so it spins faster and faster as it becomes a black hole. Ultimately that spin is preserved and can change depending on exterior circumstance. But how do we know that this spin is present and not just a bit of theory?
Black holes have earned their name because of a somewhat-misleading quality they have: an event horizon that once you have passed into you cannot escape from. This causes them to have no color, or simply put for conceptualization it is a “black” hole. Material that is around the black hole feels the gravity of it and slowly moves toward the event horizon. But gravity is just a manifestation of matter on the fabric of space-time, and so the spinning black hole will cause material near it to spin also. This disc of matter that surrounds the black hole is known as an accretion disc. As this disc spins inward, it heats up, and eventually, it may reach an energy level where X-rays are launched. These have been detected here on Earth and were the big clue to discovering black holes initially.
The First Method for Spin Measurement
For reasons which are still unclear, supermassive black holes (SMBH) are at the center of galaxies. We are still not even sure how they form, much less how they impact galaxy growth and behavior. But if we can understand the spin a bit more then maybe we stand a chance.
Chris Done recently used the European Space Agency’s XMM-Newton satellite to look at a SMBH at the center of a spiral galaxy that is over 500 million light-years away. By comparing how the disc is moving on the outer fringes and compare that to how it moves as it approaches the SMBH gives scientist a way to measure the spin, for gravity will pull on the matter as it falls in. Angular momentum must be conserved, so the closer the object gets to the SMBH the faster it spins. XMM looked at the X-rays, ultraviolet and visual waves of the material at various points on the disc to determine that the SMBH had a very low spin rate (Wall).
The Second Method for Spin Measurement
Another team led by Guido Risaliti (from the Harvard-Smithsonian Center for Astrophysics) in the February 28, 2013 issue of Nature looked at a different spiral galaxy (NGC 1365) and used a different method to calculate the spin rate of that SMBH. Instead of looking at the distortion of the overall disc, this team looked at the X-rays that were being emitted by iron atoms at different points on the disc as measured by NuSTAR. By measuring how the spectrum lines were being stretched as spinning matter in the region broadened them, they were able to find that the SMBH was spinning at about 84% the speed of light. This hints at a growing black hole, for the more the object eats, the faster it spins (Wall, Kruesi, Perez-Hoyos, Brennenan).
The reason for the discrepancy between the two SMBH’s is unclear, but several hypotheses are already in the works. The iron-line method was a recent development and utilized high-energy rays in their analysis. These would be less prone to absorption than the lower-energy ones used in the first study and may be more reliable (Reich).
One of the ways that the spin of the SMBH can increase is by matter falling into it. This takes time and will only increase the speed marginally. However, another theory says spin can increase through galactic encounters that cause SMBH’s to merge. Both scenarios increase spin rate because of the conservation of angular momentum, though the mergers would greatly increase the spin. It is also possible that smaller mergers may have occurred. Observations seem to show that merged black holes rotate faster than those which only consume matter but this can be affected by the orientation of the pre-merged objects (Reich, Brennenan, RAS).
Recently, quasar RX J1131 (which is over 6 billion light-years away, defeating the old record of furthest spin measured which was 4.7 billion light-years away) was measured by Rubens Reis and his team using the Chandra X-Ray Laboratory, the XMM, and an elliptical galaxy that magnified the distant rays using gravity. They looked at X-rays generated by excited iron atoms near the inner edge of the accretion disc and calculated the radius was only three times that of the event horizon, which means that the disc has a high spin rate to keep that material so close to the SMBH. This combined with the speed of the iron atoms determined by their excitement levels showed that RX has a spin that is 67-87% the maximum that general relativity says is possible (Redd, “Catching,” Francis).
The first study suggests that how material falls into the SMBH will affect spin. If it is counter to it, then it will slow down, but if it spins with it, then it will increase the spin rate (Redd). The third study showed that for a young galaxy there was not enough time for it to gain its spin from material falling in, so it was most likely due to mergers (“Catching”). Ultimately, spin rate shows how a galaxy grows, not only through mergers but also internally. Most SMBH’s shoot high-energy particle jets into space perpendicular to the galactic disk. As these jets leave, the gas cools and sometimes fails to return to the galaxy, hurting star production. If the spin rate helps produce these jets, then by observing these jets we can maybe learn more about the spin rate of SMBH’s, and vice versa (“Capturing”). Whatever the case may be, these results are interesting clues in the further investigations of how the spin evolves.
So we know matter falling into a black hole conserves angular momentum. But how that affects the surrounding space-time fabric of the black hole was a challenge to unfold. In 1963, Roy Kerr developed a new field equation that talked about spinning black holes, and it found a surprising development: frame dragging. Much like how a piece of clothing spins and twists if you pinch it, space-time gets swirled around a spinning black hole. And this has implications for the material falling into a black hole. Why? Because the frame dragging causes the event horizon to be closer in than a static one, meaning you can get closer to a black hole than previously thought. But is frame dragging even real or just a misleading, hypothetical idea (Fulvio 111-2)?
The Rossi X-Ray Timing Explorer provided evidence in favor of frame dragging when it looked at stellar black holes in binary pairs. It found that the gas stolen by the black hole was falling in at a rate too fast for a non-frame dragging theory to explain. The gas was too close and moving too fast for the size the black holes were, leading scientists to conclude that frame dragging is real (112-3).
What other effects does frame dragging imply? Turns out, it can make it easier for matter to escape a black hole before crossing over the event horizon, but only if its trajectory is right. The matter could split off and let one piece fall in while the other uses the energy from the break up to fly away. A surprising catch to this is how such a situation steals angular momentum from the black hole, lowering its spin rate! Obviously, this matter escape mechanism cannot go on forever, and indeed once the number crunchers got done they found the break up scenario only occurs if the speed of the infalling material exceeds half the speed of light. Not many things in the Universe move that fast, so the likelihood of such a situation occurring is low (113-4).
Brennenan, Laura. "What Does Black Hole Spin Mean and How Do Astronomers Measure It?" Astronomy Mar. 2014: 34. Print.
"Capturing Black Hole Spin Could Further Understanding of Galaxy Growth." Capturing Black Hole Spin Could Further Understanding of Galaxy Growth. Royal Astronomical Society, 29 July 2013. Web. 28 Apr. 2014.
"Chandra and XMM-Newton Provide Direct Measurement of Distant Black Hole's Spin." Astronomy.com. Kalmbach Publishing Co., 06 Mar. 2014. Web. 29 Apr. 2014.
Francis, Matthew. “6-Billion-Year-Old Quasar Spinning Nearly as Fast as Physically Possible.” ars technica. Conde Nast, 05 Mar, 2014. Web. 12 Dec. 2014.
Fulvio, Melia. The Black Hole at the Center of Our Galaxy. New Jersey: Princeton Press. 2003. Print. 111-4.
Kruesi, Liz. "Black Hole's Spin Measured." Astronomy Jun. 2013: 11. Print.
Perez-Hoyos, Santiago. "An Almost Luminal Spin For A Supermassive Black Hole." Mappingignorance.org. Mapping Ignorance, 19 Mar. 2013. Web. 26 Jul. 2016.
RAS. "Black holes spin faster and faster." Astronomy.com. Kalmbach Publishing Co., 24 May 2011. Web. 15 Aug. 2018.
Redd, Nola. "Supermassive Black Hole Spins At Half The Speed Of Light, Astronomers Say." The Huffington Post. TheHuffingtonPost.com, 06 Mar. 2014. Web. 29 Apr. 2014.
Reich, Eugene S. "Spin Rate of Black Holes Pinned." Nature.com. Nature Publishing Group, 06 Aug. 2013. Web. 28 Apr. 2014.
Wall, Mike. "Black Hole Spin Rate Discovery May Shed Light On Evolution Of Galaxies." The Huffington Post. TheHuffingtonPost.com, 30 July 2013. Web. 28 Apr. 2014.
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© 2014 Leonard Kelley