A black hole less than 8,000 light-years away is slowly devouring its companion star. As it does so, it fires jets of plasma at nearly half the speed of light. The system is V404 Cygni, and astronomers have discovered that the black hole is wobbling. But if that weren’t strange enough, it’s actually dragging the fabric of the Universe around with it. Welcome back to my blog "Mystery of Galaxy", and in this article we’re going to take a look at one of the strangest findings from a nearby black hole.
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| An artist’s impression of a black hole, similar to V404 Cygni, devouring material from an orbiting companion star. image credit: ESO / L. Calçada. |
V404 Cygni lies about 7800 light-years away in the constellation Cygnus the Swan. It’s a binary system that’s home to a black hole about 9 times the Sun’s mass. The black hole was likely formed in a core-collapse supernova. If a star's core reaches 2.8 solar masses or greater, there’s no force in the Universe that will prevent its complete and utter collapse into oblivion. To form a 9 solar mass black hole, the original star would have had to have been gargantuan, probably around 40 solar masses. The more massive the star, the shorter its lifetime. So this black hole probably formed billions of years ago. However, its companion star couldn’t be any more different. It's a K-type sub-giant star about 70% the mass of the Sun. It exhausted its supply of hydrogen fuel and is in the process of swelling up to become a red giant. The star orbits the black hole in just 6.5days. They're so close together, the black hole siphons off material from the star. Because of the star’s sideways motion relative to the black hole, matter spirals inward to form an accretion disk around the black hole. As matter in the disk draws closer to the black hole, it moves faster until it reaches nearly the speed of light. As slower matter in the surrounding disk grinds against the faster material, friction heats the disk to millions of degrees. This causes the disk to become incredibly bright, especially at X-ray wavelengths. In fact, it’s this intense X-ray signature that made it possible to detect the stellar-mass black holes that we know of today.
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| Accretion disk of black hole Source: NASA’s Goddard Space Flight Center/Jeremy Schnittman / CC BY-SA |
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| Jets from super massive black hole Source: ESA/Hubble / CC BY |
After all, black holes don’t radiate so we rely on their accretion disks to reveal their presence. As their name implies, accretion disks are thought to feed black holes, causing them to grow. But exactly how this happens isn’t completely understood. But, we do understand, the spinning disk generates a powerful magnetic field. This field rips matter off the disk and launches it along the black hole’s spin axis as a jet. But from time to time, the jet becomes unstable and unleashes violent bursts of plasma. These eruptions are extremely powerful. In 2015, NASA’s Swift satellite detected a powerful X-ray surge coming from V404 Cygni. It was its first outburst since 1989. Over the next several days, Swift detected X-rays echoing off the surrounding dust. V404 Cygni quickly became the brightest X-ray source in the sky, becoming 50 times brighter than the Crab Pulsar. It got so bright that even optical and radio telescopes could see it. Astronomers began a worldwide campaign to study the outburst in unprecedented detail. One team of astronomers used the Very Long Baseline Array to make high resolution radio images of the outburst. VLBA is a series of radio telescopes that make simultaneous observations. When their signals are linked together via interferometry, they act as a single telescope nearly the size the northern hemisphere. They hoped to catch the jets moving away from the black hole, but what they found was really weird. As expected, the black hole was ejecting blobs of plasma, each about 1000 times brighter than the Sun. But instead of firing perpendicular to the disk, the jets’ directions were changing rapidly, shifting back and forth in just a couple of hours. It was as if they were being fired from a black hole-powered X-ray water sprinkler!
The obvious question then is, "why were the jets wobbling so fast"?
It took a while to know for sure, but astronomers have figured out that the inner disk is processing like a gyroscope. And the disk is processing rapidly because- get this - the black hole is dragging spacetime around it! This is an effect predicted by Einstein’s theory of General Relativity. Massive objects distort spacetime, kind alike the way a bowling ball distorts a mattress. But as the object rotates, it drags the surrounding spacetime along with it. This is known as the Lense-Thirring effect, named for the two Austrian physicists who derived it, but it's better known as “frame dragging”. It’s kind of like a ball spinning in a bowl of syrup. As the ball spins, the syrup forms a slow-moving whirlpool around the ball. The syrup closer to the ball moving faster than the syrup further away. Relativity predicts that spacetime should behave the same way. This effect was directly measured by NASA’s Gravity Probe B. The spacecraft was fitted with a freely spinning gyroscope. As the space closer to the Earth moved faster than the space farther away, the spin axis of the gyroscope drifted sideways to point back to the Earth over time. Although the effect was real, it turned out to be a very difficult to measure because Earth has such a low mass and rotates slow. But a black hole is much more massive and rotates a lot faster. Instead of a subtle twist, it forms a tornado of spacetime! In V404 Cygni’s case, the black hole isn’t just spinning, but it’s doing so at an angle with respect to the orbit of its sub-giant companion. Matter flowing from the star into the disk stays in the same plane as the star’s orbit, but as it draws closer to the black hole it gets caught up in the spacetime tornado and gets tilted with respect to the outer disk. The intense radiation from the eruption puffs up the inner disk, forming a tall donut of plasma. This plasma donut grinds against the outer disk at an angle, creating friction. The friction between the spinning object and the surrounding surface causes the object to process. It’s the reason a spinning top wobbles due to friction with the table top. In fact, Earth processes once every 26,000years because of its axial tilt and the constant gravitational torque from the Moon. But the disk near the black hole processes in just a couple of hours. 2.6 hours at the most, but that’s a very conservative estimate. It’s probably much quicker, perhaps on the order of minutes! We’ve never seen this effect happening on such short timescales. The companion star exerts a torque on the black hole, but it’s one-billionth the torque required to produce the observed effect. On the other hand, frame dragging by the blackhole avoids torque altogether. It just twists the spacetime around it, processing the disk. The jets wobbled by about 36 degrees back and forth, which suggests that the black hole’s spin axis must be tilted by about 18 degrees.
So the next question is; "why is the black hole tilted in the first place?"
The most likely reason is that the black hole was kicked during the supernova that formed it. Supercomputer simulations show that supernovae are messy and not perfectly symmetrical. It’s very possible that was the case inV404 Cygni’s supernova as well. That could have knocked the black hole off by about 18 degrees, creating the misalignment we see today. The 2015 outburst was extremely short-lived. Hopefully in a couple of decades it’ll erupt again and we’ll be able to see if the black hole’s precession has in fact changed. Like I said, there’s a lot we don’t yet understand about how black holes accrete matter from their disks and how these jets are launched. Observations like these provide critical insights into understanding how black holes work. Oh, and you probably heard the news that astronomers took the first-ever image of a black hole last year.
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| very first image of M87 black hole Source: Event Horizon Telescope / CC BY |
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