Are there speeds in the black hole
First picture of a jet : Apparently faster than light
Almost exactly a year ago, a picture from the vastness of space fascinated people all over the world. It was recorded with the “Event Horizon Telescope” (EHT) and for the first time directly showed a phenomenon that astronomers could only paint pictures of before: the outline of a black hole. The EHT is now publishing new, no less spectacular images of an event triggered by the gravitational forces of a black hole: a jet.
The EHT researchers used radio waves that had been picked up by telescopes at eight widely spaced locations on earth. Taken together, these eight correspond to a radio telescope with a collecting surface about the size of the earth. With this trick, the radio waves captured by the EHT can depict even very distant celestial objects with astonishing sharpness - like reading a newspaper in New York from Berlin.
An image of a phenomenon five billion light years away
The black hole in galaxy M87, which the EHT imaged last year, is about 55 million light-years away. The current images come from a celestial object that is almost a hundred times farther, five billion light years away from us: the Quasar 3C 279. "Quasar" is an artificial word that summarizes the observed properties of this type of celestial object: They look like stars, which also emit radio waves are therefore “quasi-stellar radio sources”.
In truth, however, quasars are not stars, but the hot, bright centers of distant galaxies. Quasars are among the most luminous celestial objects that we know. For at least 20 years, astronomers have been almost certain that they have found the source of energy for their great luminosity: in the middle of each quasar sits a huge, supermassive black hole. The mass of the black hole of 3C 279 is believed to be about a billion times the mass of the Sun.
And this is how the power plant of a quasar works: attracted and accelerated by the gigantic gravitation in the vicinity of a black hole, gas and dust continuously rush towards the black hole. Because of its angular momentum, however, the matter does not flow directly into the black hole. Rather, a so-called accretion disk is created in which the matter swirls faster and faster towards the black hole. The swirling fall of matter in the black hole's gravitational field releases large amounts of gravitational energy.
Bursts of gamma rays
And this is exactly the energy source for the unimaginable luminosity of the radiation of a quasar. "Thanks to strong magnetic fields in the jet base, the gravitational energy of the material flowing from the accretion disk onto the central source is converted into electromagnetic waves that are emitted in a wide range of wavelengths - from short-wave X-ray and UV light to visible light and radio waves", describes Eduardo Ros from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, one of the co-authors of the study, explained the process. “One observes outbreaks of hard, extremely short-wave gamma radiation over and over again.” On average, a quasar like 3C 279 shines brighter than many trillion suns combined.
In the inner areas of the accretion disk of a quasar close to the black hole, matter consists of a hot plasma of positively charged ions and negatively charged electrons. The fast moving charge carriers build up strong magnetic fields. And these magnetic fields, conversely, influence the movements of the plasma flowing towards the black hole.
The consequence of these complex and by no means fully understood interactions between magnetic fields and plasma is: Shortly before it finally crashes into the black hole, part of the plasma is diverted. Accelerated and bundled by magnetic fields, it shoots vertically out of the accretion disk in two opposing so-called jets. The jets can grow to be several thousand light years long, and the speed of matter in a jet can be close to the speed of light.
Image sharpness never achieved before
It is precisely such a jet from the accretion disk of quasar 3C 279 that the new EHT images now show in a previously “unattained image sharpness” - the MPIfR reports. The images were based on radio waves from the jet of 3C 279. They were emitted by electrons in the jet, which were forced into curved paths by magnetic fields and thereby reach speeds close to the speed of light.
As a result, the electrons emitted what is known as synchrotron radiation in the form of radio waves, which now, five billion years later, have been picked up by the EHT's radio telescopes. In laborious data analysis, the supercomputers at the MPIfR in Bonn and the MIT Haystack Observatory in the USA were used to extract the information contained in the weak and noisy radio waves and process them into images of the jet.
"We have selected 3C 279 with its black hole in the center and the jet emanating from there as perhaps the best investigated and well-understood active galaxy nucleus so far," said Jae-Young Kim from the MPIfR, who heads the research project, told Tagesspiegel. "But only with the highest angular resolution that is currently possible in astronomy, we gain an insight into the special appearance and even more, into the complicated dynamics of the plasma in the central region of the source."
Faster than the light? A deception!
Surprisingly, some of the structures observed in the jet seem to move faster than light. Should the laws of Einstein's theory of relativity, in which the speed of light is a universally valid maximum speed, no longer apply in the vicinity of supermassive black holes, as in Quasar 3C 279? But on closer inspection, the faster than light speeds in the jet of a quasar turn out to be a kind of optical illusion. It always arises when the flow of matter from a jet happens to be almost exactly at us, as in the case of 3C 279.
Even conspicuous structures in the jet, such as bright bubbles, then come closer to us day by day. This also shortens the flight times of light from you to us. The information about the respective positions of the bubbles in the jet reaches us earlier and earlier than the day before. Because of this runtime effect of the light, we see their movement as if in time lapse. The radio waves arriving at us from the jet of Quasar 3C 279 lead us to believe that speeds are almost 20 times higher than the speed of light.
In contrast to this astonishing effect, however, many other processes in and around a quasar are still largely a mystery. It is particularly difficult to understand what is going on in those quasar regions in which the jets are catapulted out of their accretion disks. The EHT images that have now been obtained suggest, for example, that the jet from Quasar 3C 279 is twisted as it exits the accretion disk.
A counterpoint to the pandemic
"It is very unusual for us to observe the relativistic plasma jet emanating from the central black hole in this form - this requires theoretical models with a rotating central source and accretion disk in order to twist the jet to such an extent," says Jae-Young Kim. In other words, the black hole in 3C 279 may be rotating.
"I hope that our work, and especially now these results about the Quasar 3C 279, will fascinate people even in difficult times," says Anton Zensus, MPIfR Director and Chairman of the EHT Collaboration Council. "Maybe it's a small, positive counterpoint to the many important and worrying news that we all get every day from around the world."
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