Jun 9 2020
A team of researchers from Finland, Germany, Russia, and the United States has analyzed over 300 quasars, which are spinning black holes producing plasma beams.
At the center of a quasar, a black hole is surrounded by a spinning accretion disk composed of dust and gas, and there are two oppositely directed plasma jets emanating from it. The shape of the jets changes with distance from the center, and as a result, they look like the famous flared pants. In this artist’s rendering, a twisted magnetic field and clouds of interstellar gas can be seen around the jet. Image Credit: Daria Sokol/MIPT Press Office.
The researchers have now discovered that the shape of these supposed astrophysical jets differs from conical to parabolic at a certain distance from the black hole, suggestive of the historic flared jeans of the 1970s.
The scientists believe that effective measurement of these “cosmic pants” will allow them to understand the operations of the core engine that speeds up matter to almost the speed of light at the cores of distant active galaxies. The study was reported in the Monthly Notices of the Royal Astronomical Society journal.
Beacons of the Universe
In spite of being billions of light-years away from the Earth, quasars are one of the brightest celestial objects in the universe. At times, they are referred to as beacons, and they help researchers to navigate not only the complex structure of the universe and the distant cosmic past, but also enable them to traverse the planet Earth.
Since quasars are extremely remote celestial objects, they can be employed as stable reference points in the sky to quantify the rotation of the Earth and the coordinates of objects on the surface of the planet. This principle is fundamental to GLONASS, GPS, and other positioning systems.
A quasar is very bright, such that it can be detected from a vast distance. In the above image, the quasar hosts a spinning giant black hole weighing up to many billion times as that of the Sun. Matter surrounding the black hole falls onto the quasar, carrying a magnetic field.
The magnetic field lines are similar to wires with charged particles wound on them analogous to beads. When the magnetic field lines begin to rotate, plasma speeds up almost to the speed of light. The ensuing outflows are known as astrophysical jets, and they are responsible for making quasars one of the brightest objects in the universe.
Reaching for the Sky
Earlier, astronomers have believed that nearly each jet is shaped similar to a narrow cone, extending sideways once it exits the region of black hole.
After visualizing scores of quasars for 20 years through a network of radio telescopes distributed around the planet, the authors of the latest study have now confronted this theory. They created images of more than 300 quasar jets tracked by the MOJAVE program and made an automated analysis of the shapes of these jets.
Consequently, the researchers identified 10 quasars with parabola-shaped jets forming into cones. A transformation like this could be detected because of the relative proximity of the quasars involved: each of the 10 quasars was found to lie “mere” millions of light-years away from the Earth. The “bootleg flaring” took place at a distance of about several dozen light-years from the black hole region.
The mechanism behind the formation and acceleration of jets in remote active galaxies has not been fully understood so far, yet it is crucial that we figure out how these cosmic accelerators work.
Yuri Kovalev, Professor, Moscow Institute of Physics and Technology and Lebedev Physical Institute of the Russian Academy of Sciences
“The region where jets originate is difficult to discern. It is very compact, and the distance to these objects is so great that everything blurs together. So while multiple theoretical models were available, there were no observational data to test them against. Our study is the first to report the detailed geometry of jets based on observations of large numbers of quasars,” added Professor Kovalev.
Comprehending the Unseen
The shape of the jet arises from a complex interaction between the external and internal external forces, the interstellar gas, the plasma, and the magnetic field. The astrophysicists discovered an elegant method to account for such factors.
A core engine comprising a magnetic field and a spinning black hole offers a restricted power supply and is incapable of forcing particles to increasingly higher velocities indefinitely, similar to a rocket engine. It was already known that plasma can accelerate easily only up to a specific point. Beyond that, the acceleration becomes so slow that it ceases effectively. At this juncture, the cosmic pants flare.
Earlier studies pointed to the shift in quasar jet shape observed in a few galaxies. However, they did not draw the conclusion that it was a property of all quasars rather than the individual objects concerned. We have pinned this effect on the internal characteristics of jets, and that explanation turned out to be neat and intuitive.
Dr Elena Nokhrina, Moscow Institute of Physics and Technology
Now, researchers have developed a new method to assess the speed of the rotation of black hole and figure out the mechanism involved in the formation of the extraordinarily fast and focused jets of plasma in quasars, which are so dazzling that they can be visualized from billions of light-years away.
The new study was funded by grant No. 16-12-10481 of the Russian Science Foundation.
Journal Reference:
Kovalev, Y. Y., et al. (2020) A transition from parabolic to conical shape as a common effect in nearby AGN jets. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/staa1121.
Source: https://mipt.ru/english/