Predict black hole radio wave hotspots

Black holes have only three parameters – mass, rotation and charge – which can be considered one of the simplest astrophysical objects in the universe. However, the many open questions about how the two dark giants behave also set them apart as one of the more mysterious. One puzzle is why the plasma around the black hole glows so brightly. Now, in this 3D simulation of magnetic fields in plasma, Benjamin Krinkand and his colleagues at Princeton University think they have found the answer: breaking and reconnecting the magnetic field lines. [1]. The simulation predicts that, under certain conditions, the instability of the magnetic field can cause radio wave hotspots to orbit the shadow of the black hole. This prediction could be verified by a future version of the Event Horizon Telescope (EHT), the network of radio antennas used to acquire the first images of black holes (see Research News: The first images of the Milky Way’s black hole).

There are several mechanisms that physicists believe may be behind the emission of black holes. One such force is called the accretion force, in which a force such as friction in a falling plasma heats the plasma, causing photons to be emitted. This process model predicts a stable emission signal, which does not appear to match observations of high-intensity gamma-ray bursts from black holes.

Another possibility, considered by Krinkande and colleagues, is that the energy needed to produce this light is extracted from a magnetic field that passes through the plasma. When the lines associated with this field separate and then reconnect – a process known as magnetic reconnection – the energy of the magnetic field can be converted into the kinetic energy of the plasma, which is then emitted as photons. This model will not replace the accumulation model, but will work in tandem with it.

In 2D simulations, the team previously discovered that such a magnetic process could lead to the emission of gamma rays, which could explain the observed explosion. They have now moved on to 3D simulations and have considered radio wave emission, which was related to EHT’s observations of black holes. “We wanted to get a more realistic picture that we could compare with the experimental data,” said Krinkand.

The team speculates that black holes sometimes enter a so-called ignited plasma state, in which most of the plasma becomes powerless, meaning the magnetic force is so high that it masks the effects of frictional forces during accretion. . The team simulated the dynamics of plasma particles and their magnetic fields by observing the energy transfer between the particles and the field. Krinkande states that the model takes into account all currents flowing in the plasma, as well as general relativistic effects that were excluded from previous research.

The team’s simulations show that magnetic field lines constantly move, bend, divide and recombine as they move through the plasma and interact with particles. As in their previous work with 2D simulations, the researchers found that the energy of the magnetic field is converted into kinetic energy of the plasma when the field lines are reconnected.

The team modeled the radio waves emitted by the energy plasma and used ray tracking technology to visualize what this radiation would look like to observers on Earth. They found that the emission of radio waves was dominated by a ring structure, the intensity of which varied over time. These fluctuations appear as hot spots of radio waves surrounding the black hole’s shadow. In the case of a large black hole such as the one at the center of the galaxy M87, the orbital radius of the hot spot is estimated to be about 3 times the radius of the black hole and its orbital period is about 5 days.

Crinquand notes that the current version of EHT is unlikely to capture the emission patterns predicted by him and his colleagues because the spacetime resolution of the telescope is too low to capture this feature. He also noted that the transient nature of these patterns meant that they would not always be detectable, even with high-resolution imaging capabilities. “Over time, the flow of accumulation subsides and that’s when we expect the plasma to light up and these hot spots become visible,” he said. Even with the next iteration of EHT, Crinquand said, researchers will need “a lot of luck” to describe these characteristics. But he hopes the right conditions come together. “I’d like to see the EHT capture a black hole emitting a hot spot.”

The study is an “important step” in capturing the processes responsible for the radiation emitted around black holes, said Amir Levinson, a black hole physicist at Tel Aviv University in Israel. “The detailed analysis of magnetospheric dynamics and emission represents a formidable challenge which, if successfully completed, could advance our knowledge of basic physics and astrophysics.” Levinson added that while there is still a long way to go to get all the processes taking place in the plasma around the black hole. ” [by Crinquand and colleagues] It looks promising. ”

Kathryn Wright

Kathryn Wright is the magazine’s deputy editor physics magazine.