What is Black Hole Radio Wave?

First Image of Supermassive Black Hole Sagittarius A*
This image shows Sagittarius A*, the black hole at the center of the Milky Way galaxy. EHT Collaboration, CC BY-SA

Future measurements with the Event Horizon Telescope could test the hypothesis that "magnetic reconnection" could cause radio wave hotspots to orbit the black hole. This hypothesis is based on simulations of plasma surrounding a black hole.

Black holes can be considered one of the simplest astrophysical objects in the Universe because they have only three parameters: mass, spin and charge. But these black giants are also among the most mysterious because of the many unsolved mysteries surrounding their behavior. Why the plasma around black holes glows so intensely is a mystery. Now, Benjamin Crinquand of Princeton University and his colleagues believe they have discovered the solution through 3D simulations of magnetic fields within this plasma: breaking and reconnecting magnetic field lines. According to the models, magnetic field instabilities can occasionally cause radio wave hotspots swirling around the shadow of the black hole.

Future iterations of the Event Horizon Telescope (EHT), the radio dish network used to take the first pictures of a black hole, could test this prediction.

Physicists believe a number of factors may be at work when a black hole is emitting light. One is the so-called accretion force, where the plasma is heated by friction-like forces as it falls, causing photons to be released. Steady emission signals are predicted by models of this process, which do not seem to match observations of high-intensity gamma-ray bursts from black holes.

Another possibility that Crinquand and colleagues are considering is that the energy needed to create this light is derived from the magnetic field passing through the plasma. When the lines associated with this field are separated and then reconnected – a process known as magnetic reconnection – the magnetic field energy can be converted into plasma kinetic energy, which is then emitted as photons. This model will not replace the agglomeration model, but will move with it.

Scientists have discovered that such a magnetic mechanism can cause gamma-ray bursts in 2D simulations, and perhaps explain the observed bursts. They now focus on 3D simulations and take into account the EHT's radio wave emission associated with black hole observations. According to Crinquand, “We want to get more realistic images that we can compare with the experimental data.”

According to the research team, black holes periodically experience a state known as glowing plasma, where the majority of the plasma is weak and the magnetic forces are strong enough to hide the effects of friction-like forces during deposition. The team mimics the dynamics of the plasma's magnetic fields and the particle population to study the energy transfer between particles and fields. According to Crinquand, the model takes into account all plasma currents as well as general relativistic effects that have been ignored in previous research.

According to the team's models, magnetic field lines constantly bend, separate, and recombine as they move through the plasma and interact with particles. The researchers discovered that the magnetic field energy is converted into plasma kinetic energy during reconnection of the field lines, just as in their previous work with 2D models.

Black Holes Emit Radio Waves

Using ray tracing, the team simulates radio waves emitted from charged plasma and depicts what they would look like to a viewer on Earth. The team discovered that ring-like formations of varying intensities predominate in radio wave emission. Radio wave hotspots orbiting the shadow of the black hole represent these variations.

The hotspots are expected to have an orbital radius of about three times the radius of the black hole and an orbital period of about five days in the case of a massive black hole, such as at the center of galaxy M87.

Because the telescope's spatial and temporal resolutions are too low to distinguish these features, Crinquand argues that the current iteration of the EHT is unlikely to detect the emission patterns that he and his colleagues are expecting. Crinquand adds that even with advanced resolution imaging capabilities, these patterns will not always be visible due to their temporary nature. “We expect the plasma to be in a glowing state and that these hot regions will become apparent as the accumulation flow periodically recedes. According to Crinquand, it will take "a lot of luck" to display these features even in the next edition of EHT.

Still, he remains optimistic that ideal conditions will come true. I think it would be great to witness a black hole producing hotspots in the EHT.

According to black hole physicist Amir Levinson of Tel Aviv University in Israel, this discovery is an "important step" towards understanding the processes responsible for the radiation emitted around black holes. If we can successfully complete a detailed study of the dynamics and emission of the magnetosphere, we can learn more about fundamental physics and astrophysics. While there is still much to learn about the processes occurring in the plasma surrounding black holes, Levinson says that "the route taken by [Crinquand and colleagues] looks promising."

Source: physics.aps.org/articles/v15/170

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