Predicting black hole radio wave hotspots

Predicting black hole radio wave hotspots

Physics 15, 170

Plasma simulations around the black hole suggest that “magnetic reconnection” could stimulate radio-wave hotspots orbiting the black hole, a prediction that future Event Horizon Telescope measurements could test.

B. Krinkand/Princeton University

Simulations show the time evolution of magnetically induced radio wave emissions from black hole plasma. Radio wave hotspots rotate clockwise around the black hole’s shadow. For M87*, the first black hole imaged, it takes about 5 days to complete a full rotation.Simulations show the time evolution of magnetically induced radio wave emissions from black hole plasma. Radio wave hotspots rotate clockwise around the black hole’s shadow. For M87*, the first black hole imaged, it takes a full spin… Show more

Black holes have only three parameters – mass, rotation and charge – that can be considered one of the simplest astrophysical objects in the universe. However, the number of open problems related to how the two dark giants behave also distinguishes them as one of the most mysterious. One puzzle is why plasmas around black holes glow so brightly. Now, in 3D simulations of the magnetic fields within this plasma, Benjamin Krinkand and colleagues from Princeton University think they’ve found the answer: breaking and reconnecting magnetic field lines. [1]. Simulations predict that, under certain conditions, magnetic field instability can cause radio wave hotspots to orbit the black hole’s shadow. This prediction can be tested by future versions of the Event Horizon Telescope (EHT) – the radio dish network used to take the first images of a black hole (see Research News: First image of the Milky Way’s black hole).

There are several mechanisms that physicists think may be behind the black hole’s light. One of these forces is called the accretion force, whereby friction-like forces in the falling plasma heat the plasma, causing the emission of photons. Models of this process predict stable emission signals, which do not seem to fit in with observations of high-intensity gamma-ray bursts from black holes.

Another possibility – one that Krinkande and colleagues consider – is that the energy needed to generate this light is extracted from the magnetic field passing 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 kinetic energy of plasma, which is then emitted as photons. This model will not replace the accumulation model, but works in tandem with it.

In 2D simulations, the team previously found that such a magnetic process could lead to the emission of gamma-ray flares – which may explain the observed bursts. Now they turn to a 3D simulation and take into account the emission of radio waves, which correlates with the black hole observations made by the EHT. “We want to get more realistic images that we can compare with the experimental data,” says Krinkand.

The team hypothesizes that black holes sometimes enter a so-called blazing plasma state, where most of the plasma becomes devoid of force — meaning that magnetic forces are so high that they mask the effects of friction-like forces during accretion. The team simulates the dynamics of plasma particles and their magnetic fields, looking at the energy transfer between the particles and the fields. Krinkande says the model takes into account all currents flowing in the plasma – as well as general relativistic effects that were excluded from previous studies.

The team’s simulations show that magnetic field lines are constantly moving, bending, splitting, and rejoining as they move through the plasma and interact with its particles. As in their previous work with 2D simulations, the researchers found that magnetic field energy is converted into plasma kinetic energy as the field lines are reconnected.

The team models the radio waves emitted by the energetic plasma and uses ray tracing technology to visualize how this radiation would appear to an observer on Earth. They found that the emission of radio waves is dominated by ring-like structures, the intensity of which fluctuates over time. These fluctuations appear as radio wave hotspots circling the black hole’s shadow. In the case of a large black hole such as the one at the center of galaxy M87, the orbital radius of the hot spots would be expected to be about 3 times the radius of the black hole and an orbital period of about 5 days.

Crinquand notes that the current version of the EHT is unlikely to capture the emission patterns predicted by him and colleagues because the telescope’s spatio-temporal resolution is too low to elicit these features. He also notes that the transient nature of these patterns means that they will not always be detectable, even with high-resolution imaging capabilities. “From time to time, the buildup flow subsides and that’s when we would expect the plasma to be flaring and for these hot spots to become visible,” he says. Even with the next EHT iteration, Crinquand says, researchers will need “a lot of luck” to image these features. But he hopes the right conditions will come together. “I’d like to see the EHT capture a black hole emitting hot spots.”

This study is an “important step” toward capturing the processes responsible for the radiation emitted from around black holes, says Amir Levinson, a black hole physicist at Tel Aviv University, Israel. “Detailed analysis of magnetospheric dynamics and emissions is a formidable challenge that, if completed successfully, can advance our knowledge of fundamental physics and astrophysics.” Levinson adds that while there is still a long way to go to elicit all the processes that occur within the plasma surrounding a black hole, ” [by Crinquand and colleagues] It looks promising.”

Kathryn Wright

Kathryn Wright is deputy magazine editor Physics magazine.


  1. B. Crinquand et al.“Synthetic Images of Magnetosphere-Powered Radiation Around Supermassive Black Holes,” Phys. Reverend Litt. 129205101 (2022).

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