Simulations predict existence of radio wave hotspots in black holes

Black holes, regions of spacetime where gravity is so strong that nothing can escape, are among the most fascinating and studied cosmic phenomena. Although there are now countless theories about their formation and underlying physique, many questions remain unanswered.

One of the long-standing questions in the study of black holes is why the plasma around them glows so brightly, as shown by the few direct telescope images collected so far. In an article published in Physical examination lettersresearchers from Université Grenoble Alpes-CNRS, Trinity College Dublin and the University of Maryland have presented computer simulations that offer a viable explanation.

“We were very impressed with the recent release of images of the supermassive black hole M87* by the Event Horizon Telescope (EHT) collaboration,” Benjamin Crinquand, one of the researchers who led the study, told Phys. org. “This observation took place when this black hole was at a historically low luminosity (it was ‘quiescent’). However, M87* is known to produce emission bursts/flares at different wavelengths, up to to gamma rays.”

The primary goal of the recent study by Crinquand and colleagues was to make predictions about what images of the M87* black hole collected by the EHT collaboration would look like if they were collected during one of its outbursts. current broadcasts. To do this, they performed a series of kinetic plasma simulations, representing the neighborhood of a rotating black hole during such explosions.

“This new simulation tool to understand the behavior of plasma in such an extreme environment was developed very recently,” Crinquand explained. “Its purpose is to deal with plasma from first principles and to include relevant microphysics, which would be washed into the common fluid framework (magnetohydrodynamic simulations). Then one has to know how matter is coupled to radiation, which is ultimately observed from Earth.”

Theoretical and experimental studies have shown that in black hole environments, photons do not propagate in a straight line, due to their strong gravity. In their kinetic simulations, Crinquand and his colleagues attempted to account for this by implementing a ray-tracing module, which traces light emitted from the plasma around a black hole from the simulation to an observer.

Overall, the simulations carried out by this team of researchers suggest that under certain conditions, instabilities in the magnetic field can lead to the production of hot spots of radio waves, which would revolve around the shadow of a black hole. The team predicted that for large black holes, such as M87*, the orbital radius of these hotspots would be about three times larger than the radius of the black hole, and the hotspots would take about five days to orbit the hole. black.

“Our main contribution is the realization that when the black hole is in such a state, the image should show hotspots, which should rotate over time,” Crinquand said. “These hotspots are the signature of ‘plasmoids’, closed magnetic structures in the black hole’s magnetosphere. We also expect the image to shrink into the ‘photon ring’, which is commonly referred to as being the shadow observed by the EHT in 2019.”

The simulations carried out by this team of researchers introduce an interesting theoretical hypothesis that could be verified by future astronomical observations. Specifically, they predict that the radiation emission patterns seen around black holes could be due to the breakdown of magnetic fields and the resulting formation of radio wave hotspots.

The current version of the EHT may not be sensitive enough to capture the simulated emission patterns, due to its limited spatial and temporal resolution. Nevertheless, Crinquand and his colleagues hope that future versions of the telescope will help validate their theory.

“In the future, we want to pursue two lines of research,” added Crinquand. “First, we update our module to take into account the polarization of the emitted radiation, in order to increase the predictive power of our model. In 2021, the EHT published polarized observations of M87*, the weather It is therefore now ripe for theorists to make such From a theoretical point of view, we also want to better understand what drives this transition from a state of rest to a state of buckling Above all, we will need to understand the associated time scales : outbreak recurrence time, typical rise, time, etc.

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