Why do black holes spew particles into space? Scientists say shock waves may explain mystery


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An international team of scientists found that powerful shock waves may help explain why black holes spew matter into space, shedding light on one of the biggest mysteries in the universe.

Black holes are known for their ability to pull things into them, but why they shoot out jets of shining particles has long baffled scientists.

The team, which includes researchers from the United States, Italy, mainland China and Hong Kong, proposed that particles escape black holes at very high speeds, then run into surrounding materials and slow down, resulting in a shock wave that spreads outward along the jet.

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Locked inside a corkscrew-shaped magnetic field, the particles are accelerated to nearly the speed of light as they reach the front edge of the shock wave and glow bright enough to be seen by telescopes, they said in the journal Nature on Thursday.

“The study offered the first observational evidence for how exactly the particles are boosted to very high energies soon after leaving their source. This process happens within a small area and is usually difficult to observe,” said Gou Lijun, an astrophysicist with the National Astronomical Observatories in Beijing who was not involved in the research.

Supermassive black holes, which are millions to billions of times more massive than the sun, are among the most enigmatic objects in the universe.

“There’s a supermassive black hole lurking at the centre of almost all galaxies. The one in our Milky Way galaxy is currently not active,” said astrophysicist Stephen Chi-yung Ng of the University of Hong Kong, who is a co-author of the paper.

As the invisible, fast-spinning black hole swallows up nearby gas and dust, a small portion of the material is ejected because of the rapid rotation and magnetic field of the accumulated gas, he said.

Scientists knew that the particles could be accelerated to extremely high energies – millions of times higher than the energy levels reached by particles created inside particle accelerators. If the jet happens to be shooting towards Earth, it will appear very bright because of the strong electromagnetic radiation it emits, Ng said.

However, scientists had no idea how the acceleration actually takes place, though they had proposed different theories to explain the process.

In December 2021, Nasa launched a space telescope called the Imaging X-ray Polarimetry Explorer (IXPE) to study the X-ray radiation of exotic astronomical objects, including black holes and neutron stars, and better understand their magnetic fields.

The research team used IXPE to observe a bright supermassive black hole at the heart of Markarian 501, a galaxy about 400 million light years from Earth. The particles shooting from the black hole were as bright as 100 billion suns, according to the paper.

They measured a parameter called polarisation, which refers to the oscillation direction of the electric and magnetic fields of an electromagnetic wave. Then the scientists compared their measurements with previous observations in the radio and optical wavebands.

“Our study found a higher degree of polarisation in X-rays than at lower frequencies. This implies that there is a highly ordered magnetic field close to the acceleration site,” said Ng, who is part of IXPE’s science working group. “Moving downstream, where the optical and radio emission originate, the degree of polarisation drops and points to a more turbulent magnetic field.”

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“Altogether, these observations strongly favour one particular theory related to shock waves, and allow us to reject others,” he said.

IXPE is the first instrument sensitive enough to detect polarisation signals from such a supermassive black hole.

Scientists from China and Europe are jointly developing a large space telescope called the enhanced X-ray Timing and Polarimetry mission (eXTP), which is expected to be launched around 2027 to study matter with extremely high density, gravity and magnetism, Gou said.

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