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Secret behind luminous jets at centre of galaxies solved after 40 yrs, shock waves hold key

In a study in the journal Nature, scientists found that particles surrounding blackholes are accelerated by shock waves to nearly the speed of light, that look like luminous jets.

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New Delhi: A mystery that has puzzled scientists for 40 years now, has finally been solved as NASA makes a breakthrough in understanding black holes and the powerful luminous jets that occur at the center of some galaxies. These relativistic jets – made of ionized particles – are known to be powered by the supermassive blackhole itself, but how those particles are accelerated to such great energies, has been an unanswered question.

By using data from the Imaging X-ray Polarimetry Explorer (IXPE), researchers finally came to understand and explain how the blazing jets of light become so powerful. The imaging device offered insight into how shock waves essentially accelerated the strength of these luminous jets.

The IXPE satellite, which was launched in December 2021 into a 600-km circular geocentric orbit, was a collaboration between NASA and the Italian Space Agency.
“This is a 40-year-old mystery that we’ve solved,” said Yannis Liodakis, lead author of the study and astronomer at FINCA, the Finnish Centre for Astronomy with ESO. “We finally had all of the pieces of the puzzle, and the picture they made was clear.”

In a new study in the journal Nature, published on 23 November, astronomers ascertain that the best explanation for the particle acceleration is a shock wave within the jet.

Observing blazar ‘Markarian 501’

Researchers studied an exotic object called the ‘blazar’ – supermassive blackholes located at the center of active, giant elliptical galaxies – in the constellation Hercules, which is located about 460 million light years away from Earth.

There are materials or particles swirling around a blazar in the form of a disk, which can create two powerful jets perpendicular to the disk on each side. To Earth, a blazar can come off as especially bright as one of its jets points directly towards the observer. But the energy which powers these relativistic jets, has always been an unsolved mystery, until just recently.

The imaging device observed the nearby blazar, Markarian 50, for three days in March this year, and then re-observed it two weeks later. Other telescopic observations were also carried out on the ground and in space to understand the blazar better.

Previous studies have looked at polarisation (the average direction and intensity of the electric field of light waves) of low-energy light which emits from the blazar these black holes. But this time, astronomers were actually able to get a perspective of the blazar’s X-rays, which are significant as they emanate from a point much closer to the source of particle acceleration.

For the first time, scientists were able to measure the extent of polarisation of the light waves which make up the X-rays that telescopes on Earth are not equipped for, given the Blue planet’s atmosphere absorbs the rays.

“Adding X-ray polarization to our arsenal of radio, infrared, and optical polarization is a game changer,” said Alan Marscher, an astronomer at Boston University who leads the group studying giant black holes with IXPE.

Shock waves show up in polarisation data

These particles which surround the black hole, travel outward, emitting various ranges of energy. At first, they emit extremely energetic X-rays but gradually they lose energy and begin emitting less-energetic light such as optical waves and then radio waves. Even though the direction of all these lights was the same, it was clear that the X-rays were the most polarised.

Researchers found evidence that these particles became energised and emitted X-rays when they are hit with a shock wave propagating outward inside the stream.

Observations of the Markarian 501, measured the level of X-ray polarisation to be 10 per cent, which is twice as much as that of optical light which is further away from the black hole.

“Comparing these findings with theoretical models proved that a shock wave is what accelerated the jet particles. A shock wave is generated when something moves faster than the speed of sound of the surrounding material,” said Nasa.

“As the shock wave crosses the region, the magnetic field gets stronger, and energy of particles gets higher,” Marscher said. “The energy comes from the motion energy of the material making the shock wave.”

But still, several questions remain. To begin with, the origin of these shock waves is still a mystery. It has been hypothesised that this happens because of some kind of disturbance in the jet’s flow, such as particle collisions within the jet or pressure change near its boundary. These disturbances cause one section of the jet to become supersonic.

Scientists will continue observing the Markarian 501 blazar to note if polarisation of its rays changes over time. The IXPE will also look at other blazars during its mission.

Also read: NASA’s DART mission successfully knocks asteroid off course, Earth can now defend itself


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