Strange supernova with reverse shock wave

(ORDO NEWS) — A powerful shock wave passing through a cloud of gas left after the explosive death of a star has a strange feature: part of its wave is moving in the wrong direction, a new study has shown.

During the study, scientists found that the shock wave accelerates at different speeds, and one part of it goes back – towards the source of the stellar explosion, or supernova, which the authors of the study call “blowback”.

Cassiopeia A is a nebula, or gas cloud, left over from a supernova explosion in the constellation Cassiopeia, located about 11,000 light-years from Earth, making it one of the closest supernova remnants.

The nebula, which is about 16 light-years across, is made up of gas (mostly hydrogen) that was ejected both before and during the explosion that tore the original star apart.

The shock wave from this explosion is still propagating through the gas, and theoretical models show that this shock wave should expand evenly, like a perfectly rounded balloon that is constantly inflating.

But researchers have found that this is not the case.

“For a long time, we suspected that something strange was going on inside Cassiopeia A,” study lead author Jakko Wink, an astronomer at the University of Amsterdam in the Netherlands.

That the western region of the shock wave moving through the gas cloud may even be going in the wrong direction, he added.In

the new study, the scientists analyzed the movement of the shock wave using X-ray images collected by NASA’s Chandra X-ray Observatory, a telescope orbiting the Earth. collected over 19 years have confirmed that part of the western shock wave region is actually moving in the opposite direction.

But they also discovered something even more surprising: Part of the same region was still accelerating away from the epicenter of the supernova, as was the rest of the shock wave.

According to Vinck, the average velocity of the expanding gas in Cassiopeia A is about 21.6 million km/h, making it one of the fastest shock waves ever seen in a supernova remnant.

This is mainly due to the fact that the remnant is very young – light from Cassiopeia A reached Earth in 1970. But over time, shock waves lose their momentum with respect to the environment and slow down.

Cassiopeia A consists of two main expanding bands of gas: the inner and outer shells. These two shells are two halves of the same shock wave, and over most of the nebula, the inner and outer shells move at the same speed and in the same direction.

But in the western region, the two shells move in opposite directions: the outer shell continues to expand outward, while the inner shell moves back to where the exploding star should have been.

The kickback is receding at about 6.9 million km/h, about a third of the average expansion speed of the rest of the nebula. However, what really puzzled the researchers was the rate of expansion of the outer shell compared to the receding inner shell in this region.

The researchers expected the outer shell to expand at a slower rate compared to the rest of the shock wave, but they found that it accelerates faster than some other areas of the shock wave. “It was a complete surprise,” Wink said.

The unusual expansion of the western region of Cassiopeia A is not consistent with theoretical supernova models and suggests that something happened to the shock wave after the stellar explosion, Vinck said.

According to the researchers, the most likely explanation is that the shock wave collided with another shell of gas, which was likely ejected by the star before the explosion. When the shock wave collided with this gas, it could slow down and create pressure that pushed the inner shell back toward the center.

However, the outer shell could still pass through this barrier and start accelerating on the other side, Vink said. “This explains both the inward movement of the inner shell and predicts that the outer shell should be accelerating, which is what we measured,” he added.

The researchers also believe that the original star’s unique way of dying could explain the shock wave’s irregularity. Cassiopeia A is the result of a Type IIb supernova in which a massive star exploded after shedding its outer layers almost completely, Winck said.

“X-ray estimates show that at the time of the explosion, the mass of the star was about four to six times that of the Sun,” Wink said, “but, most likely, at the time of birth, the mass of the star was about 18 times that of the Sun.

This means that the star has lost about two-thirds of its mass, most of which was hydrogen, before the explosion.The shock wave may have later collided with this gas, Vinck said.

There are several theories about why Cassiopeia A lost so much mass before the explosion. In September 2020, another group of researchers suggested that the original star was part of a binary star system where two stars orbit each other.

This group of researchers said that the companion star could also go supernova before Cassiopeia A and shed the star’s hydrogen “skin” in the process, Live Science reported earlier.

However, the authors of the new study are not convinced of this theory. “The only problem is that we haven’t found the remains of another star yet,” Wink said. “So at this stage it remains speculative.”

So while no one knows exactly what caused the uneven shock wave of Cassiopeia A.

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