Sunday, April 3, 2022

Origin of antimatter arriving on Earth could be nearby pulsars

The mysterious excess of a certain antimatter particle that hits Earth may be about to be unraveled. Contrary to the conclusion of previous studies, new research shows that perhaps positrons are sent to our planet by a pulsar called PSR J2030+4415.

Antimatter, contrary to what we see in some fictional stories, does not pose any risk to us, living beings on planet Earth — not least because the amount of antimatter in the universe is almost irrelevant, considering the amount of “normal” matter.

Even so, the positron particles that travel from space and arrive here are a mystery to scientists. No one knows exactly how and where they are produced in the universe. Positrons are the “opposite version” of electrons: they have the same mass but the opposite (positive) electrical charge.

One of the positron-producing candidates is a type of neutron star called a pulsar. It is a massive star that exploded in a supernova, leaving behind an extremely dense core — more massive than the Sun and just 15 km in diameter on average.

A 2017 study showed that even nearby pulsars are too far from us to be the source of antimatter detected here on Earth. It's just that the flux emitted by these objects is too diffuse to be detected here, so the excess positrons found on our planet should have a more "exotic" origin than nearby pulsars.

However, that was not the last word on the matter. Pulsars are still very strange objects and there's a lot about them that scientists still don't know. The authors of the new paper say a more detailed study could bolster the idea that pulsars are, in fact, the source of antimatter hitting Earth.

The long filament of pulsar J2030

Pulsar PSR J2030+4415 has been observed for 10 years, with different instruments, to track its journey through space at the speed of 1.6 million km/h. In particular, the authors of the new study were interested in a beam approximately 7 light-years long, emitted by the pulsar.

 With such a large filament, the authors imagined that structures like this could be a significant source of positrons, as there is a lot of energy involved in these jet emission processes. Pulsars are extreme objects, with fast rotation and powerful magnetic fields.

These forces accelerate the particles and cause high-energy radiation, resulting in the production of electron and positron pairs — as Albert Einstein demonstrated in his equation E=mc². Positrons and electrons are contained in the pulsar's stellar wind, and powerful magnetic fields keep the wind confined. That means the particles weren't supposed to travel all the way to our planet, but something strange happened.

In the case of pulsar J2030, the wind follows it as the object moves through space. This produces a shock arc ahead of you, but that arc stopped a few decades ago. With that, the pulsar and its wind caught up with it, which led to the interaction between the pulsar and the interstellar magnetic field.

 The antimatter leak

According to study co-author Roger Romani, “this likely triggered a particle leak,” in which “the wind magnetic field from the pulsar bound to the interstellar magnetic field, and high-energy electrons and positrons squirted through a nozzle formed for that connection”.

After this particle escape, they found new magnetic field lines to follow and slowed down, moving along these lines in the interstellar medium at about one-third the speed of light. Meanwhile, the particles emitted X-rays, which were in turn detected by the Chandra telescope in the form of an extraordinarily long filament.

PSR J2030+4415 seen by two instruments (Image: Reproduction/De Vries/Romani)

The authors said they are just beginning to understand when these X-ray filaments glow, indicating increased positron emissions. If they can predict these spikes in advance, they can organize new observation campaigns with Chandra to track the whole process.

If combined with other data, the results of these X-ray images will be “a powerful tool for probing the conditions necessary for an efficient [magnetic field lines] reconnection [for positron escape].” In other words, the study is promising but further observations are still needed to prove whether antimatter does indeed come from near-Earth pulsars. 

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