Why is there so much antimatter in the Universe? Ordinary matter is far more plentiful than antimatter, but scientists keep detecting more and more antimatter in the form of positrons. More positrons reach Earth than standard models predict. Where do they come from?
Scientists think pulsars are one source, and a new study strengthens that idea.
Positrons are the antimatter equivalent to electrons. They’re the same mass, but they’re positively charged rather than negatively charged. They’re produced by decay in some naturally occurring radioactive isotopes and also by a process called pair production. But more positrons are reaching Earth than there should be.
A spectrometer on the ISS called the Alpha Magnetic Spectrometer (AMS-02) detected more positrons than expected in 2014, and it confirmed the results of previous experiments that found the same thing. Scientists have long thought that pulsars are one source of positrons, but that fact has been difficult to establish.
In a new study, researchers imaged a pulsar named PSR J2030+4415. They used the Chandra X-ray Observatory to capture images of a beam of matter and antimatter that’s 40 trillion miles long coming from the pulsar. Pulsar beams like this one could account for the excess of positrons.
“It’s amazing that a pulsar that’s only 10 miles across can create a structure so big that we can see it from thousands of light-years away.”
Martijn de Vries, lead author, Stanford University.
The study is “The Long Filament of PSR J2030+4415.” It’s published in The Astrophysical Journal Letters, and the authors are Martijn de Vries from Stanford University and Roger W. Romani from Stanford University.
A pulsar is a rapidly spinning neutron star with intense magnetic fields. As collapsed stars, they’re tiny yet very dense. They’re about the size of a large city, but they can emit jets on an epic scale.
“It’s amazing that a pulsar that’s only 10 miles across can create a structure so big that we can see it from thousands of light-years away,” said the lead author Martijn de Vries. “With the same relative size, if the filament stretched from New York to Los Angeles, the pulsar would be about 100 times smaller than the tiniest object visible to the naked eye.”
The size of this filament has the authors thinking that structures like it could be a significant source of positrons. Pulsars are extreme objects that exhibit a combination of rapid rotation and powerful magnetic fields. These extreme forces accelerate particles and cause high-energy radiation resulting in electron and positron pair production. Einstein’s E=mc2 equation explains how this works. His equation shows how mass can convert into energy, but the process is reversed in this case.
The positrons, along with electrons, are contained in the pulsar’s stellar wind, and usually, the pulsar’s powerful magnetic fields keep the wind confined. But something else is happening with PSR J2030+4415.
It’s travelling through space at about 1.6 million km/h (one million mp/h.) The pulsar’s wind trails behind the pulsar, and a bow shock is in front of it. But a couple of decades ago, the bow shock stalled, and the pulsar and its wind caught up to it. That led to an interaction between the pulsar and the interstellar magnetic field.
This figure from the study shows the pulsar travelling through space for about ten years. The solid red line is the bow shock, and the dotted red line is the bubble that contains the pulsar itself. The pulsar is the cyan circle. While the bow shock hardly shifts, the pulsar bubble at the apex grows over ten years. The image on the right shows the bubble growing as a yellow, green, and red circle. Eventually, the pulsar wind’s magnetic field linked up with the interstellar magnetic field. Then high-energy particles broke out from the bubble and travelled along the interstellar magnetic field, creating the long filament seen in the Chandra x-ray images. Image Credit: De Vries and Romani 2022.
“This likely triggered a particle leak,” said co-author Roger Romani. “The pulsar wind’s magnetic field linked up with the interstellar magnetic field, and the high-energy electrons and positrons squirted out through a nozzle
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