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.”
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 formed by connection.”
After the particles escaped, they found new magnetic field lines to follow. They slowed and moved along the interstellar magnetic field lines at about one-third the speed of light. They emitted x-rays, and that’s what Chandra sees as the extraordinarily long filament.
This image from NASA’s Chandra X-ray Observatory and ground-based optical telescopes show an extremely long beam, or filament, of matter and antimatter extending from a relatively tiny pulsar named PSR J2030+4415. The image on the left shows a portion of the filament as particles flow along the magnetic lines of the interstellar magnetic field. The image on the right shows X-rays created by particles moving around the pulsar itself. Image Credit: X-ray: NASA/CXC/Stanford Univ./M. de Vries; Optical: NSF/AURA/Gemini Consortium.Researchers observed positrons at pulsars before, but they weren’t leaking into the galaxy. Instead, they existed in a kind of halo around the pulsar. The halo is what remains of a pulsar wind nebula that appeared shortly after the core-collapse supernova created the pulsar. The bow shock observed around J2030 is the halo’s edge, and the positrons were confined to the halo.
Those observations went against the idea that pulsars are a source of antimatter in the Universe. But the tremendously long filament coming from PSR J2030+4415 shows that positrons can come from pulsars and that some could even reach Earth. This helps explain the excess of positrons detected by the AMS-02 instrument on the ISS.
This image from the study shows PSR J2030+4415 as seen by two instruments. The red, green, and blue are H-alpha spectral emissions as imaged with the Gemini Multi-Object Spectrograph on the Gemini Telescope North. The smoothed green contours show x-ray emissions observed with the Advanced CCD Imaging Spectrometer on the Chandra X-ray Observatory. Some of the x-ray emissions are from field stars and background sources, but the pulsar wind nebula and the filament are clearly visible. Image Credit: De Vries and Romani 2022.The pair of researchers say that J2030 has more to teach us about antimatter positrons and the complex structure of pulsars and their filaments. The filament’s morphology is of particular interest because its at the root of positron escape. While J2030’s filament is similar to other known pulsar filaments, the authors point out that it has some “special features.”
“It is purely one-sided, initially narrow, with an approximate linear expansion, and its flux varies little with distance,” the authors write.
To understand what this means, the authors referred to a previous study from 2020. That study paid particular attention to the morphology of bowshocks created by pulsars moving through the ISM, the same way J2030 does.
In that study, the authors write, “The interaction of a fast-moving pulsar wind with the ISM produces a bow-shock nebula with extended tail. In the apex part of the PWN <pulsar wind nebula> the ISM <interstellar medium> ram pressure confines the pulsar wind producing two shocks – a forward shock in the ISM and the reverse/termination shock in the pulsar wind; the contact discontinuity separates the two shocks.” A contact discontinuity is a transition layer where properties change, in this case, a layer between the pulsar wind and the interstellar medium.
This is critical to the positrons emitted by PSR J2030+4415.
This image is a cartoon of magnetic geometry near the magnetopause at the contact discontinuity. Image Credit: De Vries and Romani 2022.“J2030’s extreme one-sidedness may be connected with the unusually small spin-velocity angle inferred from the H-alpha bubble velocity,” the authors of the newer study write. “… this ensures that one magnetic hemisphere is much closer to the swept-up magnetic field at the apex.” This fosters the reconnection to the ISM field lines. That reconnection allows the positrons to escape along the ISM field lines, creating the extraordinarily long x-ray filaments seen in the Chandra images.
For decades scientists have wondered about the source of positrons that reach Earth. This research peels back some of the layered mystery around antimatter, but it’s not definitive. It just outlines one potential source. Other studies say pulsars can’t be the source.
A study from 2017 showed that even nearby pulsars are too far away to be the source of antimatter detected here at Earth. The flow is too diffuse to be detected when it reaches Earth. “The excess positrons detected on Earth must therefore have a more exotic origin than nearby pulsars,” that study concluded.
That leads us to another candidate source: the mysterious dark matter. Positrons could come from the annihilation and decay of dark matter particles. But there’s no way of observing that and it’s largely theoretical.
That brings us back to pulsars.
The 2017 study showing that pulsars can’t be the source of antimatter reaching Earth isn’t the last word on the subject. There’s a lot that astronomers don’t know about pulsars, bow shocks, and ISM magnetic field lines. The authors of this new paper say that a more detailed study could strengthen the idea that pulsars like PSR J2030+4415 are the source of antimatter reaching Earth.
Scientists need more detailed measurements to understand if pulsars are a significant source of positrons. Unlike some astronomical objects, J2030 changes rapidly, and that’s part of the problem in understanding it. “Our revised picture of the J2030 bow shock and filament suggests rapid evolution of these structures on a decade time scale …” the authors write. “This makes precision measurements of the shock structure and filament connection very difficult with ground-based seeing.”
The authors say they’re beginning to understand when these x-ray filaments flare brightly, indicating increased positron emissions. If they can predict the flaring in advance, they can organize extensive x-ray imaging campaigns. When combined with other data, the results of that x-ray imaging will be “… a powerful tool for probing the conditions needed for efficient reconnection…” and positron escape.
All of that work “… may have important implications for the propagation of pulsar cosmic ray positrons through the nearby ISM to Earth detectors,” they conclude.
Though antimatter sounds like the stuff of science fiction weaponry, it’s not dangerous. It’s entirely natural and has been around as long as the Universe itself.
But the source of the excess positrons reaching Earth is a mystery, and scientists are drawn to those.
