A faint yet widespread glow of high-energy light surrounding a neighboring pulsar has been detected by NASA’s Fermi Gamma-ray Space Telescope. If this gamma-ray “halo” is visible to the human eye, it would seem nearly 40 times bigger in the sky when compared to a full Moon.
This structure could be the key to a long-pending puzzle related to the quantity of antimatter in Earth’s neighborhood.
Our analysis suggests that this same pulsar could be responsible for a decade-long puzzle about why one type of cosmic particle is unusually abundant near Earth. These are positrons, the antimatter version of electrons, coming from somewhere beyond the solar system.
Mattia Di Mauro, Astrophysicist Catholic University of America
Di Mauro is also associated with NASA’s Goddard Space Flight Center in Greenbelt, Maryland. A paper describing the findings of the study has been published in the Physical Review D journal on December 17th, 2019, and is available online.
A neutron star is the crumbled core that remains when a star, considerably more enormous than the Sun, runs out of fuel, crashes due to its own weight, and explodes as a supernova. Some neutron stars can be observed as pulsars, which are fast-spinning objects that emit beams of light regularly sweeping across the line of sight of humans, quite similar to a lighthouse.
Geminga (pronounced geh-MING-ga), which was found out by NASA’s Small Astronomy Satellite 2 in 1972, is one of the brightest pulsars in gamma rays. This pulsar is located nearly 800 light-years away in the Gemini constellation.
Geminga’s name is both a play on the phrase “Gemini gamma-ray source” and the expression “it’s not there”—denoting that astronomers have not been able to detect the object at other energies—in the dialect of Milan, Italy.
It was in March 1991 that Geminga was finally spotted, when Germany’s ROSAT mission picked up flickering X-rays, revealing its source to be a pulsar that spins 4.2 times per second.
In general, a cloud of electrons and positrons surrounds a pulsar because the intense magnetic field of the neutron star attracts the particles from the surface of the pulsar, accelerating them to almost the speed of light.
Electrons and positrons are part of the fastest particles called cosmic rays, originating beyond the solar system. Since an electrical charge is carried by cosmic ray particles, their paths turn scrambled upon encountering magnetic fields on their journey toward Earth. This implies that it would be impossible for astronomers to directly track them back to their sources.
In the last 10 years, cosmic ray measurements by Fermi, NASA’s Alpha Magnetic Spectrometer (AMS-02) aboard the International Space Station, and other space experiments close to Earth have observed more positrons at energies higher than expected by researchers. Neighboring pulsars such as Geminga were the main suspects.
Subsequently, in 2017, researchers from the High-Altitude Water Cherenkov Gamma-ray Observatory (HAWC) near Puebla, Mexico, confirmed previous ground-based observations of a small gamma-ray halo surrounding Geminga. This structure was detected at energies from 5 to 40 trillion electron volts—light that has trillions of times more energy compared to what human eyes can see.
Researchers consider this emission emerges when accelerated electrons and positrons bump into nearby starlight. The collision increases the light up to considerably higher energies. On the basis of the halo’s size, the HAWC researchers inferred that positrons from Geminga with these energies only rarely make it to Earth. If this is true, it would imply that the detected positron excess must have a more peculiar interpretation.
However, with the continued interest in the origin of a pulsar, Geminga was front and center. Di Mauro headed a review of a decade of gamma-ray data from Geminga collected by Fermi’s Large Area Telescope (LAT), which observes light of lower energy compared to HAWC.
To study the halo, we had to subtract out all other sources of gamma rays, including diffuse light produced by cosmic ray collisions with interstellar gas clouds. We explored the data using 10 different models of interstellar emission.
Silvia Manconi, Study Co-Author and Postdoctoral Researcher, RWTH Aachen University
Upon removing these sources, the remnant was an enormous, oblong glow with an energy of 10 billion electron volts (GeV), and spanning nearly 20° in the sky. That is analogous to the size of the well-known Big Dipper constellation—and at lower energies, the halo is much bigger.
Lower-energy particles travel much farther from the pulsar before they run into starlight, transfer part of their energy to it, and boost the light to gamma rays. This is why the gamma-ray emission covers a larger area at lower energies. Also, Geminga’s halo is elongated partly because of the pulsar’s motion through space.
Fiorenza Donato, Study Co-Author, Italian National Institute of Nuclear Physics and University of Turin
The researchers found out that the Fermi LAT data matched well with the previous HAWC observations. Geminga as such could be accountable for nearly 20% of the high-energy positrons observed by the AMS-02 experiment. The researchers extrapolated this to the overall emission from all pulsars in the Milky Way galaxy and report that evidently, pulsars are still the best interpretation for the excess positrons.
“Our work demonstrates the importance of studying individual sources to predict how they contribute to cosmic rays,” stated Di Mauro. “This is one aspect of the exciting new field called multimessenger astronomy, where we study the universe using multiple signals, like cosmic rays, in addition to light.”
The Fermi Gamma-ray Space Telescope, which is a particle physics and astrophysics collaboration, is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Fermi was created in partnership with the U.S. Department of Energy, with significant contributions from academic institutions and partners in Germany, France, Japan, Italy, Sweden, and the United States.
Astronomers using data from NASA’s Fermi mission have discovered a pulsar with a faint gamma-ray glow that spans a huge part of the sky. Watch to learn more. Video Credit: NASA’s Goddard Space Flight Center.