This animation shows a region of the sky centered on the pulsar Geminga. The first image shows the total number of gamma rays detected by Fermi’s Large Area Telescope at energies from 8 to 1,000 billion electron volts (GeV) — billions of times the energy of visible light — over the past decade. By removing all bright sources, astronomers discovered the pulsar’s faint, extended gamma-ray halo. Credit: NASA/DOE/Fermi LAT Collaboration
NASA’s Fermi Gamma-ray Space Telescope has discovered a faint but
sprawling glow of high-energy light around a nearby pulsar. If visible
to the human eye, this gamma-ray “halo” would appear about 40 times
bigger in the sky than a full Moon. This structure may provide the
solution to a long-standing mystery about the amount of antimatter in
our 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,” said Mattia Di Mauro, an astrophysicist at the
Catholic University of America in Washington and NASA’s Goddard Space
Flight Center in Greenbelt, Maryland. “These are positrons, the
antimatter version of electrons, coming from somewhere beyond the solar
system.”
A paper detailing the findings was published in the journal Physical Review D on Dec. 17 and is available online.
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.Credits: NASA’s Goddard Space Flight Center. Download additional multimedia from NASA Goddard's Scientific Visualization Studio
A neutron star is the crushed core left behind when a star much more
massive than the Sun runs out of fuel, collapses under its own weight
and explodes as a supernova. We see some neutron stars as pulsars,
rapidly spinning objects emitting beams of light that, much like a
lighthouse, regularly sweep across our line of sight.
Geminga (pronounced geh-MING-ga), discovered in 1972 by NASA’s Small Astronomy Satellite 2,
is among the brightest pulsars in gamma rays. It is located about 800
light-years away in the constellation Gemini. Geminga’s name is both a
play on the phrase “Gemini gamma-ray source” and the expression “it’s
not there” — referring to astronomers’ inability to find the object at
other energies — in the dialect of Milan, Italy.
Geminga was finally identified in March 1991, when flickering X-rays picked up by Germany’s ROSAT mission revealed the source to be a pulsar spinning 4.2 times a second.
A pulsar naturally surrounds itself
with a cloud of electrons and positrons. This is because the neutron
star’s intense magnetic field pulls the particles from the pulsar’s
surface and accelerates them to nearly the speed of light.
Electrons and positrons are among the speedy particles known as
cosmic rays, which originate beyond the solar system. Because cosmic ray
particles carry an electrical charge, their paths become scrambled when
they encounter magnetic fields on their journey to Earth. This means
astronomers cannot directly track them back to their sources.
For the past decade, cosmic ray measurements by Fermi, NASA’s Alpha Magnetic Spectrometer (AMS-02)
aboard the International Space Station, and other space experiments
near Earth have seen more positrons at high energies than scientists
expected. Nearby pulsars like Geminga were prime suspects.
Then, in 2017, scientists with the High-Altitude Water Cherenkov Gamma-ray Observatory (HAWC)
near Puebla, Mexico, confirmed earlier ground-based detections of a
small gamma-ray halo around Geminga. They observed this structure at
energies from 5 to 40 trillion electron volts — light with trillions of
times more energy than our eyes can see.
Scientists think this emission arises when accelerated electrons and
positrons collide with nearby starlight. The collision boosts the light
up to much higher energies. Based on the size of the halo, the HAWC team
concluded that Geminga positrons at these energies only rarely reach
Earth. If true, it would mean that the observed positron excess must
have a more exotic explanation.
This model of Geminga's gamma-ray halo shows how
the emission changes at different energies, a result of two effects. The
first is the pulsar's rapid motion through space over the decade
Fermi's Large Area Telescope has observed it. Second, lower-energy
particles travel much farther from the pulsar before they interact with
starlight and boost it to gamma-ray energies. This is why the gamma-ray
emission covers a larger area at lower energies. One GeV represents 1
billion electron volts — billions of times the energy of visible light. Credits: NASA’s Goddard Space Flight Center/M. Di Mauro.
Scientists think this emission arises when accelerated electrons and
positrons collide with nearby starlight. The collision boosts the light
up to much higher energies. Based on the size of the halo, the HAWC team
concluded that Geminga positrons at these energies only rarely reach
Earth. If true, it would mean that the observed positron excess must
have a more exotic explanation.
But interest in a pulsar origin continued, and Geminga was front and
center. Di Mauro led an analysis of a decade of Geminga gamma-ray data
acquired by Fermi’s Large Area Telescope (LAT), which observes
lower-energy light than 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,” said co-author Silvia Manconi, a postdoctoral
researcher at RWTH Aachen University in Germany. “We explored the data
using 10 different models of interstellar emission.”
What remained when these sources were removed was a vast, oblong glow
spanning some 20 degrees in the sky at an energy of 10 billion electron
volts (GeV). That’s similar to the size of the famous Big Dipper star
pattern — and the halo is even bigger at lower energies.
“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 ,” explained co-author Fiorenza Donato at
the Italian National Institute of Nuclear Physics and the University of Turin. “Also, Geminga’s halo is elongated partly because of the pulsar’s motion through space.”
The team determined that the Fermi LAT data were compatible with the
earlier HAWC observations. Geminga alone could be responsible for as
much as 20% of the high-energy positrons seen by the AMS-02 experiment.
Extrapolating this to the cumulative emission from all pulsars in our
galaxy, the scientists say it’s clear that pulsars remain the best
explanation for the positron excess.
“Our work demonstrates the importance of studying individual sources
to predict how they contribute to cosmic rays,” Di Mauro said. “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 is an astrophysics and particle
physics partnership managed by NASA's Goddard Space Flight Center in
Greenbelt, Maryland. Fermi was developed in collaboration with the U.S.
Department of Energy, with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.
Illustration of NASA’s Fermi Gamma-ray Space Telescope in orbit.
Credits: NASA's Goddard Space Flight Center
Source: NASA/Fermi Space Telescope
