A new study of observations from NASA's Fermi Gamma-ray Space Telescope has discovered a faint but sprawling glow around a nearby pulsar. If visible to the human eye, this gamma-ray "halo" would appear larger in the sky than the famed Big Dipper star pattern.
The halo suggests this same pulsar could be responsible for a decade-long puzzle about one type of cosmic particle arriving from beyond the solar system that is unusually abundant near Earth -- positrons, the antimatter version of electrons.
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 radio waves, light, X-rays and gamma rays that, much like a lighthouse, regularly sweep across our line of sight from Earth.
Geminga (pronounced geh-MING-a) is among the brightest pulsars at gamma-ray energies.
To study its halo, scientists had to subtract out all other sources of gamma rays, including diffuse light produced by cosmic ray collisions with interstellar gas clouds. Ten different models of interstellar emission were evaluated.
What remained when these sources were removed was a vast, oblong glow spanning some 20 degrees -- about 40 times the apparent size of a full Moon -- at an energy of 10 billion electron volts (GeV), and even larger at lower energies.
The team determined that Geminga alone could be responsible for as much as 20% of the high-energy positrons seen by other space experiments. Extrapolating this to the cumulative emission of positrons from all pulsars in our galaxy, the scientists say it's clear that pulsars remain the best explanation for the observed excess of positrons.
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