Halo Around Pulsar Could Answer Mystery of Antimatter

Larger than the Big Dipper as seen from Earth, a faint halo surrounding an energetic dead star could answer one of the great mysteries of cosmic rays and help reveal the behavior of pulsars.

The Fermi Space Telescope, launched in 2008, first recorded a faint haze of high-energy light around a nearby stellar corpse called a pulsar. Now, a new analysis of the data suggests this object may play a critical role in the production of the cosmic rays which bathe our Solar System.

Pulsars are the remains of massive dead stars, which emit powerful beams of radiation that, under ideal conditions, can be seen from Earth. One of these stellar corpses, Geminga (geh-MING-ga), was recently found to be encompassed by an enormous, although tenuous, region of high-energy radiation.

An animation of the sky surrounding Geminga, viewing energies between eight and one trillion electron volts. Image credit: NASA/DOE/Fermi LAT Collaboration

If it were possible to see this haze of light with the human eye, it would stretch 20 degrees across the night sky — as large as 40 full moons, lined up side-by-side.

“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,” said Mattia Di Mauro, an astrophysicist at NASA’s Goddard Space Flight Center.

A Cosmic Conundrum

When stars significantly larger than the Sun run out of fuel, they collapse under their own weight, and explode as a supernova. This can leave behind a neutron star, one of the densest objects in the Universe. If these neutron stars are aligned correctly, beams of radiation from these bodies can be seen from Earth as they spin, creating a pulsing effect.

The makeup of neutron stars, a class of stellar corpses which include rapidly-spinning pulsars. Image credit: NASA’s Goddard Space Flight Center

Neutron stars are surrounded by clouds of electrons, and their anti-matter equivalents, positrons. These particles travel through space, and when they encounter the area near Earth, they are known as cosmic rays. Because they hold an electromagnetic charge, their paths are altered by magnetic fields, making it impossible for astronomers to determine their source.

Observations by a range of instruments reveal that the region around our planet is flooded with significantly more cosmic rays than researchers predicted. Astrophysicists suspected nearly pulsars, like Geminga, may be the source of this radiation.

In 2017, astronomers in Puebla, Mexico confirmed an earlier observation showing evidence of a small gamma-ray halo surrounding Geminga. The energy in this halo is measured between five and 40 trillion electron volts — trillions of times more energetic than light seen by the human eye.

Astrophysicists believe that cosmic rays may be the product of collisions between electrons and photons of light. However, the small size of the Geminga halo, as measured in 2017, was not large enough to account for the excess radiation, putting a damper on the idea that the source of the cosmic rays may have been nearly pulsars like Geminga.

However, some astronomers continued to examine Geminga and other pulsars in our stellar neighborhood, searching for the source of this energetic radiation.

A look at the halo surrounding the Gemina pulsar. Video credit: NASA Goddard

“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,” explained Silvia Manconi from RWTH Aachen University in Germany.

When researchers removed the obscuring radiation, the data left behind revealed a massive glow at 10 billion electron volts, filling an area of the sky larger then the constellation of the Big Dipper. At lower energies, the phenomenon fills an even-larger portion of the sky. This is due to the fact that at lower energies, particles need to travel greater distances before interacting with photons of light, producing gamma rays.

Finding a Signal in the Darkness

“My eureka moment was in the dead of night, the early hours of the morning, on a cold, cold night, and my feet were so cold, they were aching. But when the result poured out of the charts, you just forget all that. You realize instantly how significant this is — what it is you’ve really landed on — and it’s great!” Jocelyn Bell Burnell

Geminga, discovered by NASA’s Small Astronomy Satellite 2 in 1972, is one of the most energetic pulsars known, emitting vast amounts of gamma radiation. This object lies 800 light years from Earth, and is seen in the constellation of Gemini. Spinning 4.2 times a second, Geminga was first identified as a pulsar in 1991.

Pulsars were first discovered in 1967 by British astronomer Jocelyn Bell Burnell, a graduate student Cambridge University. The regular beams of energy, looking like a homing beacon, suggested to astronomers that these sources may have been the product of alien intelligence, and they were originally dubbed LGM’s (little green men).

“We did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem — if one thinks one may have detected life elsewhere in the universe how does one announce the results responsibly? Who does one tell first?” Burnell writes.

Before long, Thomas Gold determined that pulsars were rapidly-rotating neutron stars. Today, astronomers know of more than 2,000 pulsars, some of which spin faster than a household blender.

Although pulsars were not evidence of space-faring civilizations, these bodies did play a significant role in our understanding of the Cosmos. In January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12 (a third has since been detected). These were the first alien worlds ever found outside our Solar System.

This new study suggests that Geminga alone could account for 20 percent of the high-energy protons seen by astronomers in the cosmic rays bathing our cosmic neighborhood.

Analysis of the finding was published in the journal Physical Review D.

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