The Earth is being slammed with an immense amount of mysterious cosmic rays from outer space. However, scientists still don't know why our planet is being showered with these anti-matter particles and are yet to discover the source of the particles.
The cosmic rays, called positrons, are powerful particles, which when encountering matter can often result in the production of energy, including gamma-rays. Scientists detected positrons in 2008, using the space-based PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) detector. At the time, researchers uncovered that the Earth is hit with three times more number of energy particles than previously thought.
These findings were later confirmed by the Fermi Gamma-ray Space Telescope as well as the Alpha Magnetic Spectrometer (AMS-02), aboard the International Space Station, Scientific American reported. Scientists have since attempted to uncover the mystery surrounding the origins of these positrons.
Pulsars or dark matter?
Scientists previously believed that the Earth is showered by these positrons because of nearby pulsars — energy-emitting neutron stars. However, a new study, published in the journal Science, points away from previous theories, suggesting that the cosmic particles may be more "exotic" in nature.
A team of international scientists attempted to solve the mystery posed by positrons with recent observations that were made using the recently activated High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) detector in Mexico. The observatory contains numerous steel vats filled with water. When cosmic rays pass through, a flash of blue light appears, given that light travels slower in water than air.
"We know that high-energy particles of cosmic rays, travelling through our galaxy, quickly dissipate their energy by interacting with other radiation and magnetic fields. This is how particles of primary cosmic rays behave. Positrons are secondary, they come from interactions in which primary radiation is involved. We would therefore expect a similar dependence: a marked decrease in the number of high-energy positrons", Professor Sabrina Casanova (IFJ PAN) said in a statement.
"The reality is different. Satellite and terrestrial observatories record many more high-energy positrons than they should," Casanova added. "Our aim was to check whether the source of the positron excess was astronomical objects in our vicinity, such as pulsars and their surrounding nebulae".
The new HAWC observations appear to rule out the theory of pulsars causing these cosmic rays to rain down on Earth. Scientists made calculations using HAWC's recent data and suggested that the positrons, while travelling to the magnetic fields between pulsars and the Earth, would likely have lost too much energy, Gizmodo reported.
Researchers now believe that a more exotic theory could be the key to solving the mystery surrounding the origin of excess positron, such as the interactions of dark matter.
"When I started this work, I really believed it was pulsars," said study author Rubén López Coto of the Max Planck Institute for Nuclear Physics, National Geographic reported. "But these two pulsars actually cannot provide enough positrons in order to account for this positron excess."
"Since the involvement of close-by pulsars in the generation of high-energy positrons reaching us is so modest, other explanations become more and more likely. The most interesting is the hypothesis about the origin of excess positrons from the decay or annihilation of dark matter", Casanova said.
However, there appears to be some contention regarding the source of these mysterious cosmic rays. Some scientists argue that pulsars are the contributing factor in excess positrons.
According to Dan Hooper of the Fermi National Accelerator Laboratory, Chicago, whose work has reportedly been key in attributing pulsars for the anti-matter, the interpretation of the new observations is ambiguous.
"I am as convinced as ever that these pulsars are contributing very significantly to the local positron excess, and very well could dominate it," Hooper said, according to the National Geographic.
Regardless of the fact that the new observations appear to have resulted in raising more questions than answers, researchers are still reportedly excited about the findings.
The new observations "has the potential to be transformative in our understanding of how high energy gamma rays are produced and moreover how high energy cosmic rays, in particular cosmic ray electrons [and positrons] propagate throughout the galaxy," Tim Linden from The Ohio State University, told Gizmodo. "HAWC is going to be the telescope that tells us what produces the positron excess."