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| Cosmic rays propagating through Milky Way interact with matter producing excess antimatter counterpart of electron, Image By NASA Via Unsplash |
New Delhi (Lisbon Times):- High energy
particles are generally lower in number in the cosmic universe. But the excess
number of high energy particles of the antimatter counterpart of the electrons,
called positrons have intrigued scientists for long. Now they have found an
explanation for this mystery.
Over the years
astronomers have observed an excess of antimatter
counterpart of the electron or positrons having an energy of more than 10
giga-electronvolts, or 10 GeV. For an estimate, this is the energy of a
positively charged electron accelerated across a 10,000,000,000 volt battery!
Positrons with energy more than 300 GeV, however, are lower in comparison to
what astronomers expect. This behaviour of positrons between 10 and 300 GeV is
what astronomers call the ‘positron excess’.
Researchers from the Raman
Research Institute (RRI), Bengaluru, an autonomous institution of
the Department of Science and
Technology have resolved the mystery in a new study published in the Journal of High Energy Astrophysics. Their
proposal is simple –– cosmic rays while propagating through the Milky Way
galaxy interact with matter producing other cosmic rays, primarily electrons
and positrons. The authors Agnibha
De Sarkar, Sayan Biswas and Nayantara Gupta argue that these new cosmic rays are the origin of
the ‘positron excess’ phenomenon.
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The Milky Way consists of giant clouds of molecular
hydrogen. They are the seats of the formation of new stars and can be as
massive as 10 million times the Sun’s mass. They can extend up to 600
light-years, the distance that would take light 600 years to travel. Cosmic
rays, produced in supernovae explosions propagate through these clouds before
they reach the Earth. Cosmic rays interact with molecular hydrogen and can give
rise to other cosmic rays. As they propagate through these clouds, they decay
from their original forms and intermix, lose their energy by energising the
clouds, and may also get re-energised. The researchers from RRI studied all
these astrophysical processes via a code they set up on the computer, using a
publicly available code.
The code considers 1638 molecular hydrogen clouds in the
Milky Way that other astronomers have observed across different wavelengths of
the electromagnetic spectrum. “We have followed three different catalogues to
construct a comprehensive one,” explains Agnibha De Sarkar, PhD student at RRI
and one of the authors of the study.
The combined catalogue consists of ten molecular clouds
in the immediate neighbourhood of our Sun. These galactic clouds provide the
astronomers a crucial input –– the number of giga-electronvolt cosmic rays.
These help them determine the excess number of positrons that reach the Earth.
The computer code the researchers used, by taking into account the exact number
of nearby galactic molecular clouds, was successfully able to reproduce the
observed number of positrons at giga-electronvolt energies. “We consider all
mechanisms via which cosmic rays interact with the molecular clouds to show
that nearby molecular clouds can be a viable contributor to the positron excess
phenomenon,” said Agnibha De Sarkar.
Not only the positron excess, the computer code
accurately reproduces the spectra of protons, antiprotons, boron, carbon, and
all other components of cosmic rays. “Our method explains all the observed
numbers without running into any contradiction,” said Agnibha De Sarkar,
comparing it with the currently available explanations invoking pulsars that
run into contradictions.
Nevertheless, the researchers considered simple
geometrical structures of the molecular clouds, whereas real molecular clouds
have complex geometries. They plan to address these shortcomings in their
future work. “Along with a more realistic environment inside the molecular
clouds, we plan to include more cosmic ray data from other satellites to
establish our idea beyond any doubt,” he pointed out.
Publication link: https://doi.org/10.1016/j.jheap.2020.11.001
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