Clues about the origin of cosmic rays in the Crab Nebula

Composite image of the Crab Nebula (Image NASA, ESA, G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; A. Loll et al.; T. Temim et al.; F. Seward et al.; VLA/NRAO/AUI/NSF; Chandra/CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; and Hubble/STScI)
Composite image of the Crab Nebula (Image NASA, ESA, G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; A. Loll et al.; T. Temim et al.; F. Seward et al.; VLA/NRAO/AUI/NSF; Chandra/CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; and Hubble/STScI)

An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes a research that revealed a surprise in the origin of electromagnetic radiation from the Crab Nebula that can influence the research on cosmic rays. Federico Fraschetti of the University of Arizona, USA, and Martin Pohl of the University of Potsdam, Germany, believe that the model created by Enrico Fermi in 1949 is to be partially revised because those radiation are produced in a way different from what was thought.

The Crab Nebula is what remains after the supernova sighted on Earth and recorded in 1054 and is about 6,500 light-years away from Earth. At its center there’s the Crab Pulsar, also known as PSR B0531+21 or PSR J0534+2200, which has a mass approximately 1.5 times the Sun’s concentrated in a volume of about 10 kilometers (a little more than 6 miles) in diameter.

The studies of the Crab Nebula have become more and more sophisticated since it was discovered, independently in 1731 by the English astronomer John Bevis and in 1758 by the French one Charles Messier. In May 2017 an article published in “The Astrophysical Journal” described a research in which that supernova remnant were photographed by five telescopes.

The possibility to combine the observations of several telescopes carried out in different parts of the electromagnetic spectrum allows for a more complete view of the processes at work within the Crab Nebula, which emits radiation throughout the spectrum. Those radiation come from cosmic rays, specifically very high-energy electrons, and astrophysicists can create detailed models to reproduce them.

According to Enrico Fermi’s model, once the electrons reach a shock boundary, they bounce back and forth many times because of the magnetic turbulence gaining energy. This model doesn’t include what happens when the particles reach their highest energy.

Federico Fraschetti explained that only when a different acceleration process is included in which the number of higher energy particles decreases faster than at lower energy it’s possible to explain the radiation seen in the whole electromagnetic spectrum.

In essence, the shock wave caused by the supernova is the source of the particle acceleration but there are other mechanisms that generate all those electromagnetic radiation. Similar processes occur as a result of plasma explosions on the surface of the Sun with the creation of shock waves and charged particles that are projected into space.

The origin of cosmic rays is a complex research topic and this research confirms that there are still many questions waiting for answers. More than a century after their discovery, the new clues gathered may perhaps indicate more precisely where to investigate to understand the processes that generate them.

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