Cosmic rays, we begin to shed light on the mystery

Cosmic rays, we begin to shed light on the mystery

Cosmic rays

The Large Hadron Collider - or Lhc - of CERN is one of the most ambitious enterprises in the field of particle physics. With nearly five billion dollars, scientists have been able to build a ring of superconducting magnets cooled to temperatures lower than those found in space, to be used to accelerate subatomic particles to speeds close to those of light.

But nature is able to do the same job even more efficiently. For over a century, physicists have been amazed at the existence of cosmic rays, charged particles - mostly protons - from outer space that bombard the Earth, thousands per square meter every second. Cosmic rays can reach our planet at a speed greater than one petaelectronvolt, or Pev, of energy (that is, one trillion electronvolts, a hundred times faster than the speed that can be obtained with Lhc). Despite the large availability of cosmic rays to study, scientists still do not have a precise idea of ​​what can push particles to such extreme speeds.

The Cosmic Ray Factory

All ' early August a new article published in Physical Review Letters has begun to shed light on this mystery. By combining data from NASA's Fermi gamma-ray space telescope with observations from nine other experiments, a team of five scientists has definitively identified a source of Pev protons in the remnants of a supernova. The discovery of these cosmic ray "factories" - the most powerful accelerators in the galaxy, called Pevatron by the scientists who study them - will help characterize the environmental conditions that drive these particles and the role they play in the evolution of the cosmos.

"The identification of the Pevatrons is a first step towards understanding the energy of the universe," says Ke Fang, astrophysicist at the University of Wisconsin-Madison, who leads the discovery. So far, only a couple of potential Pevatrons have been identified in the Milky Way: the supermassive black hole at the center of our galaxy and a star-forming region on its periphery. In theory, supernova remnants - the gas and dust left behind by the explosion that leads to the "death" of stars - should also be able to generate Pev protons, Fang argues. But so far there was no evidence to support the theory.

"When massive stars explode, they produce these shock waves that propagate in the interstellar medium," explains Matthew Kerr, physicist at the US Naval research laboratory and co-author of the study. Protons are hypothesized to become trapped in the magnetic field of supernova remnants, spinning in close proximity to the shockwaves and receiving a "push" with each turn - "almost as if they were surfing," notes Kerr - until they gain enough energy to run away.

Although many Pev protons fall back to Earth, scientists have no way of determining which direction, let alone which source these particles come from. This is because cosmic rays zigzag through the universe, bouncing off matter like ping pong balls and spinning through magnetic fields, a path that makes it impossible to trace their origins. However, in the case of this supernova remnant, scientists have noticed the bright glow of gamma rays which, unlike charged particles, travel in a straight line from their place of origin to Earth. This was an important clue: if Pev protons were present, it was possible that they interacted with the interstellar gas and produced unstable particles called pions, which rapidly decay into gamma rays, with wavelengths too small to be seen by the human eye. .

The study

The gamma rays coming from this supernova remnant have been observed by telescopes since 2007. Until 2020, however, no light source with an energy had been detected unusual, when it was then captured by the Hawc Observatory, in Mexico, arousing the interest of scientists hunting for galactic Pevatrons. When gamma rays reach our atmosphere, they are capable of producing showers of charged particles that can be measured by ground-based telescopes. Thanks to the Hawc data, the scientists were able to go back and determine that these rains come from the gamma rays emitted by the remnants of the supernova. However, it had not been possible to establish whether the light was generated by protons or by electrons, which can themselves radiate gamma rays, as well as X-rays and lower-energy radio waves.

To demonstrate the presence of Pev protons, Fang's research team has collected data on a wide range of energies and wavelengths, collected from ten different observers over the past decade. Subsequently, the researchers relied on computer simulations. By changing different values, such as the strength of the magnetic field or the density of the gas cloud, they tried to reproduce the conditions necessary to explain all the different wavelengths of the observed light, determining that electrons could not be the only source. The simulations matched the data relating to the maximum energy levels only by including the protons Pev as an additional source of light.

"We were able to exclude that this emission was mainly produced by electrons since the spectrum obtained did not correspond observations, "explains Henrike Fleischhack, an astronomer at the Catholic University of America who first attempted this analysis two years ago with the Hawc dataset alone. Multi-wavelength analysis was crucial, explains Fleischhack, because it showed, for example, that increasing the number of electrons at a wavelength led to a mismatch between the data and the simulation at a wavelength. 'other wavelength, which is to say that the only way to explain the entire spectrum of light was the presence of Pev protons.

"The result required great attention to the energy balance - says David Saltzberg, astrophysicist at the University of California at Los Angeles, who did not participate in the study -. The research shows that many experiments and many observers are needed. to answer the most complex questions ".

The next steps

Looking ahead, Fang hopes that more Pevatrons will be found in the supernova remnants, which would help to understand if the researchers' discovery represents a isolated case or if all stellar bodies have the ability to accelerate particles to such speeds: "It could be the tip of the iceberg", highlights astrophysics. Emerging instruments such as the Cherenkov Telescope Array, the largest and most sensitive gamma-ray telescope complex on the planet, may be able to locate Pevatrons even outside our own galaxy.

Saltzberg also believes that next-generation experiments will be able to see neutrinos (tiny neutral particles that can result from pion decay) from supernova remnants. Detecting them with the IceCube Neutrino Observatory, which looks for traces of them at the South Pole, would be even more overwhelming evidence of the presence of Pevatron, since it would indicate the presence of pions.

Ultimately, finding the Pevatrons in the our universe is fundamental to understanding how the remains generated by the death of stars pave the way for the birth of new ones, and how higher-energy particles help to fuel this cosmic cycle. Cosmic rays affect pressure and temperature, guide galactic winds and ionize molecules in regions suitable for star birth, such as the remnants of supernovae. Some of these stars may go on to form their own planets or one day explode and form themselves into supernovae, starting the process again. "Studying cosmic rays is almost as important for understanding the origins of life as studying exoplanets - Kerr says -. It is a very complicated energy system. And we are only beginning to understand it now".

This article originally appeared on US.

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