Origin of cosmic rays: supernovae would be PeVatrons

As you can see from the previous Futura article below, it’s been a century since the noosphere discovered the existence of cosmic rays. This has made it possible to advance our knowledge of elementary particles and, in passing, to demonstrate the existence of antimatter before the particles exoticand the fleeting existence demonstrated in cosmic rays, are produced by collisions of particles at higher and higher energies.

The study of cosmic rays continues, already because some of the particles present have been accelerated to energies impossible to reach even with today’s LHCbut also because they provide information on phenomena astrophysics. The study of cosmic neutrinosfor example, can help us understand the active galactic nucleisupplied with energy by supermassive black holes in rotation accreting matter.

But there is a catch, as Futura already explained. Cosmic rays are overwhelmingly charged particles, which means that in magnetic fields turbulent inside the galaxies they are deflected by these fields and move there by carrying out a movement Brownian and therefore stochastic. Clearly, the direction from which a very energetic proton seems to come from on the celestial vault, creating a shower of secondary particles by colliding with a nucleus of the high atmospheremay have nothing to do with its place of origin on the same vault of heaven.

Fortunately, the astrophysicists are clever and they have equipped themselves with a tool and a strategy allowing them to trace the origin of some of these protons high energy in the Milky Way. They have just published an article on this subject, an open-access version of which can be found on arXiv.

Protons more than 100 times more energetic than at the LHC

This tool is the telescope gamma in the space of the Nasa baptized Fermi, in honor of the famous physicist Italian who proposed the first of the mechanisms of cosmic ray acceleration, mechanisms which one finds associated with the shock waves of the explosions of supernovae in the interstellar medium.

A few years ago, as explained in detail in the Futura article below, Fermi’s observations of supernova remnants had already confirmed the existence of the advanced mechanisms for cosmic protons, which are elsewhere the major component of cosmic rays, even if one can find positrons and nuclei.

Today, astrophysicists therefore explain that they have similarly put to use about 12 years of gamma flux measurements by Fermi concerning a remnant of supernova and that these measurements confirmed that at least this remainder was indeed a proton accelerator allowing to give them energies at least equal to the PeV, that is to say at least 100 times the energy of an accelerated proton in the LHC.

This supernova remnant, named G106.3+2.7, is therefore an authentic PeVatrons and it is in the constellation of Cepheus, a circumpolar constellation of thenorthern hemisphereat about 2,600 light years from Solar system. It contains in its heart a pulsar called J2229+6114 which we have every reason to think that like all the other pulsars, it is a neutron star left by the explosion of a star at the origin of the supernova remnant G106.3+2.7.

The researchers established the spectrum in gamma photon energy between 100 GeV and 100 TeV by studying the data collected by Fermi. This spectrum is not compatible with that of gamma photons which would be mainly produced by electrons high energies colliding with photons from the cosmic radiation by giving them part of its energy according to an inverse Compton effect (we know that pulsars are accelerators of electrons and positrons). If it were electrons, it would contradict the shape of the spectrum in the domain radio and X associated with G106.3+2.7.

As a few years ago, we therefore come to the conclusion that the gamma photons observed by Fermi come from the decay of mesons Neutral π, π mesons produced by collisions involving protons at energies up to and exceeding the PeV.

Origin of cosmic rays: Fermi confirms the track of supernovae

Article by Laurent Sacco, published on 02/18/2013

It has been assumed for decades: at least some of the cosmic rays come from proton acceleration mechanisms in supernova remnants. After years of observations in the field of gamma rays with the Fermi telescope, astrophysicists have just confirmed the existence of accelerated protons at large gears in two supernova remnants, IC 443 and W44.

In 1912, the Austrian physicist Victor Franz Hess discovered the existence of cosmic rays. Using experiments carried out in ballhe finds that the rate ofions present in the atmosphere increases with altitude whereas until then we imagined the opposite, since it is the Earth’s crust which houses the radioactive elements. These measurements at altitude therefore demonstrate that there is ionizing radiation coming from space and striking the upper layers of the atmosphere.

In the decades that followed, the study of cosmic rays made it possible to discover new elementary particles, such as pions and muonsbefore we built after the Second World War accelerators powerful enough to produce them directly in the laboratory.

There must be particle accelerators in space

The question of the origin of these rays naturally arose and, as early as 1949, the great physicist Enrico Fermi proposed mechanisms for the acceleration of charged particles in interstellar clouds magnetized. Subsequently, it was generally accepted that cosmic rays probably owe their existence to supernovae explosions and that Fermi mechanisms, collectively referred to as Fermi acceleration, must be at work in supernovae remnants. Basically, successive passages of charged particles through the front of the shock wave caused by the explosion of a supernova, due to Brownian movements, can sometimes lead to a clear acceleration for some of them.

Unfortunately, these hypotheses are difficult to test. Cosmic rays are made up of 90% protons, the rest being electrons and nuclei. They are subject to the effect of magnetic fields sometimes turbulent during their movements in the Milky Way, which has the effect of making their trajectories very complex, a bit like that, again, of a particle following a Brownian movement. It is therefore difficult to associate a precise source on the celestial vault with showers of secondary particles, produced by cosmic rays hitting nuclei in the upper atmosphere.

Two supernova remnants under Fermi’s gamma eye

A recently published article on arxiv by members of the Fermi collaboration, using the gamma telescope bearing the name of the great Italian physicist, has nevertheless just made a significant contribution to the elucidation of the enigma of the origin of cosmic rays. To do this, the researchers took advantage of the fact that gamma rays are not deflected by galactic magnetic fields. As a result of which they observed, over a period of 4 years, two supernova remnants, CI 443 and W44.

The shock waves associated with the explosions of the two supernovae that produced these remnants propagate in cold molecular clouds. As a result, gamma rays are emitted from these clouds, visibly bombarded by energetic particles from supernova remnants. But, problem, a priori, electrons and protons can both be responsible for these emissions gamma. If they are due to accelerated electrons then one should not seek in the supernova remnants natural proton accelerators, which constitute 90% of cosmic rays as we have said.

The pion gamma decay test

However, there is a way to decide between the hypotheses. If protons are indeed the cause of gamma emissions, part of their spectrum must be slightly different from that caused by electrons. The reason for this is that sufficiently energetic protons, upon collision with nuclei, produce neutral pions which decay into gamma photons, whereas very fast electrons emit these photons directly. The precise measurements carried out with Fermi ended up showing that the trace of the pions producing the gamma emissions was indeed there. The protons accelerated to very high speeds in the remains of supernovae are indeed responsible for the observed gamma radiation.

The thesis explaining the origin of at least a non-negligible part of cosmic rays by supernovae explosions therefore comes out very reinforced. The riddle is still not completely solved because there are cosmic rays at very high energies that cannot be explained by invoking supernova remnants. We tried to involve supermassive black holes at the heart of galaxies, but this explanation remains problematic to this day.