Highest energy nebula. Cosmic rays have been a window into high energy physics and astrophysical phenomena for almost a century. A Sino-Japanese detector, Tibet ASgamma, has just broken the energy record for cosmic gamma rays with a measured value of 450 TeV.
It was more than a century ago, in 1912 precisely, that Austrian physicist Victor Franz Hess discovered the existence of cosmic rays using balloon experiments. He observed that the rate of ions present in the atmosphere increases with altitude, whereas until then we had imagined the opposite, since it is the earth’s crust that houses the radioactive elements. These measurements at altitude thus demonstrated that there was ionizing radiation coming from space and striking the upper layers of the atmosphere.
The following decades will be marked by a growing interest in the study of these cosmic rays because they made it possible to discover, by studying their composition, new particles that could not initially be produced with accessible energies in the form of radioactive sources and the first particle accelerators. Thus the positron, the antiparticle of the electron, and the Yukawa pawn were discovered, giving a beginning of explanation for the nuclear forces sticking the nucleons in the nuclei.
The study of cosmic rays was also motivated by the information they were able to give on their sources, necessarily astrophysical, even cosmological. The great physicist Enrico Fermi, at the end of the Second World War, was very interested in these questions, at the root of modern high energy physics and the young discipline of astroparticles.
We seek in particular to precisely determine the nature of sources and processes accelerating the particles detected in cosmic rays, especially if they are very high energies. In recent years, this issue has become even more hot because it has been realized that cosmic rays of high energy could reflect the existence of the famous dark matter that remains elusive. But still it is necessary for that to identify much more ordinary sources, in order to clearly highlight and without possible contestation a component coming from the dark matter.
The task is difficult because part of these cosmic rays are charged particles. They will therefore undergo a Brownian motion in the turbulent magnetic fields of the interstellar medium so that it is no longer possible to determine an initial direction of emission. This lock jumps if one chooses to study gamma photons because they are not deviated by the interstellar magnetic fields, even intergalactic. We can thus try to verify the models that make black holes or neutron stars the sources of certain cosmic rays. You can also play this game with neutrinos that are also electrically neutral.
There is thus an instrument, the result of a collaboration between Chinese and Japanese researchers, named Tibet ASgamma which, as its name suggests, is on the Tibetan plateau and aims to detect gamma photons from beyond of the Solar System. As a Chinese video shows, hundreds of detectors are scattered over tens of thousands of square meters in Yangbajain, in the Autonomous Region of Tibet (southwest China). High energy astrophysicists and physicists, in charge of this experiment, have just reported via an article published in Physical Review Letters and available for free access on arXiv that he had broken a record as regards the measured energy of a gamma photon: 450 TeV, which equates to 45 billion times the energy of X-rays for a medical diagnosis.
This record fell during observation campaigns that lasted from February 2014 to May 2017, during which the detectors observed the gamma ray array ranging from 100 to 450 TeV and especially from a specific source on the vault heavenly: the Crab Nebula.
It is known that, about 6,500 light years from Earth, remains of a supernova that was observed in 1054 on Earth, in the constellation Taurus. At its heart is a neutron star, the famous crab pulsar, which has an intense magnetic field producing, via high energy electrons that circulate, synchrotron radiation and radio. In summary, this pulsar is a formidable particle accelerator producing radiation of all kinds. The fact that it is a gamma source is not a surprise and in broad outline we understand very well why. The relativistic electrons associated with this pulsar collide with the surrounding photons, in particular fossil radiation, according to a process called inverse Compton. The high energies of the electrons are then transferred to the photons that become gamma rays at the energies now highlighted on Earth with Tibet ASgamma, which confirms that cosmic rays are well associated with pulsars in the Milky Way.
The previous record for gamma photons was once held by the late Hegra experiment (High Energy Gamma Ray Astronomy) carried out in La Palma (Canary Islands), ie 75 TeV against 450 TeV today. This type of research will continue with even better instruments hoping to detect gamma sources of 1,000 TeV and more.