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Gamma halo of pulsars clarified the nature of excess antimatter

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The Fermi Large Area Telescope has discovered extensive sources of high-energy radiation around the nearby pulsars of Geminga and Monogem. This halo should be sources not only of photons but also of massive particles, such as electrons and their antiparticles, positrons. The analysis shows that these objects are responsible for a substantial part of the recorded on Earth positron flux, and the total contribution of all the pulsars may be responsible for has not yet received a definitive explanation for the excess of these particles, write the authors in the Journal of the Physical Review, D

Pulsars are neutron stars, the emission of hot spots on the surface of which periodically hits the Earth, due to which the stream from them experiences regular oscillations (pulsations). Like all neutron stars, such bodies are characterized by extreme densities and magnetic fields, as a result of which they turn out to be sources of electromagnetic radiation in a wide range of wavelengths, as well as significant flows of charged particles of different energies that can be observed on Earth in the form of cosmic rays.

In recent years, several cosmic ray detectors, such as AMS onboard the International Space Station and PAMELA on the Resource-DK1 satellite, have recorded an excess of high-energy positrons (above a couple of tens of gigaelectronvolts), that is, an excess of their number relative to theoretical expectations. Astrophysicists immediately hypothesized that this could be due to the influence of nearby pulsars, but there was no convincing evidence. Also, supernova remnants and the annihilation of dark matter particles were considered as causes.

Another indication of the involvement of the pulsars was obtained by the Milagro and HAWK ground-based cosmic ray detectors. These devices detect gamma-ray electromagnetic waves that can appear in the vicinity of pulsars due to Compton backscattering of background photons (relict and infrared radiation, as well as starlight) on high-energy electrons and positrons accelerated by the fields of a neutron star. Nevertheless, the HAWK collaborators came to the conclusion that the contribution of pulsars to radiation in this range is small, although the work was based on a number of assumptions.

Astrophysicists from Italy and the USA, with the participation of Mattia Di Mauro from the NASA Goddard Space Flight Center, processed the entire gamma-ray data array of the nearby pulsars Geminga and Monogem (B0656 + 14) and were able to distinguish a weak extended halo around objects. This work is difficult, since these sources, although located relatively close to the Earth, lie in the plane of the Galaxy, which is why besides them a large amount of noise is recorded. In this regard, the authors used ten different models of interstellar radiation, but in all cases, they were able to confidently distinguish the halo of pulsars.

The halo found are slightly elongated in shape, and their size is very large and reaches about 20 degrees in the sky, which is many times larger than the apparent diameter of the Moon and is comparable to the size of the Big Dipper bucket – the main asterism of the constellation. According to the authors, such a large size is due to the fact that electrons and positrons of not the highest energies manage to fly away at considerable distances from the pulsars before they experience scattering by surrounding photons.

One of the key features of the analysis was the accounting of the pulsars’ own motion. In particular, Geminga moves in the plane of the sky at a speed of 178 microseconds of arc per year, which corresponds to a linear speed of 211 kilometres per second, and the radial velocity is negligible. As a result, on a scale of millions of years, during which emitted electrons and positrons will spend energy on the production of gamma rays, the source manages to cover a distance of tens of parsecs, which significantly affects the shape of the observed halo.

In addition to this, the authors constructed a two-component model of particle motion, which took into account different environmental conditions in the plerion (the nebula immediately surrounding the pulsar) and more distant interstellar space. Such a model is devoid of a number of shortcomings in the previous analysis of HAWK scientists, but it also agrees well with the data they obtained.

As a result, based on the brightness of the halo in the gamma range, astrophysicists were able to estimate the number of electrons and positrons emitted by the pulsars. It turned out that only one Geminga pulsar can account for up to 20 per cent of the recorded excess of positrons when converting one per cent of the energy of charged particles into gamma rays. A generalization taking into account the known concentration of pulsars in this part of the Milky Way shows that they remain the most likely explanation for the excess of positrons and may be responsible for the entire effect under discussion.

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