Searching for Astrophysical Sources of Cosmic Rays, Gamma-Rays and Neutrinos: Real-Time Multimessenger Programs and Theoretical Models

Dear Colleagues,

The first seed of multi-messenger astronomy dates back to 1987, when neutrinos from the supernova SN1987A were recorded a few hours before the light was detected by conventional telescopes. In the last few years, this field has been enormously boosted by two key discoveries: (1) The detection of gravitational waves from the merging of a binary system of neutron stars, in coincidence with the electromagnetic emission from the associated gamma-ray burst and the subsequent kilonova; (2) the detection of a high-energy neutrino in coincidence with a powerful active galactic nucleus (AGN) named TXS 0506+056, during a flare in the gamma-ray band.

These two observations changed and are changing the study approaches in high-energy astrophysics. The messengers of the high-energy non-thermal universe are not only energetic photons and cosmic rays, but also neutrinos and gravitational waves, each of them carrying information of at least one of the fundamental forces. Uncovering the high-energy universe, therefore, requires exploring it from very different perspectives, sharing these different views and trying to interpret the observational facts in a unique picture.

This Special Issue is focused on the connection between cosmic messengers and on the search for their sources. There is still no clear proof regarding the astrophysical objects that produce ultra-high energy cosmic rays (E>1018 eV), whose arrival directions are scrambled because of interactions with magnetic fields. The joint study of gamma-rays and neutrinos, not suffering from deflections, aims to solve this long-standing mystery, as demonstrated by the association between a high-energy neutrino and TXS0506+056. Among all the messengers, neutrinos play a special role: they arrive undeflected and unabsorbed to the Earth. Current neutrino telescopes, characterized by a large duty cycle, developed programs for sending out real-time alerts, whenever events with high astrophysical probability and good angular accuracy are recorded. These alerts promptly trigger observations and follow-ups from partner experiments. Strategies also exist for the follow-up of gravitational waves. The latter are, in turn, expected to be related not only to electromagnetic counterparts, but also to high-energy neutrinos. A common point for all existing programs is the need for real-time work and analysis. On the other hand, from the theoretical point of view, models explaining all the possible connections among these messengers are crucial for a meaningful interpretation of the observations.

This Special Issue will be divided into four main sections. The first section will be dedicated to a state-of-the-art understanding of high-energy cosmic rays, gamma-rays, and neutrinos. The second section will be focused on the existing infrastructures developed by current observatories for real-time multi-messengers programs. In addition to the experimental aspects, in the third section special attention is given to theoretical studies and modeling of the most plausible cosmic sources. Finally, the last chapter will give a panoramic view of the future of multi-messenger astronomy, in the light of the planned new UHECRs, neutrinos, and gamma-rays facilities.

Dr. Manuela Mallamaci
Prof. Elisa Bernardini
Guest Editors

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• non-thermal universe
• multi-messenger astronomy
• neutrino astronomy
• gamma-ray astronomy
• real-time multi-messenger work and analysis
• theoretical models