Speaker
Description
From the detection by John Linsley (1963) of a cosmic ray of energy ∼ 1.0 × $10^{20}$ eV to the recent detection (2023) of the shower called Amaterasu by the Telescope Array experiment at ∼ 2.4 × $10^{20}$ eV (i.e. 40 joules!), our community has been striving to build the largest astroparticle observatories in the world to identify the sources of ultra-high-energy protons and nuclei. The Pierre Auger Observatory has made phenomenal progress in this area and has now collected nearly three thousand events with an energy per nucleus of more than 5 joules. These observations, combined with those from the Telescope Array on the one hand and information from the spectrum and composition of cosmic rays on the other hand, now provide evidence for excesses on angular scales of the order of 15-25°. The most recent analyses disfavor the hypothesis of an isotropic arrival-direction distribution with a confidence level approaching 5 sigma. In this talk, we propose to explain the spectrum, composition and evolution of the ultra-high-energy sky by a population of transient extragalactic sources of stellar size, a hypothesis well justified by the Hillas-Lovelace-Waxman-Blandford criterion and by the low helium content observed in the cosmic rays. We show that the ultra-high-energy sky in the flux suppression region can be reproduced from the three-dimensional distribution of star formation rates obtained from extensive optical and infrared surveys on scales from 10 kpc to several Gpc. Two conditions are necessary and sufficient for this agreement: a) the presence of a magnetic field of a few nG in the structure of the cosmic web that embeds us: the Local Sheet, b) that only one massive star in ten to a hundred thousands of newly formed stars ends its life by accelerating nuclei to ultra-high energy. We discuss the relevance of these two conditions in the context of the emerging fields of cosmic magnetism, especially in the radio bands, and time-domain astronomy, especially in X-rays. Only one class of stellar-sized transient sources satisfies condition b) and has a kinetic energy ejecta capable of producing the observed cosmic-ray flux: long-duration gamma-ray bursts. As the 5sigma detection of excesses on intermediate angular scales gets within reach, could the ultra-high-energy community be on the verge of solving a 60-year-old puzzle?
