Speaker
Description
The highest-energy cosmic rays (CRs) with energy above $10^{20}$ eV$=100$ EeV are one of the most mysterious particles in the Universe. Recently, Telescope Array Collaboration (2024) detected the second highest energy CR in history, $244\pm29{\rm (stat.)}~^{+51}_{-76}{\rm (sys.)}$~EeV, which is named as `Amaterasu.' Unger and Farrar (2024) reported no existence of candidates among the radio galaxies by cross-matching the astronomical source catalog and backtrack analysis of Amaterasu. They pointed out the most straightforward possibility that Amaterasu was accelerated in an astrophysical transient event in undistinguished galaxies. Moreover, the $\sim120^{+110}_{-60}$ PeV muons event, KM3-230213A, recently discovered by KM3Net Collaboration, implying the arrival of $35$-$380$ PeV cosmic neutrino (KM3Net Collaboration 2025). This event is thought to imply the existence of CR sources other than those responsible for lower-energy neutrinos because the estimated energy flux exceeds the flux in the lower-energy range observed by IceCube Collaboration. To explain the above observations, we need to consider the possibilities of CR acceleration processes in a broad sense, however challenging they may be.
Transient phenomena in magnetars have been considered as possible acceleration sites of CRs. However, the CR acceleration and the trigger mechanism of magnetar transients are still unclear. In particular, the ion injection mechanism is the most significant problem. Wada and Shimoda (2024) recently suggested a scenario for the activity that the magnetar's rotation axis suddenly flips due to the Dzhanibekov effect, resulting in a sudden rise of the Euler force. The material in the outer layer plastically flows due to the force and finally fractures in this scenario. We study the possibilities of ion acceleration along with this scenario and find that the electron stream from the fractured region possibly induces a strong electric field for a moment. The Fe-ions from the same fractured region can be accelerated by this electric field up to $\sim1$ ZeV within a timescale of $\sim1$ ps.
The nuclear spallation reactions limit the acceleration timescale, and therefore, high energy CR `neutrons' ($\sim$20 EeV) from the parenteral nuclei become proper observational predictions of this scenario: their arrival time and direction will be correlated with the bursting photon emissions of the host magnetars. The nuclear spallation of $\sim$ ZeV nuclei is preferred to explain $\sim100$ PeV neutrino events observed by IceCube and KM3Net.
In this presentation, we will review the novel scenarios of magnetar transients, iron-ion acceleration, and proper observational predictions of our model in the sense of multimessenger astrophysics.
