Searching for Anisotropy in Electron+Positron Cosmic Rays with CALET

Abstract

The ISS-based Calorimetric Electron Telescope (CALET) is directly measuring the energy spectrum and direction distribution of electron+positron cosmic-rays up to 20 TeV. A main goal of CALET is to identify a signature of a nearby supernova remnant (SNR) in electron+positron cosmic-rays. The Vela SNR has the highest potential to cause a spectral feature in the TeV region and/or a detectable anisotropy. Using the numerical cosmic-ray propagation code DRAGON, the spectrum and expected anisotropy of the Vela SNR together with background from more distant SNR was calculated depending on injection and propagation conditions. The results of these calculations were used to simulate CALET event sky-maps on which several analysis methods were employed to estimate the CALET sensitivity. Assuming that there is no anisotropy, the expected limits on the dipole amplitude from an all-sky search were calculated as a function of the selected energy range and the shape of the predicted spectra. However for the detection of a dipole anisotropy, the direction towards Vela is predetermined, and sensitivity is strongly boosted by a directed search. It is shown that with this method, CALET has a significant probability to identify an anisotropy signature from Vela. As it may disturb the Vela signature, the contribution to the local cosmic-ray anisotropy from several other nearby SNR and pulsars, as well as from the general source distribution in the galaxy was studied. It was found that Vela is expected to dominate and have a detectable signature, though there is some influence from other sources on direction and strength of the anisotropy. Furthermore, the implications of detecting an dipole anisotropy directed towards Vela for the local propagation parameters, such as the diffusion coefficient, are explained.