Cosmic-ray variations in the solar neighbourhood

Abstract

The γ radiation produced in cosmic-ray and gas interactions has been used to probe the local cosmic-ray spectrum in several clouds to distances of ∼1 kpc around the Sun. Early measurements made with Fermi LAT data showed consistent gas emissivities across this region and good agreement with the cosmic-ray spectra reaching the Solar System, given the 10-15% uncertainty in gauging the atomic gas mass pervaded by the cosmic rays. The local cosmic-ray uniformity has recently been challenged by the finding of a significant loss of particles in the nearby Eridu cirrus with respect to the 1-kpc-wide average. We now aim to compare the CR flux in Eridu with that near the Sun and in clouds at comparably short distances. We have therefore revisited the γ-ray emissivity spectrum one would expect per gas nucleon from hadronic and bremsstrahlung emission using the latest cosmic-ray spectra inferred near the Sun, outside the heliosphere. We have also used improved photon production spectra in nucleus-nucleus interactions. We find that, because of the current uncertainty in the γ-ray yield per nuclear interaction, one cannot conclude whether the average cosmic-ray spectrum measured within ∼1 kpc from the Sun is comparable or superior to that inferred just outside the Solar System, nor whether the low cosmic-ray flux pervading the Eridu cirrus is consistent or not with the flux reaching the Sun. The Eridu cirrus and the Sun both lie in a hollow valley of the local interstellar medium. We show that the cosmic-ray flux in Eridu is about half that observed in two other more massive clouds flanking the valley, 200-300 pc away from Eridu.

Rapporteur Talk: Cosmic Ray Direct

“…An important input for such studies are of course the -ray production cross-section which are known with less precision that one would hope for. In fact, comparing the predictions for the -ray emissivity for a fixed proton flux plus contributions from heavy nuclei and the inverse-Compton scattering of electrons, the predictions differ by 20 − 30 % between different hadronic models [65]. Yet, comparing the predicted -ray emissivities with measurements, none of the models can explain all the data.…”

ICRC 2021: Gamma-ray Direct Rapporteur

“…Nine nearby superluminous stars were observed in LAT data [42]. Cosmic-ray variations [43] Finally, the LAT and CTA are and will be respectively sensitive to protostellar jets [44] In terms of new physics, searches for evidence of antimatter annihilation in gamma-ray data place constraints on anti-stars [45].…”