Cosmic-ray spectrum in the local Galaxy

¹ Astronomy Department, University of Geneva, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
e-mail: denys.malyshev@astro.uni-tuebingen.de
² Institut für Astronomie und Astrophysik Tübingen, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³ APC, Université Paris Diderot, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris, Sorbonne Paris Cité, 75014 Paris, France
⁴ National Research Nuclear University MEPHI (Moscow Engineering Physics Institute), 115409 Moscow, Russia

Received: 10 May 2017
Accepted: 10 August 2017

Abstract

Aims. We study the spectral properties of the cosmic-ray spectrum in the interstellar medium within 1 kpc distance from the Sun.

Methods. We used eight-year exposure data of molecular clouds of the Gould Belt obtained with the Fermi-LAT telescope to precisely measure the cosmic-ray spectrum at different locations in the local Galaxy. We compared this measurement with the direct measurements of the cosmic-ray flux in and around the solar system obtained by Voyager and AMS-02 or PAMELA.

Results. We find that the average cosmic-ray spectrum in the local Galaxy in the 1–100 GeV range is well described by a broken power-law in rigidity with a low-energy slope of 2.33^+0.06_-0.08 and a break at 18⁺⁷_-4 GV, with a slope change by 0.59 ± 0.11. This result is consistent with an earlier analysis of the γ-ray signal from the Gould Belt clouds based on a shorter exposure of Fermi-LAT and with a different event selection. The break at 10–20 GV is also consistent with the combined Voyager + AMS-02 measurements in/around the solar system. The slope of the spectrum below the break agrees with the slope of the average cosmic-ray spectrum in the inner part of the disk of the Milky Way that was previously derived from the Fermi-LAT γ-ray data. We conjecture that it is this slope of 2.33 and not the locally measured softer slope of 2.7–2.8 that is determined by the balance between a steady-state injection of cosmic rays with a power-law slope of 2–2.1 that is due to Fermi acceleration and the energy-dependent propagation of cosmic-ray particles through the turbulent interstellar magnetic field with a Kolmogorov turbulence spectrum. The approximation of a continuous-in-time injection of cosmic rays at a constant rate breaks down, which causes the softening of the spectrum at higher energies.