Propagation of Secondary Antiprotons and Cosmic Rays in the Galaxy

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratios (e.g., B/C) produce too few antiprotons. In the present paper we discuss one possibility to overcome these difficulties. Using the measured antiproton flux and B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local “unprocessed” component at low energies perhaps associated with the Local Bubble, thus decreasing the measured secondary to primary nuclei ratio. The independent evidence for SN activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni (Z ≤ 28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three “radioactive clock” isotopes in CR, 10Be, 26Al, 36Cl, are all consistent and indicate a halo size z h ∼ 4 kpc based on the most accurate data taken by the ACE spacecraft. INTRODUCTION The result of our recent analysis (Moskalenko et al., 2001b, 2002), in agreement with calculations of other authors (Molnar and Simon, 2001), was that matching the secondary/primary nuclei ratio B/C using reacceleration models leads to values of the spatial diffusion coefficient apparently too large to produce the required p̄ flux, when the propagated nucleon spectra are tuned to match the local p and He flux measurements. Assuming the current heliospheric modulation models are approximately right, we have the following possibility (Moskalenko et al., 2003) to reconcile the B/C ratio with the measured flux of secondary p̄’s. The spectra of primary nuclei as measured in the heliosphere may contain a fresh local “unprocessed” component at low energies thus decreasing the measured secondary to primary nuclei ratio. This component would have to be local in the sense of being specific to the solar neighbourhood, so that the well-known “Local Bubble” phenomenon is a natural candidate. The low-density region around the Sun, filled with hot H I gas, is called the Local Bubble (hereafter LB) (?, e.g.,)]sfeir. The size of the region is about 200 pc, and it is likely that it was produced in a series of supernova (SN) explosions, with the last SN explosion occuring approximately 1–2 Myr ago, or 3 SN occuring within the last 5 Myr. Most probably its progenitor was an OB star association (?, e.g.,)]maiz,berghofer. Knie et al. (1999) report about an excess of 60Fe measured in a deep ocean core sample of ferromanganese crust suggesting a SN explosion about 5 Myr ago at 30 pc distance and the deposition of SN-produced iron on earth. Sonett et al. (1987) report an enhancement in the CR intensity dated about 40 kyr ago, which is interpreted as the passage across the solar system of the shock wave from a SN exploding about 0.1 Myr ago. It could also be that “fresh” LB contributions from continuous acceleration in the form of shock waves (Bykov and Fleishman, 1992), and/or energetic particles coming directly from SN remnants still influence the spectra and abundances of local CR.