[Paper Review] Comment on AMS02 results support the secondary origin of cosmic ray positrons

S. Dado, Arnon Dar|arXiv (Cornell University)|Apr 13, 2015

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper challenges the claim that secondary cosmic ray positrons produced in the interstellar medium (ISM) can explain the AMS-02 positron flux. Using a simplified model based on priors and including energy loss effects, it shows that secondary production in the ISM alone fails to match observations, but when combined with production inside cosmic ray sources, the total flux agrees well with the measured data, supporting a source-origin for the excess positrons.

ABSTRACT

We present a simple calculation of the flux of secondary positrons produced in the ISM that is based only on priors. Our calculated ISM flux agrees very well with that calculated with the elaborate GALPROP code. It confirms that secondary production of positrons in the ISM by the primary cosmic rays cannot explain the observed sub-TeV flux of CR positrons. Moreover, we show that once energy loss of positrons in source and in the ISM are included, secondary production inside the CR sources plus the ISM does explain the measured near-Earth flux of cosmic ray positrons.

Motivation & Objective

• To test the claim that secondary positrons from cosmic ray interactions in the ISM can explain the high-energy positron flux measured by AMS-02.
• To assess the validity of the upper limit on ISM-produced positrons proposed by Blum, Katz, and Waxman.
• To evaluate whether secondary positron production inside cosmic ray sources, combined with ISM production, can account for the observed flux.
• To incorporate energy loss effects of positrons in both the ISM and sources into the flux calculation.

Proposed method

• A simple analytical model is developed based on priors, using the primary cosmic ray nucleon flux with spectral index β = 2.7.
• The model calculates the secondary positron flux in the ISM using the formula φe⁺(E) ≤ fe⁺ σin(pp) nism c τ φn / (β - 1), where fe⁺ ≈ 7×10⁻³.
• Energy loss timescales are computed using τ = τesc τrad / (τesc + τrad), with τrad ≈ 10¹⁶ (E/GeV)⁻¹ s from inverse Compton and synchrotron losses.
• The escape time τesc ≈ 10¹⁵ (E/GeV)⁻¹/³ s is derived from diffusion in Kolmogorov-type magnetic fields.
• The total flux is recalculated by replacing the ISM column density with the total grammage X = 8.7 (E/10 GeV)⁻⁰.⁴ g/cm², accounting for source and ISM contributions.
• The resulting flux from Eq. (3) is compared to AMS-02 data, showing good agreement when source and ISM production are both included.

Experimental results

Research questions

• RQ1 Can secondary positron production in the ISM alone explain the observed AMS-02 positron flux above 10 GeV?
• RQ2 Does the inclusion of positron energy losses in the ISM and in cosmic ray sources alter the conclusion about the origin of the positron excess?
• RQ3 Is the upper limit on ISM-produced positrons, as claimed by Blum, Katz, and Waxman, robust when energy losses are considered?
• RQ4 Can the total observed positron flux be explained by secondary production occurring both inside cosmic ray sources and in the ISM?
• RQ5 What is the required total grammage traversed by cosmic ray nucleons to reproduce the measured positron flux?

Key findings

• The secondary positron flux produced solely in the ISM falls significantly short of the AMS-02 measurements, both in magnitude and spectral shape.
• The inclusion of positron energy losses—especially radiative losses via inverse Compton scattering and synchrotron emission—reduces the effective ISM production flux below the observed level.
• When secondary production is considered both inside cosmic ray sources and in the ISM, the total predicted flux matches the AMS-02 data very well.
• The total grammage X = 8.7 (E/10 GeV)⁻⁰.⁴ g/cm², derived from the B/C ratio, provides a consistent and accurate estimate for the total column density traversed.
• The calculated flux from Eq. (3), incorporating source and ISM contributions, shows excellent agreement with the measured positron flux, supporting a secondary origin from cosmic ray sources.
• The study concludes that secondary positrons produced in cosmic ray sources, combined with ISM production and energy loss effects, can explain the observed positron excess.