The Most Likely Sources of High‐Energy Cosmic‐Ray Electrons in Supernova Remnants

Abstract

Evidence of nonthermal X-ray emission and TeV gamma rays from supernova remnants (SNRs) have strengthened the hypothesis that primary Galactic cosmic-ray electrons are accelerated in SNRs. High-energy electrons lose energy via synchrotron and inverse Compton processes during propagation in the Galaxy. Because of these radiative losses, TeV electrons liberated from SNRs at distances larger than $1 kpc, or times older than $10 5 yr, cannot reach the solar system. We investigated the cosmic-ray electron spectrum observed in the solar system using an analytical method and considered several candidate sources among nearby SNRs that may contribute to the high-energy electron flux. In particular, we discuss the effects for the release time from SNRs after the explosion, as well as the deviation of a source spectrum from a simple power law. From this calculation, we found that some nearby sources, such as Vela, Cygnus Loop, or Monogem, could leave unique signatures in the form of identifiable structure in the energy spectrum of TeV electrons and show anisotropies toward the sources, depending on when the electrons were liberated from the remnant. This suggests that, in addition to providing information on the mechanisms of acceleration and propagation of cosmic rays, specific cosmic-ray sources can be identified through the precise electron observation in the TeV region.

On possible interpretations of the high energy electron–positron spectrum measured by the Fermi Large Area Telescope

“…As Fig. 1 shows, a simple power-law fit of the Fermi-LAT electron energy spectrum is possible giving: J e ± = (175.40 ± 6.09) E 1 GeV −(3.045±0.008) GeV −1 m −2 s −1 sr −1 (1) with χ 2 = 9.7 (for 23 d.o.f.) where statistical and systematic (dominant) errors have been, conservatively, added in quadrature.…”

“…Prior to 2008, the high energy electron spectrum was measured by balloon-borne experiments (Kohayashi et al 2004 [1]) and by a single space mission AMS-01 (Aguilar et al 2002 [2]). Those data are compatible with a featureless power-law spectrum within their errors.…”

“…Statistical fluctuations in the injection spectrum and spatial distribution of those nearby sources may produce significant deviations in the most energetic part of the observed spectrum compared to the conventional homogeneous and steady state scenario (see e.g. [4], Pohl & Esposito 1998 [5], Strong & Moskalenko 2001 [6], Kobayashi et al 2004 [1]). …”

The Origin of Cosmic Ray Electrons and Positrons

“…1, Table 1) have been interpreted in a number of different ways. Atoyan et al (1995) considered the overall electron spectrum as the sum of a Galactic average and a single source component, and Kobayashi et al (2001) concluded that Vela (distance 250 pc, age 20 kyr) should have produced this component. However, Guetta & Amato (2003) emphasized that the high-energy photons coming from Vela are likely produced by hadronic processes instead of inverse Compton scattering.…”

“…Another possible source could be the Monogem pulsar ( Thorsett et al 2003), whose distance and age match the prediction of the model by Erlykin & Wolfendale (1997), who attributed the knee feature to a single nearby source. A few other nearby supernova remnants are considered by Kobayashi et al (2004). Pohl & Esposito (1998) considered the measured spectrum as the superposition of many ''single-source'' components, producing several spectral breaks and a time-dependent electron spectrum.…”