The interstellar cosmic-ray electron spectrum from synchrotron radiation and direct measurements

Abstract

Aims. We exploit synchrotron radiation to constrain the low-energy interstellar electron spectrum, using various radio surveys and connecting with electron data from Fermi-LAT and other experiments. Methods. The GALPROP programme for cosmic-ray propagation, gamma-ray and synchrotron radiation is used. Secondary electrons and positrons are included. Propagation models based on cosmic-ray and gamma-ray data are tested against synchrotron data from 22 MHz to 94 GHz. Results. The synchrotron data confirm the need for a low-energy break in the cosmic-ray electron injection spectrum. The interstellar spectrum below a few GeV has to be lower than standard models predict, and this suggests less solar modulation than usually assumed. Reacceleration models are more difficult to reconcile with the synchrotron constraints. We show that secondary leptons are important for the interpretation of synchrotron emission. We also consider a cosmic-ray propagation origin for the low-energy break. Conclusions. Exploiting the complementary information on cosmic rays and synchrotron gives unique and essential constraints on electrons, and has implications for gamma rays. This connection is especially relevant now in view of the ongoing Planck and Fermi missions.

Planck2015 results

“…The Galactic synchrotron spectrum curves significantly below a few GHz (e.g., de Oliveira-Costa et al 2008;Strong et al 2011). To generate a useful spectral constraint from just one low-frequency map, we cannot afford to fit this curvature independently at each pixel, as otherwise the low-frequency point would always fit perfectly and give no constraint on the highfrequency spectrum; instead we force the curvature to be constant across the sky, and we have tried several approaches to regularize the fit.…”

“…As expected, simple power-law fits are inconsistent with our data, generating large and spurious "gain corrections" at 408 MHz. Strong et al (2011) and Orlando & Strong (2013) model the observed synchrotron emission for given Galactic magnetic Fig. 15.…”

“…At lower energies, where solar modulation makes the local spectrum hard to determine, the lepton spectrum is based on synchrotron observations. In order to reproduce the curvature of the spectrum below a few GHz, Strong et al (2011) found that the injected electron spectrum (before propagation effects) should have a break at approximately 4 GeV and should be harder than p = 1.6 below 4 GeV. Accounting for all these constraints, the resulting injected spectral index used in those works are (N(E) ∝ E −p ) with p = 1.6/2.5/2.2, respectively, below/between/above the breaks at 4 and 50 GeV.…”

Modelling the cosmic ray electron propagation in M 51

“…Furthermore, advances in computing now enable us to solve the diffusion energy loss equation via approximation rather than analytical modelling. While analytical solutions for simple cases can give insight into the relations between the quantities involved and are good for rough estimates, they can become so complicated that no insight is gained (Strong et al 2011). On the other hand, numerical models are intuitive because they are able to generate the cosmic ray distribution over the galaxy for all species, the best example being the Galactic Propagation (GALPROP) code .…”

“…While our own Galaxy has been modelled extensively using the GALPROP code, including recent work using the observed synchrotron spectral index to constrain the parameters governing CRE propagation (Strong et al 2011), external galaxies have not been modelled with both diffusion and energy losses of CREs since Segalovitz (1977;M 51), Strong (1978;NGC 891), and Pohl & Schlickeiser (1990;NGC 4631). Additionally, these were analytical models.…”