[Paper Review] Multi-wavelength constraints on cosmic-ray leptons in the Galaxy

E. Orlando, A. W. Strong+8 more|arXiv (Cornell University)|Jul 21, 2015

Astrophysics and Cosmic Phenomena7 references3 citations

TL;DR

This paper uses multi-wavelength observations—radio, microwave, and gamma-ray data—combined with the GALPROP cosmic ray propagation model to constrain the interstellar cosmic-ray lepton spectrum in the Milky Way. It demonstrates that synchrotron emission from radio to microwave frequencies, especially from WMAP and Planck maps, provides critical low-energy constraints on lepton spectra, breaking degeneracies in solar modulation and propagation models, and improves understanding of diffuse Galactic gamma-ray emission via inverse Compton and bremsstrahlung processes.

ABSTRACT

Cosmic rays (CRs) interact with the gas, the radiation field and the magnetic field in the Milky Way, producing diffuse emission from radio to gamma rays. Observations of this diffuse emission and comparison with detailed predictions are powerful tools to unveil the CR properties and to study CR propagation. We present various GALPROP CR propagation scenarios based on current CR measurements. The predicted synchrotron emission is compared to radio surveys, and synchrotron temperature maps from WMAP and Planck, while the predicted interstellar gamma-ray emission is compared to Fermi-LAT observations. We show how multi-wavelength observations of the Galactic diffuse emission can be used to help constrain the CR lepton spectrum and propagation. Finally we discuss how radio and microwave data could be used in understanding the diffuse Galactic gamma-ray emission observed with Fermi-LAT, especially at low energies.

Motivation & Objective

• To improve constraints on the interstellar cosmic-ray lepton spectrum at low energies (below a few GeV), where direct measurements are obscured by solar modulation.
• To reduce degeneracies in cosmic ray propagation models by combining radio, microwave, and gamma-ray observations with detailed propagation simulations.
• To assess the role of synchrotron emission from cosmic-ray leptons in the Galactic disk and halo, using high-latitude data to isolate the lepton contribution.
• To evaluate the impact of component separation uncertainties in microwave data (e.g., WMAP and Planck) on lepton spectrum inference, particularly from synchrotron, anomalous microwave emission (AME), and free-free components.
• To explore the potential of upcoming radio surveys (e.g., C-BASS, SKA) and low-energy Fermi-LAT data to further refine lepton spectrum constraints below 100 MeV.

Proposed method

• Utilizes the GALPROP code to simulate cosmic-ray propagation, including energy losses, diffusion, re-acceleration, and interactions with the interstellar medium and radiation field.
• Employs a 3D Galactic model with spatially resolved distributions of gas, magnetic fields, and interstellar radiation field (ISRF) to compute synchrotron and inverse Compton emission.
• Compares predicted synchrotron emission across radio and microwave frequencies (from 408 MHz to 23 GHz) with multi-frequency surveys, WMAP 9-year synchrotron temperature maps, and Planck component-separated maps.
• Uses Fermi-LAT diffuse gamma-ray data (above 100 MeV) to validate predictions of lepton-induced emission via inverse Compton and bremsstrahlung processes.
• Applies component separation techniques using external priors (e.g., from ancillary data) to isolate synchrotron emission from other foregrounds like AME and free-free emission in WMAP and Planck data.
• Incorporates PAMELA and LAT lepton measurements to anchor the high-energy end of the lepton spectrum and constrain the low-energy extrapolation via synchrotron modeling.

Experimental results

Research questions

• RQ1 To what extent can radio and microwave observations of diffuse Galactic synchrotron emission constrain the low-energy interstellar cosmic-ray lepton spectrum, especially below a few GeV?
• RQ2 How do uncertainties in component separation (e.g., synchrotron vs. AME vs. free-free) in WMAP and Planck data affect the inferred lepton spectrum and propagation model parameters?
• RQ3 Can multi-wavelength constraints from radio, microwave, and gamma-ray data jointly reduce degeneracies in cosmic ray propagation models and improve the accuracy of lepton spectrum reconstruction?
• RQ4 What is the contribution of secondary leptons to the observed diffuse gamma-ray emission, and how can it be disentangled from primary lepton emission using multi-wavelength data?
• RQ5 How do upcoming radio surveys (e.g., C-BASS, SKA) and low-energy Fermi-LAT data enhance constraints on the lepton spectrum below 100 MeV?

Key findings

• Synchrotron emission from high-latitude regions (|b| > 10°) shows a spectral index consistent with a lepton spectrum that turns over sharply below a few GeV, independent of propagation effects.
• The WMAP 9-year synchrotron temperature maps and Planck component-separated maps show a discrepancy in absolute intensity, suggesting possible overestimation in WMAP and underestimation in Planck synchrotron maps, likely due to component separation degeneracies.
• The Planck synchrotron map intensity is lower than WMAP's, while AME and free-free intensities are higher in Planck, indicating potential contamination or underestimation of synchrotron emission in Planck component separation.
• The inclusion of a 3.6 K offset in the 408 MHz map (as used in prior work) increases the low-frequency excess in diffusive re-acceleration models, highlighting the sensitivity of results to calibration and offset assumptions.
• Gamma-ray emission models below 100 MeV predict that inverse Compton and bremsstrahlung processes dominate, with leptonic origin, but these energy regimes remain poorly constrained due to lack of deep observations post-COMPTEL.
• Future data from C-BASS (3–15 GHz) and low-energy Fermi-LAT pass 8 data are expected to significantly improve component separation and provide tighter constraints on the low-energy lepton spectrum.