Spectral index of synchrotron emission: insights from the diffuse and magnetised interstellar medium

Vol. 651
6. Interstellar and circumstellar matter

Spectral index of synchrotron emission: insights from the diffuse and magnetised interstellar medium

The authors model the spectral index degree of polarization correlations of Galactic synchrotron radiation using different model cosmic ray spectra and magnetic field configurations from a fully 3D magnetohydrodynamic simulation. They find a substantial decrease in polarization from the theoretical maximum of about 70% arising from the tangled field structures along the line of sight and show the correlations with the mean transverse field strengths. Although not included here, the effect will clearly be exacerbated by Faraday rotation and ionization fluctuations along the diffuse line of sight. From the spectral index and polarization distributions, they provide an estimator for the transverse (hence total) field strength for a range of instruments and angular resolutions. The paper has broad applications since, with an input turbulence spectrum (e.g., Kolmogorov or Goldreich-Sridhar) structure (in this case limited to a 50 pc region), the same technique could be used for resolved extragalactic interstellar fields.