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Abstract

For synchrotron radiation from relativistic electrons having a power-law energy distribution with a power-law index $p$ in an optically thin medium with a locally uniform magnetic field, it is generally adopted that the spectral index $\alpha = (p-1)/2$ and the degree of linear polarisation $\Pi_{\rm L,pl} = (p +1)/(p+7/3)$, and hence that $\Pi_{\rm L,pl} = (\alpha +1)/(\alpha+5/3)$. These ($\alpha$, $\Pi_{\rm L,pl}$; $p$) relations are derived assuming that the electrons have an isotropic momentum distribution and a power-law energy distribution, and they have been used, almost universally, in the interpretation of polarimetry observations. In this study, we assess the validity of the ($\alpha$, $\Pi_{\rm L,pl}$; $p$) relations in different scenarios, such as anisotropic electron momenta and non-power-law distributions of electron energy. We calculate the synchrotron radiation and polarisation spectra for power-law, kappa and log-parabola electron-energy distributions, with isotropic and two anisotropic (beamed and loss cone) electron-momentum distributions. Our calculations show that when the electron momenta are isotropic, the usual ($\alpha$, $\Pi_{\rm L,pl}$; $p$) relations are generally applicable. However, if the electrons are anisotropic, the usual ($\alpha$, $\Pi_{\rm L,pl}$; $p$) relations could break down. Applying the usual ($\alpha$, $\Pi_{\rm L,pl}$; $p$) relations indiscriminately without caution on anisotropy in the electron momentum distribution, would lead to incorrect interpretations of polarimetry data.