Pyrolysis of methane is a promising, new, greenhouse gas-free production method of hydrogen. Here, we present optical emission spectra of a microwave plasma torch operated in an argon methane mixture. Detailed spatial resolution is achieved by means of Abel inversion. The emission spectra are dominated by dicarbon Swan bands and black body radiation from carbon nanoparticles. Both spectral features are utilized to estimate the gas temperature. In the center of the plasma, gas temperatures of up to \SI{4300}{\kelvin} are reached with large gradients (\SI{500}{\kelvin\per\milli\meter}) in radial direction. The thermal equilibrium chemistry and the kinetics of methane pyrolysis are analyzed to explain the observed coupling between the local gas temperature and the local emission.