Unraveling the nature of coherent pulsar radio emission

Forty years have passed since the discovery of pulsars, yet the physical mechanism of their coherent radio emission is a mystery. Recent observational and theoretical studies strongly suggest that the radiation outcoming from the pulsar magnetosphere consists mainly of extraordinary waves polarized perpendicular to the planes of pulsar dipolar magnetic field. However, the fundamental question whether these waves are excited by maser or coherent curvature radiation, remains open. High quality single pulse polarimetry is required to distinguish between these two possible mechanisms. Here we showcase such {\it decisive} strong single pulses from 10 pulsars observed with the GMRT, showing extremely high linear polarization with the position angle following locally the mean position angle traverse. These pulses, which are relatively free from depolarization, must consist of exclusively single polarization mode. We associate this mode with the extraordinary wave excited by the coherent curvature radiation. This crucial observational signature enables us to argue, for the first time, in favor of the coherent curvature emission mechanism, excluding the maser mechanism.