Those sources which have the power to emit the radio signals after fixed time intervals. The discovery of the PULSAR was not done by a single person. It was a team work. The discovery was completed in 1967 at Cambridge University of England.
The value of the regular interval after which the pulsars emit radio signals is 1.3372795secs. Pulsars are basically neutron stars having high densities. The speed of rotation and magnetic field of pulsars is very high. This is the main reason of the radio signals emission.