Constraining the Vela Pulsar's Radio Emission Region Using   Nyquist-Limited Scintillation Statistics
  Using a novel technique, we achieve ~100 picoarcsecond resolution and set an upper bound of less than 4 km for the characteristic size of the Vela pulsar's emission region. Specifically, we analyze flux-density statistics of the Vela pulsar at 760 MHz. Because the pulsar exhibits strong diffractive scintillation, these statistics convey information about the spatial extent of the radio emission region. We measure both a characteristic size of the emission region and the emission sizes for individual pulses. Our results imply that the radio emission altitude for the Vela pulsar at this frequency is less than 340 km. 
Michael D. Johnson, Carl R. Gwinn, and Paul Demorest
astro-ph.SR astro-ph.IM