Interstellar Scintillation od Pulsar B0809+74

Abstract

Weak interstellar scintillations of pulsar B0809+74 were observed at two epochs using a 30m EISCAT antenna at 933 MHz. These have been used to constrain the spectrum, the distribution and the transverse velocity of the scattering plasma with respect to the local standard of rest (LSR). The Kolmogorov power law is a satisfactory model for the electron density spectrum at scales between 20 megameters and 1 gigameter. We compare the observations with model calculations from weak scintillation theory and the known transverse velocities of the pulsar and the Earth. The simplest model is that the scattering is uniformly distributed along the 310 pc line of sight (l=140 deg, b=32 deg) and is stationary in the LSR. With the scattering measure as the only free parameter, this model fits the data within the errors and a range of about +/-10 km/s in velocity is also allowed. The integrated level of turbulence is low, being comparable to that found toward PSR B0950+08, and suggests a region of low local turbulence over as much as 90 deg. in longitude including the galactic anti-center. If, on the other hand, the scattering occurs in a compact region, the observed time scales require a specific velocity-distance relation. In particular, enhanced scattering in a shell at the edge of the local bubble, proposed by Bhat et al. (1998), near 72 pc toward the pulsar, must be moving at about ~ 17 km/s; however, the low scattering measure argues against a shell of enhanced scattering in this direction. The analysis also excludes scattering in the termination shock of the solar wind or in a nebula associated with the pulsar.