Interstellar Scintillation of Pulsar B0809+74

Weak interstellar scintillations of pulsar B0809+74 were observed at twoepochs using a 30m EISCAT antenna at 933 MHz. These have been used to constrainthe spectrum, the distribution and the transverse velocity of the scatteringplasma with respect to the local standard of rest (LSR). The Kolmogorov powerlaw is a satisfactory model for the electron density spectrum at scales between20 megameters and 1 gigameter. We compare the observations with modelcalculations from weak scintillation theory and the known transverse velocitiesof the pulsar and the Earth. The simplest model is that the scattering isuniformly distributed along the 310 pc line of sight (l=140 deg, b=32 deg) andis stationary in the LSR. With the scattering measure as the only freeparameter, 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 foundtoward PSR B0950+08, and suggests a region of low local turbulence over as muchas 90 deg. in longitude including the galactic anti-center. If, on the otherhand, the scattering occurs in a compact region, the observed time scalesrequire a specific velocity-distance relation. In particular, enhancedscattering 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 enhancedscattering in this direction. The analysis also excludes scattering in thetermination shock of the solar wind or in a nebula associated with the pulsar.Comment: Accepted for ApJ, 20 page