Diffractive interstellar scintillation timescales and velocities

We derive general relationships between the observed timescale of diffracti ve interstellar scintillations and the physical velocities of the observer, the source, and the scattering medium. Our treatment applies exclusively t o saturated scintillations of point sources in the strong scattering regime . We show how scintillation observations may be combined with other observa bles (groper motion and dispersion measure) to yield (1) improvements in ga lactic models for the free-electron density and (2) estimates of the distan ce and transverse velocity of individual pulsars. We explicitly consider ca ses of current astrophysical interest, including hypervelocity pulsars too far above the Galactic plane to allow distance estimates from dispersion me asures alone. We also briefly consider scintillations of extragalactic sour ces, including gamma-ray burst sources at great distances from the Galaxy.