Deflection of pulsar signal reveals compact structures in the galaxy

We have detected a strong deflection of radio waves from the pulsar PSR B0834 + 06 in scintillation observations. Interference between the undeflected pulsar image and deflected subimages allows single-dish interferometry of the interstellar medium with submilliarcsecond resolution. We infer the presence of scattering structure(s) similar to those that are thought to cause extreme scattering events in quasar flux monitoring programs: size similar to 0.2 AU ( an angular size of 0.1 mas) with an electron overdensity of greater than or similar to 10(3) compared to the warm ionized medium. The deflectors are nearly stationary in a scattering screen that is thin ( less than or similar to 5% of the pulsar-observer distance in extent), is located 70% of the way from the Earth to the pulsar, and has been seen consistently in observations dating back 20 years. The pulsar scans the scattering screen at a velocity of 110 km s(-1) with a detection radius of 15 mas. Pulsar observations such as these - particularly with a new generation of low-frequency radio telescopes with large collecting areas - hold promise for improving constraints on the poorly understood physical characteristics and space density of the deflecting structures. Such observations may also prove useful in correcting deviations that the deflectors produce in high-precision timing of millisecond pulsars.