Interstellar interferometry of the pulsar PSR 1237+25

An episode of exceptionally prominent double imaging of the pulsar PSR 1237+25 caused by refraction in the interstellar medium has been detected with the Arecibo telescope at 430 MHz and used to resolve the pulsar magnetosphere for the first time. Multiple imaging causes quasi-periodic modulation of the pulsar intensity in time and frequency that represents the fringe pattern of an "interstellar interferometer" with a baseline of the order of 1 AU. From the detected fringe phase shift across the pulse profile of PSR 1237+25, the authors have estimated a typical transverse separation between the emitting regions to be ≡108cm. This estimate depends on the assumed distance to the refracting region and may be much smaller. It is shown that the observed fringe shifts and their dependence on pulse phase are inconsistent with a simple magnetic dipole model of pulsar emission.