DISCONTINUITIES IN THE DYNAMIC SPECTRUM OF THE PULSAR 0823+26

The 1400 MHz dynamic spectrum of the pulsar 0823 + 26 exhibits sharp discontinuities which are characterized by changes in the diffractive scintillation pattern over transition times of less-than-or-similar-to 2 minutes. The discontinuities occur during periods in which the pulsar's broad-band (40 MHz) light curve is undergoing strong oscillations with periods of approximately 15 minutes. We believe these effects are due to coherent interference between multiple images created by strong refraction in the interstellar medium. In effect, we observe the discontinuities in the dynamic spectrum when the scintillation patterns of different images (each traveling different optical paths) alternately dominate the received intensity. The light curve oscillations are caused by coherent interference between the various images over bandwidths greater than the observing bandwidth. Based on the time scale for the oscillations at 1400 MHz, we derive a characteristic angular offset between images of approximately 0.5 mas. We cannot determine if the strong refraction is a consequence of the statistical nature of the medium along this line-of-sight, or whether a discrete refractor (i.e., an interstellar lens) is responsible. If the refractor is a discrete object, its dimension transverse to the line-of-sight is greater-than-or-similar-to 0.19f AU and its electron column density is greater-than-or-equal-to 1.2 x 10(-4)f cm - 3 pc, where f is the fractional distance of the refractor along the line-of-sight to the pulsar. Its electron number density is approximately 123/eta cm-3, where eta is the elongation of the refractor along the line-of-sight relative to its transverse dimension. The pulsar 0823 + 26 is a promising candidate for interstellar interferometry since it apparently undergoes extended periods of multiple imaging.