Identification of PSR1758 - 23 as a runaway pulsar from the supernova remnant W28

The discovery of the pulsar 1758 – 23 in the vicinity of the radio supernova remnant W28 has prompted speculation that the two might be physically associated. Although the brightening of the supernova remnant in the direction of the pulsar suggests such an association, the large column density of electrons in the interstellar medium towards PSR1758 – 23 (evident from the pronounced dispersion of the radio signal) and the anomalously large scattering observed in the pulses seem to indicate a distance much greater than that to W28. Here we present new observations which indicate that the large dispersion and scattering of the pulses is instead caused by a dense screen of ionized material located along the line of sight, and thus the distances to the pulsar and W28 can be reconciled. Similar scattering screens have been reported previously and appear to be associated with HII regions; the properties of these screens seem, however, to be inconsistent with current models of turbulent structure in the interstellar medium.