Pulsar NP 0532: Variability of Dispersion and Scattering

Abstract

Observations of the Crab Nebula pulsar NP 0532 over a period of 22 months are analyzed for effects of interstellar scattering and dispersion. The two phenomena are distinguished on the basis of a simple model which considers the possible contributions of two scattering regions following Counselman and Rankin. Variations are observed in dispersion and in one of the scattering regions which are imperfectly correlated. The two regions exhibiting appreciable scattering can plausibly be identified as the general intervening galactic medium and the Crab Nebula itself, respectively. The variability, which occurs on a scale of months, is thus presumed to occur within the nebula. Scargle and Pacini's suggestion of an association between dispersion "events," "wisps" in the Crab Nebula, and timing irregularities ("glitches") of the pulsar is evaluated in the light of the observations. Subject headings: Crab Nebula - pulsars