Trompe L'Oeil 'binary' pulsars

Abstract

A freely precessing pulsarproduces pulse phase residuals which can mimic those of a pulsar in abinary orbit. In particular, discrete sets of phase residuals due toprecessional motion of an isolated pulsar are sampled; it is shownthat this data is well fit by residuals from a binary pulsar in asufficiently tight orbit. Analytic and numerical relationships between theprojected orbital size, a(p) sin i, and the orbital eccentricity, e, of amisidentified binary pulsar; are found the observations that woulddistinguish between these models are discussed. Regardless of themechanism that causes the precession, the maximum amplitude of the phaseresidual is pi/2: consequently, a(p)sin i is (approximately) bounded bycP(puls)/4. The newly discovered 'binary' millisecond pulsars in theglobular cluster 47 Tuc is discussed, and it is shown that the periodicfrequency modulation reported cannot be explained by free precession.