Trompe l'oeil "binary" pulsars

We show that a freely processing pulsar produces pulse phase residuals which can mimic those of a pulsar in a binary orbit. In particular, we sample discrete sets of phase residuals due to precessional motion of an isolated pulsar, and show that this data is well fit by residuals from a binary pulsar in a sufficiently tight orbit. We find analytic and numerical relationships between the projected orbital size, a_p sin i, and the orbital eccentricity, e, of a misidentified binary pulsar and discuss the observations that would distinguish between these models. Regardless of the mechanism that causes the precession, the maximum amplitude of the phase residual is π/2: consequently, a_p sin i is (approximately) bounded by cP_puls/4. We discuss the newly discovered "binary" millisecond pulsars in the globular cluster 47 Tuc and in the remnant of SN 1987A, and show that the periodic frequency modulation reported for these pulsars cannot be explained by free precession.