On the Orbital Decay of the PSR J0045−7319 Binary

Abstract

Recent observations of PSR J0045-7319, a radio pulsar in a close eccentric orbitwith a massive main-sequence B star companion, indicate that thesystem's orbital period is decreasing on a timescale ~5 x 105 yr.Timing observations of PSR J0045-7319 also indicate that the B staris rotating rapidly, perhaps close to its breakup rotation rate.For rapid (supersynchronous) prograde rotation of the B star, tidaldissipation leads to an increasing orbital period for the binary system,while for retrograde rotation of any magnitude, the orbital perioddecreases with time. We show that if tidal effects are to accountfor the observed orbital decay of the PSR J0045-7319 binary, the Bstar must have retrograde rotation. This implies that the supernovathat produced the pulsar in this binary system likely had a dipoleanisotropy. For a reasonably wide range of retrograde rotation rates, theenergy in the dynamical tide of the B star needs to be dissipated inabout one orbital period in order to account for the observed orbitalevolution time. We show, however, that the radiative dissipation of thedynamical tide in a rigidly rotating B star is too inefficient by afactor of ~103, regardless of the magnitude of the rotation rate.We describe how, when the surface of the B star is rotating nearlysynchronously, the energy in the dynamical tide is dissipated inless than an orbital period, thus reconciling the theoretical andobserved rates of orbital evolution. Nonlinear parametric decay of theequilibrium tide, for rigid retrograde rotation of the B star, may also beable to explain the observed rate of orbital evolution, although themargin of instability is too small to draw definitive conclusionsabout the relevance of this process for the PSR J0045-7319 binary.