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J.M. Weisberg (Carleton College Physics & Astron. Dept.), J.H. Taylor (Princeton U. Physics Dept.)
General relativistic spin-orbit coupling leads to ``geodetic'' precession of the spin axis of binary pulsar B1913+16. This precession will cause the observed pulse profile to evolve as our line of sight cuts across different parts of the emission beam. Weisberg, Romani, and Taylor (1989, APJ 347, 1030) found that the {\it{intensity ratio}} of the two principal profile components changed in a secular fashion from 1981 to 1987, but that the expected change in their {\it{separation}} did not occur. They suggested that the latter null observation resulted from our line of sight fortuitously passing across the {\it{middle}} of a hollow cone of emission. Kramer (1998, APJ 509, 856) found that the intensity ratio continued its secular change for another decade, and also found that the conal component separation was narrowing.
We have measured the pulse profile at 21 cm for almost twenty years. We confirm that the profile is narrowing as precession finally carries our line of sight away from the beam axis. Our two-dimensional emission beam map shows that the beam is elongated in latitude. Our modelling indicates that the elongation grows with distance from the magnetic axis.
This research was supported by the National Science Foundation.