Orbital Variability in the Eclipsing Pulsar Binary PSR B1957+20

Abstract

We have conducted timing observations of the eclipsing millisecond binary pulsar PSR~B1957+20, extending the span of data on this pulsar to more than five years. During this time the orbital period of the system has varied by roughly $\hat{I}”P_b/P_b=1.6\times 10^{-7}$, changing quadratically with time and displaying an orbital period second derivative ${\ddot{P}}_b=(1.43\pm 0.08)\times 10^{-18}$s${}^{-1}$. The previous measurement of a large negative orbital period derivative reflected only the short-term behavior of the system during the early observations; the orbital period derivative is now positive and increasing rapidly. If, as we suspect, the PSR~B1957+20 system is undergoing quasi-cyclic orbital period variations similar to those found in other close binaries such as Algol and RS CVn, then the $0.025M{}_{\odot }$ companion to PSR~B1957+20 is most likely non-degenerate, convective, and magnetically active.

9 pages, 3 figures, LaTeX, submitted ApJL 13 Dec. 1993, arz-001