Is There a Planet in the PSR 1620-26 Triple System?

Abstract

The unusually large eccentricity ($e_1=0.025$) of the low-mass binary millisecond pulsar PSR B1620-26 can be explained naturally as arising from the secular perturbation of a second, more distant companion. Such a triple configuration has been proposed recently as the most likely cause of the anomalous second period derivative of the pulsar. The current timing data are consistent with a second companion mass $m_2$ as low as $\sim10^{-3}\,M_\odot$, i.e., comparable to that of Jupiter. However, {\em if\/} the eccentricity is indeed produced by secular perturbations, then the second companion must be another star, most likely of mass m_2\lo1M_\odot and in a very eccentric (e_2\go0.5) orbit of period $P_2\sim10^2$--$10^3\,$yr. A second companion of planetary mass cannot induce the observed eccentricity. Independent of the mass of the second companion, small changes in the binary pulsar's orbit should become detectable with just a few additional years of timing data. This detection would provide direct confirmation of the triple nature of the system, and an accurate measurement of the effects would place important new constraints on the orbital parameters.Comment: 11 pages, uuencoded compressed postscript includes figures, IAS-AST-94-209