Gravitationally bound three-body systems have been studied for hundreds of years1,2 and are common in our Galaxy3,4. They show complex orbital interactions, which can constrain the compositions, masses and interior structures of the bodies5 and test theories of gravity6, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 (refs 7, 8; with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13), where is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15) and 0.4101(3)), as well as the inclinations of the orbits (both about 39.2°). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity.
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We thank D. Levitan and R. Simcoe for providing optical and infrared observations; J. Deneva for early Arecibo telescope observations; P. Bergeron for use of his white dwarf photometry models; K. O’Neil and F. Camilo for approving discretionary time observations on the GBT and the Arecibo telescope, respectively; J. Heyl, E. Algol, and P. Freire for discussions; and G. Kuper, J. Sluman, Y. Tang, G. Jozsa, and R. Smits for their help supporting the WSRT observations. The GBT and VLBA are operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Arecibo Observatory is operated by SRI International in alliance with Ana G. Méndez-Universidad Metropolitana and the Universities Space Research Association, under a cooperative agreement with the National Science Foundation. The WSRT is operated by the Netherlands Institute for Radio Astronomy (ASTRON). This paper made use of data from the WIYN Observatory at Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy under cooperative agreement with the National Science Foundation. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. I.H.S., V.M.K., M.H.v.K. and A.B. acknowledge support from NSERC. A.M.A. and J.W.T.H. acknowledge support from a Vrije Competitie grant from NWO. J.B., D.R.L, V.I.K. and M.A.M. were supported by a WV EPSCoR Research Challenge Grant. V.M.K. acknowledges support from CRAQ/FQRNT, CIFAR, the Canada Research Chairs Program and the Lorne Trottier Chair.