Strong-Field Equivalence from the Pulsar Triple System: Self-Energy Coupling at Neutron-Star Compactness

In the hierarchical triple system PSR J0337+1715, a neutron star and a white dwarf orbit together and fall together in the field of an outer white dwarf. Their accelerations agree to within 2.60e-06 (Archibald et al. 2018) - the strongest strong-field equivalence test available, because the neutron star's gravitational self-energy fraction, of order 0.15, exceeds the Earth's by a factor 3.37e+08. Expressed as a coefficient of the self-energy difference, the bound is 1.73e-05, improving on the lunar Nordtvedt coefficient bound of 1.34e-03 by a factor of about 77 despite the looser raw acceleration agreement. In the frame, whose density bound is a strong-field statement, the pulsar result certifies that self-energy couples with its standard weight where gravity is genuinely strong. Computed from published results, not measured.