Multimessenger tests of the weak equivalence principle from GW170817 and its electromagnetic counterparts

The coincident detection of a gravitational-wave (GW) event GW170817 with electromagnetic (EM) signals (e.g., a short gamma-ray burst SGRB 170817A or a macronova) from a binary neutron star merger within the nearby galaxy NGC 4933 provides a new, multimessenger test of the weak equivalence principle (WEP), extending the WEP test with GWs and photons. Assuming that the arrival time delay between the GW signals from GW170817 and the photons from SGRB 170817A or the macronova is mainly attributed to the gravitational potential of the Milky Way, we demonstrate that the strict upper limits on the deviation from the WEP are Δγ < 1.4 × 10⁻³ for GW170817/macronova and Δγ < 5.9 × 10⁻⁸ for GW170817/SGRB 170817A. A much more severe constraint on the WEP accuracy can be achieved (∼ 0.9 × 10⁻¹⁰) for GW170817/SGRB 170817A when we consider the gravitational potential of the Virgo Cluster, rather than the Milky Way's gravity. This provides the tightest limit to date on the WEP through the relative differential variations of the γ parameter for two different species of particles. Compared with other multimessenger (photons and neutrinos) results, our limit is 7 orders of magnitude tighter than that placed by the neutrinos and photons from supernova 1987A, and is almost as good as or is an improvement of 6 orders of magnitude over the limits obtained by the low-significance neutrinos correlated with GRBs and a blazar flare.