A new test of the weak equivalence principle on the galaxy level based on observations of binary neutron star merger GW170817

Lulu Yao, Zonghua Zhao, Yu Han, Jingbo Wang, Tong Liu, Molin Liu

Published 2019-09-10Version 1

Because the binary neutron star (BNS) merger GW170817 and its electromagnetic (EM) counterparts were successfully detected, there is a chance to explore the joint effect of the host galaxy and the Milky Way on the weak equivalence principle (WEP) test by the gravitational-waves (GWs) and photons. In this paper, a new test of WEP is presented by modelling each galaxy composed by the Navarro-Frenk-White (NFW) halos and the Hernquist stellar. It is found that the WEP test is significantly enhanced by the joint action of the Milky Way and the NGC 4993. The constraints on the differences of the parametrized post-Newtonian (PPN) parameter $\Delta \gamma$ is under $10^{-9}$ for GW170817/GRB 170817A with total mass, and is under $10^{-4}$ for GW170817/AT 2017gfo. Following the conservative emission time difference given by \citet{LVFI}, we can obtain that $\Delta \gamma$ for GW170817/GRB 170817A is in the range of $[-5.3\times10^{-9}, 2.4\times10^{-8}]$ which is tighter by 2 orders of magnitude than previous range $[-2.6 \times 10^{-7}, 1.2\times 10^{-6}]$. Finally, we also consider potential impacts due to uncertainties in the halos mass of NGC 4993 and the position of AT2017gfo within it, and conclude that they do not affect our results at the order of magnitude level.