Pure polarization test of GW170814 and GW170817 using waveforms consistent with modified theories of gravity
We can test a gravity theory by searching for gravitational-wave (GW) polarization modes beyond general relativity. The LIGO-Virgo Collaboration analyzed several GW events in the O1 and O2 observing runs in the pure polarization framework, in which only scalar or vector polarization modes are allowed. In this paper, we reanalyze the polarizations of GW170814 (binary black hole merger) and GW170817 (binary neutron star merger) in the improved framework of pure polarizations including the angular patterns of nontensorial radiation. We find logarithms of the Bayes factors of 2.775 and 3.636 for GW170814 in favor of the pure tensor polarization against pure vector and scalar polarizations, respectively. These Bayes factors are consistent with the previous results from the LIGO-Virgo Collaboration, though the estimated parameters of the binaries should be different. For GW170817 with the priors on the location of the binary from NGC4993, we find logarithms of the Bayes factors of 21.078 and 44.544 in favor of the pure tensor polarization against pure vector and scalar polarizations, respectively. These support general relativity more strongly than the previous results by the LIGO-Virgo Collaboration due to the location prior. In addition, by utilizing the orientation information on the binary from a gamma-ray burst jet, we find logarithms of the Bayes factor of 51.043 and 60.271 in favor of the pure tensor polarization against pure vector and pure scalar polarizations, much improved from those without the jet prior.