Since gravitational and electromagnetic waves from a compact binary coalescence carry independent information about the source, the joint observation is important for understanding the physical mechanisms of the emissions. Rapid detection and source
localization of a gravitational wave signal are crucial for the joint observation to be successful. For a signal with a high signal-to-noise ratio, it is even possible to detect it before the merger, which is called early warning. In this letter, we estimate the performances of the early warning for neutron-star black-hole binaries, considering the precession effect of a binary orbit, with the near-future detectors such as A+, AdV+, KAGRA+, and Voyager. We find that a gravitational wave source can be localized in $100 ,mathrm{deg^2}$ on the sky before $sim 10$--$40 ,mathrm{s}$ of time to merger once per year.
Axion is a promising candidate for ultralight dark matter which may cause a polarization rotation of laser light. Recently, a new idea of probing the axion dark matter by optical linear cavities used in the arms of gravitational wave detectors has be
en proposed [Phys. Rev. Lett. 123, 111301 (2019)]. In this article, a realistic scheme of the axion dark matter search with the arm cavity transmission ports is revisited. Since photons detected by the transmission ports travel in the cavity for odd-number of times, the effect of axion dark matter on their phases is not cancelled out and the sensitivity at low-mass range is significantly improved compared to the search using reflection ports. We also take into account the stochastic nature of the axion field and the availability of the two detection ports in the gravitational wave detectors. The sensitivity to the axion-photon coupling, $g_{agamma}$, of the ground-based gravitational wave detector, such as Advanced LIGO, with 1-year observation is estimated to be $g_{agamma} sim 3times10^{-12}$ GeV$^{-1}$ below the axion mass of $10^{-15}$ eV, which improves upon the limit achieved by the CERN Axion Solar Telescope.
An additional scalar degree of freedom for a gravitational wave is often predicted in theories of gravity beyond general relativity and can be used for a model-agnostic test of gravity. In this letter, we report the first direct search for the scalar
-tensor mixed polarization modes of gravitational waves from compact binaries in a strong regime of gravity by analyzing the data of GW170814 and GW170817, which are the merger events of binary black holes and binary neutron stars, respectively. Consequently, we obtain the constraints on the ratio of scalar-mode amplitude to tensor-mode amplitude: $lesssim 0.18$ for GW170814 and $lesssim 0.069$ for GW170817, which are the strongest constraints obtained so far on the presence of the scalar polarization in a strong regime of gravity.
Rapid localization of gravitational-wave events is important for the success of the multi-messenger observations. The forthcoming improvements and constructions of gravitational-wave detectors will enable detecting and localizing compact-binary coale
scence events even before mergers, which is called early warning. The performance of early warning can be improved by considering modulation of gravitational wave signal amplitude due to the Earth rotation and the precession of a binary orbital plane caused by the misaligned spins of compact objects. In this paper, for the first time we estimate localization precision in the early warning quantitatively, taking into account an orbital precession. We find that a neutron star-black hole binary at $z=0.1$ can typically be localized to $100,mathrm{deg}^2$ and $10,mathrm{deg^2}$ at the time of $12$ -- $15 ,mathrm{minutes}$ and $50$ -- $300,mathrm{seconds}$ before merger, respectively, which cannot be achieved without the precession effect.
The physical degrees of freedom of a gravitational wave (GW) are imprints of the nature of gravity. We can test a gravity theory by searching for 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, where they perform the Bayesian model selection between general relativity and the theory allowing only scalar or vector polarization modes. 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 by the LIGO-Virgo collaboration, though the estimated parameters of the binaries are significantly biased. 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 more strongly support GR, epecially compared to the scalar polarization, than the previous results by the LIGO-Virgo collaboration due to the location prior. In addition, by utilizing the orientation information of 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 polarization, much improved from those without the jet prior.
