We present possible observing scenarios for the Advanced LIGO, Advanced Virgo and KAGRA gravitational-wave detectors over the next decade, with the intention of providing information to the astronomy community to facilitate planning for multi-messenger astronomy with gravitational waves. We estimate the sensitivity of the network to transient gravitational-wave signals, and study the capability of the network to determine the sky location of the source. We report our findings for gravitational-wave transients, with particular focus on gravitational-wave signals from the inspiral of binary neutron star systems, which are the most promising targets for multi-messenger astronomy. The ability to localize the sources of the detected signals depends on the geographical distribution of the detectors and their relative sensitivity, and credible regions can be as large as thousands of square degrees when only two sensitive detectors are operational. Determining the sky position of a significant fraction of detected signals to areas of 5– requires at least three detectors of sensitivity within a factor of of each other and with a broad frequency bandwidth. When all detectors, including KAGRA and the third LIGO detector in India, reach design sensitivity, a significant fraction of gravitational-wave signals will be localized to a few square degrees by gravitational-wave observations alone.

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An intermediate-mass black hole (IMBH) may have a dark-matter (DM) minihalo around it and develop a spiky structure within less than a parsec from the IMBH. When a stellar mass object is captured by the minihalo, it eventually infalls into such an IMBH due to gravitational wave backreaction which in turn could be observed directly by future space-borne gravitational wave experiments such as eLISA and NGO. In this Letter, we show that the gravitational wave (GW) detectability strongly depends on the radial profile of the DM distribution. So if the GW is detected, the power index, that is, the DM density distribution, would be determined very accurately. The DM density distribution obtained would make it clear how the IMBH has evolved from a seed black hole and whether the IMBH has experienced major mergers in the past. Unlike the γ-ray observations of DM annihilation, GW is just sensitive to the radial profile of the DM distribution and even to noninteracting DM. Hence, the effect we demonstrate here can be used as a new and powerful probe into DM properties.

Pre-DECIGO (DECihertz laser Interferometer Gravitational wave Observatory) consists of three spacecraft arranged in an equilateral triangle with 100 km arm lengths orbiting 2000 km above the surface of the earth. It is hoped that the launch date will be in the late 2020s.Pre-DECIGO has one clear target: binary black holes (BBHs) like GW150914 and GW151226. Pre-DECIGO can detect ∼ 30M ⊙ -30M ⊙ BBH mergers like GW150914 up to redshift z ∼ 30. The cumulative event rate is ∼ 1.8 × 10 5 events yr −1 in the Pop III origin model of BBHs like GW150914, and it saturates at z ∼ 10, while in the primordial BBH (PBBH) model, the cumulative event rate is ∼ 3 × 10 4 events yr −1 at z = 30 even if only 0.1% of the dark matter consists of PBHs, and it is still increasing at z = 30. In the Pop I/II model of GW150914-like BBHs, the cumulative event rate is (3-10) × 10 5 events yr −1 and it saturates at z ∼ 6. We present the requirements on orbit accuracy, drag-free techniques, laser power, frequency stability, and the interferometer test mass. For BBHs like GW150914 at 1 Gpc (z ∼ 0.2), SNR ∼ 90 is achieved with the definition of Pre-DECIGO in 0.01-100 Hz band. Since for z ≫ 1 the characteristic strain amplitude h c for a fixed frequency band weakly depends on z as z −1/6 , ∼ 10% of BBHs near face-on have SNR > 5 (7) even at z ∼ 30 (10). Pre-DECIGO can measure the mass spectrum and the z-dependence of the merger rate to distinguish various models of BBHs like GW150914, such as Pop III BBH, Pop II BBH and PBBH scenarios.Pre-DECIGO can also predict the direction of BBHs at z = 0.1 with an accuracy of ∼ 0.3 deg 2 and a merging time accuracy of ∼ 1 s at about a day before the merger so that ground-based GW detectors further developed at that time as well as electromagnetic follow-up observations can prepare for the detection of merger in advance like a solar eclipse. For intermediate mass BBHs such as ∼ 640M ⊙ -640M ⊙ at a large redshift z > 10, the quasinormal mode frequency after the merger can be within the Pre-DECIGO band so that the ringing tail can also be detectable to confirm the Einstein theory of general relativity with SNR ∼ 35.