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Recent population synthesis simulations of Pop III stars suggest that the event rate of coalescence of 30M_-30M_ binary black holes can be high enough for the detection by the second generation gravitational wave detectors. The frequencies of chirp signal as well as quasinormal modes are near the best sensitivity of these detectors so that it would be possible to confirm Einstein's general relativity. Using the WKB method, we suggest that for the typical value of spin parameter a/M0. 7 from numerical relativity results of the coalescence of binary black holes, the strong gravity of the black hole space-time at around the radius 2M, which is just 1. 17 times the event horizon radius, would be confirmed as predicted by general relativity. The expected event rate with the signal-to-noise ratio >35 needed for the determination of the quasinormal mode frequency with a meaningful accuracy is 0. 17-7. 2 events yr^-1 (SFR/ (10^-2. 5M_ yr^-1 Mpc^-3) ) ([f₁/ (1+f₁) /0. 33) ], where SFR and f₁ are the peak value of the Pop III star formation rate and the fraction of binaries, respectively. As for the possible optical counterpart, if the merged black hole of mass M60M_ is in the interstellar matter with n100 cm^-3 and the proper motion of the black hole is 1 km s^-1, the luminosity is 10^40 erg s^-1 which can be detected up to 300 Mpc, for example, by Subaru-HSC and LSST with the limiting magnitude 26.
Nakamura et al. (Wed,) studied this question.