Kazuki Sakai

Estimation of starting times of quasi-normal modes in ringdown gravitational waves with the Hilbert-Huang transform

It is known that a quasinormal mode (QNM) of a remnant black hole dominates a ringdown gravitational wave (GW) in a binary black hole (BBH) merger. To study properties of the QNMs, it is important to determine the time when the QNMs appear in a GW signal as well as to calculate its frequency and amplitude. In this paper, we propose a new method of estimating the starting time of the QNM and calculating the QNM frequency and amplitude of BBH GWs. We apply it to simulated merger waveforms by numerical relativity and the observed data of GW150914. The results show that the obtained QNM frequencies and time evolutions of amplitudes are consistent with the theoretical values within 1% accuracy for pure waveforms free from detector noise. In addition, it is revealed that there is a correlation between the starting time of the QNM and the spin of the remnant black hole. In the analysis of GW150914, we show that the parameters of the remnant black hole estimated through our method are consistent with those given by LIGO and a reasonable starting time of the QNM is determined.

Toward classification of swimming style by using underwater wireless accelerometer data

The use of technical refinement is playing an important role in the process of optimizing training efficiency and improving results of athletes. Therefore, we aim to develop an online monitoring and feedback tool to follow the swimming training using a single waterproofed and wireless motion sensor attached on the swimmers center of body (back). In this paper, as the first step, we investigated the feature quantities to discriminate four swimming styles using the data of a single three-axis accelerometer. We found that there was a possibility that the combination of simple feature quantities could be used to discriminate four swimming styles.