(a) Normalized power spectrum of the phase time series data. Red line shows the power spectrum applied to the whole phase time series data of the received signal, and black line shows that the spectrum applied to a part of the time series corresponding to the gray region in the upper panel of Figure 2. The power spectrum is normalized by multiplying f^8/3. The 95% confidence interval of the chi-squared distribution is overplotted as the error bar for each spectrum. Panel b provides an enlarged view of the spectrum shown in panel a for the frequency range of 10⁻² − 10⁻¹ Hz.