(a) Hardness-ratio history of No. 8a, CR(3.0-10.0keV) |$/$| CR(1.5-3.0keV). (b) Softness-ratio history of No. 8a, CR(0.5-1.5keV) |$/$| CR(1.5-3.0keV). The |$\chi ^2_{\nu}$| values from a constant hypothesis are indicated in the panels. The broken lines are the best-fit sinusoidal curves.