The light curve of the outburst of Aql X-1 in 1998 via ASM data (upper panel, black points) with the time evolution of the hardness ratio estimated by using the ratio of count rates in the energy ranges of 5.0–12.0 keV and 1.5–5.0 keV (bottom panel, black points). The times of the pointing RXTE observations and the detected pulsation are shown via vertical grey lines and the blue dashed line, respectively.