2000ApJ...530..916R

Based on Rossi X-Ray Timing Explorer data, we discuss the classification of the soft X-ray transient Aquila X-1 in the Z/atoll scheme and the relation of its kilohertz quasi-periodic oscillations (kHz QPOs) properties to the X-ray colors. The color-color diagram shows one elongated (``banana'') structure and several ``islands'' of data points. The power spectra of the island are best represented by a broken power law whereas those of the banana by a power law below ∼1 Hz plus an exponentially cutoff component at intermediate frequencies (30-60 Hz). The parameters of these two components change in correlation with the position of the source in the color-color diagram. Based on the pattern that the source shows in the color-color diagram and its aperiodic variability, we conclude that Aquila X-1 is an atoll source. We have also investigated the possible correlation between the frequency of the kHz QPOs and the position of the source in the color-color diagram. The complexity seen in the frequency versus count rate diagram is reduced to a single track when the frequency is plotted against hard or soft color.