Correlated timing and spectral variations of the soft X-ray transient Aquila X-1: Evidence for an atoll classification

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") struct ure and several "islands" of data points. The power spectra of the island a re best represented by a broken power law whereas those of the banana by a power law below similar to 1 Hz plus an exponentially cutoff component at i ntermediate frequencies (30-60 Hz). The parameters of these two components change in correlation with the position of the source in the color-color di agram. Based on the pattern that the source shows in the color-color diagra m and its aperiodic variability, we conclude that Aquila X-1 is an atoll so urce. We have also investigated the possible correlation between the freque ncy of the kHz QPOs and the position of the source in the color-color diagr am. The complexity seen in the frequency versus count rate diagram is reduc ed to a single track when the frequency is plotted against hard or soft col or.