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 QPO) 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 cut-off 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 QPO 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. Subject headings: accretion, accretion disks — stars: neutron — stars: individual (Aquila X–1) — X-rays: stars