2004ApJ...602..918R

We have analyzed five X-ray outbursts of the neutron star soft X-ray transient Aquila X-1 and investigated the timing properties of the source in correlation with its spectral states as defined by different positions in the color-color and hardness-intensity diagrams. The hard color and the source count rate serve as the distinguishing parameters, giving rise to three spectral states: a low-intensity hard state, an intermediate state, and a high-intensity soft state. These states are respectively identified with the extreme island, island, and banana states that characterize the atoll sources. The large amount of data analyzed allowed us to perform, for the first time, a detailed timing analysis of the extreme island state. Differences in the aperiodic variability between the rise and the decay of the X-ray outbursts are found in this state: at the same place in the color-color diagram, the source exhibits more power at low frequencies (≲1 Hz) during the rise, whereas during the decay the source is more variable at high frequencies (≳100 Hz). The very low frequency noise that characterizes the banana-state power spectra below 1 Hz cannot be described in terms of a single power law: a two-component model is required. In two outbursts, a new 6-10 Hz quasi-periodic oscillation (QPO) has been discovered and tentatively identified with the normal/flaring branch-like oscillation observed only at the highest inferred mass accretion rates. We have compared the spectral and timing properties of Aql X-1 with those of other atoll and Z sources. Our results argue against a unification scheme for these two types of neutron star X-ray binaries.