Evolution of spectral states of Aquila X-1 during the 2000 outburst

We present the results of detailed analysis of X-ray data in the 3-20 keV range from a similar to70 day outburst of the neutron star transient Aquila X-1 during 2000 September-November. Optical monitoring with the Yale 1 m telescope was used to trigger X-ray observations with Rossi X-Ray Timing Explorer (RXTE) in order to follow the outburst from a very early stage. In this paper we discuss the correlated evolution in time of features in the spectral and temporal domains for the entire outburst. The state transition from the low/hard state to the high/soft state during the rise of the outburst occurs at higher luminosity than the transition back to the low/hard state during the decay, as has also been observed in other outbursts. Fourier power spectra at low frequencies show a broken power law continuum during the rising phase, with the break frequency increasing with time. During the decline from maximum, the source evolves to a position in the hardness-intensity plane, as well as in the color-color diagram, that is similar to, but distinct from, the canonical high/soft state. High-frequency quasi-periodic oscillations from 636 to 870 Hz were seen only during this transitional state.