Evolution of Spectral States of Aql X-1 during the 2000 Outburst

We present the results of detailed analysis of X-ray data in 3-20 keV range from a ~70 day outburst of the neutron star transient Aquila X-1 during September-November 2000. Optical monitoring with the YALO 1m 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 low/hard state to high/soft state during the rise of the outburst occurs at higher luminosity than the transition back to 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 which is similar to, but distinct from, the canonical high/soft state. High frequency quasi-periodic oscillations from 636-870 Hz were seen only during this transitional state.