Evolution of Spectral States of Aquila X‐1 during the 2000 Outburst

We present the results of detailed analysis of X-ray data in 3-20 keV rangefrom a ~70 day outburst of the neutron star transient Aquila X-1 duringSeptember-November 2000. Optical monitoring with the YALO 1m telescope was usedto trigger X-ray observations with Rossi X-ray Timing Explorer (RXTE) in orderto follow the outburst from a very early stage. In this paper we discuss thecorrelated evolution in time of features in the spectral and temporal domains,for the entire outburst. The state transition from low/hard state to high/softstate during the rise of the outburst occurs at higher luminosity than thetransition back to low/hard state during the decay, as has also been observedin other outbursts. Fourier power spectra at low frequencies show a brokenpower law continuum during the rising phase, with the break frequencyincreasing with time. During the decline from maximum the source evolves to aposition in the hardness-intensity plane as well as in the color-color diagramwhich is similar to, but distinct from, the canonical high/soft state. Highfrequency quasi-periodic oscillations from 636-870 Hz were seen only duringthis transitional state.Comment: 25 pages, 11 figures. Accepted for publication in Ap