Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst

We present results from 16 snapshots of Aql X-1 with RXTE during the rising phase of its recent outburst. The observations were carried out at a typical rate of once or twice per day. The source shows interesting spectral evolution during this period. Phenomenologically, it bears remarkable similarities to ``atoll sources. Shortly after the onset of the outburst, the source is seen to be in an ``island state, but with little X-ray variability. It then appears to have made a rapid spectral transition (on a time scale less than half a day) to another ``island state, where it evolves slightly and stays for 4 days. In this state, the observed X-ray flux becomes increasingly variable as the source brightens. Quasi-period oscillation (QPO) in the X-ray intensity is detected in the frequency range 670--870 Hz. The QPO frequency increases with the X-ray flux while its fractional rms decreases. The QPO becomes undetectable following a transition to a ``banana state, where the source continues its evolution by moving up and down the ``banana branch in the color-color diagram as the flux (presumably, the mass accretion rate) fluctuates around the peak of the outburst. Throughout the entire period, the power density spectrum is dominated by very-low frequency noises. Little power can be seen above ~1 Hz, which is different from typical ``atoll sources. In the ``banana state, the overall X-ray variability remains low (with fractional rms ~3--4%) but roughly constant. The observed X-ray spectrum is soft with few photons from above $sim$25 keV, implying the thermal origin of the emission. The evolution of both spectral and temporal X-ray properties is discussed in the context of disk-instability models.