Evolution of Aquila X-1 during the rising phase of its 1998 outburst

We present results from 16 snapshots of Aquila X-1 with the Rossi X-Ray Timing Explorer 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 timescale of 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-periodic 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 similar to 1 Hz, which is different from typical atoll sources. In the banana state, the overall X-ray variability remains low (with a fractional rms similar to 3%-4%) but roughly constant. The observed X-ray spectrum is soft, with few photons from above similar to 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.