A Hard-to-Soft State Transition during A Luminosity Decline of Aquila X-1

We have discovered a spectral transition from the low/hard (LH) state to the high/soft (HS) state when Aquila X-1 was {\em declining} in observations made with the {\it Rossi X-Ray Timing Explorer (RXTE)}. The 2--200 keV energy flux corresponding to the state transition is $1.1\times{10}^{-9} ergs cm^{-2} s^{-1}$, an order of magnitude lower than observed in the past. The 2--200 keV peak flux of the following HS state is $1.6\times{10}^{-9} ergs cm^{-2} s^{-1}$. The relation between the luminosity of the hard-to-soft state transition and the peak luminosity of the following HS state confirms the linear relation found previously. This implies that the luminosity of the hard-to-soft state transition is not determined solely by the mass accretion rate, but appears to be determined by the peak luminosity of the soft X-ray outburst. We also found that the LH-to-HS state transition occurred at a luminosity similar to that of the corresponding HS-to-LH state transition, i.e., there is no apparent hysteresis. These results provide additional evidence that the mass in the accretion disk affects the luminosity of the hard-to-soft state transition, and that the accretion flow that powers the LH state is related to the accretion flow that powers the HS state at a later time.