We have discovered a spectral transition from the low/hard ( LH ) state to the high/soft ( HS ) state when Aquila X-1 was declining in observations made with the 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 .