Outburst Morphology in the Soft X-Ray Transient Aquila X-1

We present optical and near-IR (OIR) observations of the major outbursts of the neutron star soft X-ray transient binary system Aquila X-1, from 1998 summer to 2007 fall. The major outbursts of the source over the observed timespan seem to exhibit two main types of light-curve morphologies, (1) the classical fast rise and exponential decay (FRED) type outburst seen in many soft X-ray transients and (2) the low-intensity state (LIS), where the optical-to-soft X-ray flux ratio is much higher than that seen during a FRED. Thus there is no single correlation between the optical (R band) and soft X-ray (1.5-12 keV, as seen by the ASM on board RXTE) fluxes even within the hard state for Aquila X-1, suggesting that LISs and FREDs have fundamentally different accretion flow properties. Time evolution of the OIR fluxes during the major LIS and FRED outbursts is compatible with thermal heating of the irradiated outer accretion disk. No signature of X-ray spectral state changes or any compact jet are seen in the OIR, showing that the OIR color-magnitude diagram (CMD) can be used as a diagnostic tool to separate thermal and nonthermal radiation from X-ray binaries where orbital and physical parameters of the system are reasonably well known. We suggest that the LIS may be caused by truncation of the inner disk in a relatively high \dot{M} state, possibly due to matter being diverted into a weak outflow.