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We present optical and near-IR (OIR) observations of the major outbursts ofthe neutron star soft X-ray transient binary system Aquila X-1, from summer1998 -- fall 2007. The major outbursts of the source over the observed timespanseem to exhibit two main types of light curve morphologies, (a) the classicalFast-Rise and Exponential-Decay (FRED) type outburst seen in many soft X-raytransients and (b) the Low-Intensity State (LIS) where theoptical-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 onboard RXTE) fluxes even within the hard statefor Aquila X-1, suggesting that LISs and FREDs have fundamentally differentaccretion flow properties. Time evolution of the OIR fluxes during the majorLIS and FRED outbursts is compatible with thermal heating of the irradiatedouter accretion disk. No signature of X-ray spectral state changes or anycompact 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 non-thermalradiation from X-ray binaries where orbital and physical parameters of thesystem are reasonably well known. We suggest that the LIS may be caused bytruncation of the inner disk in a relatively high mass accretion state,possibly due to matter being diverted into a weak outflow.

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