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

TL;DR

This study analyzes optical and X-ray observations of Aquila X-1's outbursts, revealing two distinct outburst types with different accretion properties and suggesting the LIS results from inner disk truncation.

Contribution

It identifies two main outburst morphologies in Aquila X-1 and demonstrates the use of OIR CMDs to distinguish thermal and non-thermal emission components.

Findings

LIS and FRED outbursts have different flux correlations.

OIR flux evolution aligns with thermal heating of the accretion disk.

OIR CMDs can diagnose emission origins in X-ray binaries.

Abstract

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 summer 1998 -- fall 2007. The major outbursts of the source over the observed timespan seem to exhibit two main types of light curve morphologies, (a) the classical Fast-Rise and Exponential-Decay (FRED) type outburst seen in many soft X-ray transients and (b) 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 onboard 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 non-thermal 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 mass accretion state, possibly due to matter being diverted into a weak outflow.