A Hard-to-Soft State Transition of Aquila X-1 Observed with Suzaku

TL;DR

This study observed a clear spectral transition in Aquila X-1 from hard to soft state with Suzaku, revealing continuous parameter changes and supporting a unified disk plus Comptonized blackbody model for different states.

Contribution

First detailed Suzaku observation capturing the spectral transition of Aquila X-1 from hard to soft state with comprehensive spectral modeling.

Findings

Inner disk radius decreased from 31 km to 18 km

Electron temperature of Comptonization decreased from 10 keV to 3 keV

Spectral shape changed continuously during the transition

Abstract

The recurrent soft X-ray transient Aquila X-1 was observed with Suzaku for a gross duration of 79.9 ks, on 2011 October 21 when the object was in a rising phase of an outburst. During the observation, the source exhibited a clear spectral transition from the hard state to the soft state, on a time scale of ~30 ks. Across the transition, the 0.8-10 keV XIS count rate increased by a factor ~3, that of HXD-PIN in 15-60 keV decreased by a similar factor, and the unabsorbed 0.1-100 keV luminosity increased from 3.5 times 10^37 erg/s to 5.1 times 10^37 erg/s. The broadband spectral shape changed continuously, from a power-law like one with a high-energy cutoff to a more convex one. Throughout the transition, the 0.8-60 keV spectra were successfully described with a model consisting of a multi-color blackbody and a Comptonized blackbody, which are considered to arise from a standard accretion disk and a closer vicinity of the neutron star, respectively. All the model parameters were confirmed to change continuously, from those typical in the hard state to those typical of the soft state. More specifically, the inner disk radius decreased from 31 km to 18 km, the effects of Comptonization on the blackbody photons weakened, and the electron temperature of Comptonization decreased from 10 keV to 3 keV. The derived parameters imply that the Comptonizing corona shrinks towards the final soft state, and/or the radial infall velocity of the corona decreases. These results reinforce the view that the soft and hard states of Aql X-1 (and of similar objects) are described by the same "disk plus Comptonized blackbody" model, but with considerably different parameters.