Broad-band spectral analysis of Aql X-1

Abstract

We present the results of a broadband spectral study of the transient Low Mass X-ray Binary Aql X-1 observed by Suzaku and Rossi X-ray Timing Explorer satellites. The source was observed during its 2007 outburst in the High/Soft (Banana) state and in the Low/Hard (Extreme Island) state. Both the Banana state and the Extreme Island state spectra are best described by a two component model consisting of a soft multi-colour blackbody emission likely originating from the accretion disk and a harder Comptonized emission from the boundary layer. Evidence for a hard tail (extending to ~50 keV) is found during the Banana state; this further (transient) component, accounting for atleast ~1.5% of the source luminosity, is modeled by a power-law. Aql X-1 is the second Atoll source after GX 13+1 to show a high energy tail. The presence of a weak but broad Fe line provides further support for a standard accretion disk extending nearly to the neutron star surface. The input photons for the Comptonizing boundary layer could either be the disk photons or the hidden surface of the star or both. The luminosity of the boundary layer is similar to the disk luminosity in the banana state and is about six times larger in the extreme island state. The temperature of the Comptonizing boundary layer changes from ~2 keV in the banana state to ~20 keV in the extreme island state.