2011MNRAS.416..637R

We present the results of a broad-band spectral study of the transient low-mass X-ray binary Aql X-1 observed by the 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 multicolour blackbody emission likely originating from the accretion disc 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 at least ∼ 1.5 per cent of the source luminosity, is modelled 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 disc extending nearly to the neutron star surface. The input photons for the Comptonizing boundary layer could be either the disc photons or the hidden surface of the star or both. The luminosity of the boundary layer is similar to the disc 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. Fast optical and X-ray variability in the UCXB 4U0614+09