Classification and Spectral Evolution of Outbursts of Aql X-1

We present a broad classification of all outbursts detected with the All-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) and the Monitor of All Sky X-Ray Image (MAXI) of Aql X-1. We identify three types of outbursts; long-high, medium-low, and short-low, based on the duration and maximum flux. We analyse the trends in the "phase-space" of flux-derivative versus flux to demonstrate the differences in the three identified outburst types. We present a spectral analysis of the observations of Aql X-1 performed by the Proportional Counter Array (PCA) onboard RXTE during the 2000 and 2011 outbursts of the long-high class and the 2010 outburst of the medium-low class. We model the source spectrum with a hybrid thermal/non-thermal hot plasma emission model (EQPAIR in XSPEC, Coppi 2000) together with a Gaussian component to model the Fe K_alpha emission line. We construct time histories of the source flux, the optical depth of the corona (tau), the seed photon temperature (kT_bb) and the hard state compactness (l_h) for these three outbursts. We show that the physical parameters of either classes reach the same values throughout the outbursts, the only difference being the maximum flux. We discuss our results in the terms of modes of interaction of the star with the disc and size of the disc kept hot by irradiation. We conclude that irradiation is the dominant physical process leading to the different classes of outbursts.Comment: MNRAS accepted. 12 pages, 9 figures, 3 table