On the X-ray outburst light curves and recurrence characteristics of X-ray transients with neutron stars

It was proposed earlier that the knee-like morphology followed by a sharp decay in the outburst light curves of some magnetised stars, including the prototypical neutron star X-ray transient (NSXT) Aql X-1, could be an indication of the transition to the propeller phase. In this thesis, we first investigate the effects of the disc parameters, namely the X-ray irradiation strength, the kinematic viscosity, the initial mass distribution, and the disc mass on the decay characteristics of the Xray outburst light curves of NSXTs in the frame of the disc instability model. Our results show that the typical features of the outburst light curves of Aql X-1 cannot be obtained by simply changing one or some of these parameters. Subsequently, with detailed numerical analysis, we have shown that the entire X-ray outburst light curve can be reproduced by taking the weak disc mass flow rate dependence of the disc aspect ratio and its effect on the X-ray irradiation strength into account in the calculations. When the mass flow from the companion into the outer disc is included in the calculations, the same model can also yield recurrent outbursts with morphology and recurrence times similar to those observed from Aql X-1. In our model, the X-ray light curve of the source is produced by mass accretion onto the neutron star throughout the outburst phase without including any propeller effect or partial accretion regime, as proposed in earlier work to explain the decay features of the outburst light curve of Aql X-1