Multiwavelength light-curve evolution of Swift J1357.2-0933 during its 2011 outburst

Swift J1357.2-0933 underwent an episodic accretion in 2011 and provided very regular temporal and spectral evolution, making it an ideal source for exploring the nature of very faint X-ray transients. In this work, we present a detailed analysis on both X-ray and near-ultraviolet (NUV) light curves. The fluxes at all wavelengths display a near-exponential decays in the early phase and transit to a faster decay at late times. The e-folding decay time-scales monotonically decrease with photon energies, and the derived viscous time-scale is tau((M) over dot) similar to 60 d. The time-scale in the late faster decay stage is about a few days. The high ratio of NUV luminosity to X-ray luminosity indicates that the irradiation is unimportant in this outburst, while the near-exponential decay profile and the long decay time-scales conflict with the disc thermal-viscous instability model. We thus suggest that the disc is thermally stable during the observations. Adopting the truncated disc model, we obtain a lower limit of peak accretion rate of 0.03 (M) over dot(Edd) and the X-ray radiative efficiency eta < 5 x 10(-4), which decreases as the luminosity declines. The low X-ray radiative efficiency is caused by the combined action of advection and outflows, and naturally explains that the X-ray reprocessing is overwhelmed by the viscous radiation of the outer standard disc in the NUV regime. We also propose a possibility that the outer standard disc recedes from the central black hole, resulting in the faster decay at late times.