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Swift J1357.2-0933 underwent an episodic accretion in 2011 and provided veryregular temporal and spectral evolution, making it an ideal source forexploring the nature of very faint X-ray transients (VFXTs). In this work, wepresent a detailed analysis on both X-ray and near-ultraviolet (NUV) lightcurves. The fluxes at all wavelengths display a near-exponential decays in theearly phase and transits to a faster-decay at late times. The e-folding decaytime-scales monotonically decrease with photon energies, and the derivedviscous time-scale is $\tau_{\rm \dot{M}} \sim 60$ days. The time-scale in thelate faster-decay stage is about a few days. The high ratio of NUV luminosityto X-ray luminosity indicates that the irradiation is unimportant in thisoutburst, while the near-exponential decay profile and the long decaytime-scales conflict with the disc thermal-viscous instability model. We thussuggest that the disc is thermally stable during the observations. Adopting thetruncated disc model, we obtain a lower limit of peak accretion rate of $0.03\dot{M}_{\rm Edd}$ and the X-ray radiative efficiency $\eta < 5\times10^{-4}$,which decreases as the luminosity declines. The low X-ray radiative efficiencyis caused by the combined action of advection and outflows, and naturallyexplains that the X-ray reprocessing is overwhelmed by the viscous radiation ofthe outer standard disc in the NUV regime. We also propose a possibility thatthe outer standard disc recedes from the central black hole, resulting in thefaster-decay at late times.

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