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 ( VFXTs ) .
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 transits 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 _ { \dot { M } } \sim 60 days .
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 \dot { M } _ { Edd } and the X-ray radiative efficiency \eta < 5 \times 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 .