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

Abstract

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 ${\tau }_{\dot{\mathrm{M}}}\sim 60$ 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 $0.03{\dot{M}}_{\mathrm{Edd}}$ 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.

6 pages, 4 figures, published in MNRAS

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• Optical Precursors to Black Hole X-ray Binary Outbursts: An evolving synchrotron jet spectrum in Swift J1357.2-0933
• Time-lag between disk and corona radiation leads to hysteresis effect observed in black-hole X-ray binary MAXI J1348-630
• Puzzling Blue Dips in the Black Hole Candidate Swift J1357.2-0933, from ULTRACAM, SALT, ATCA, Swift and NuSTAR
• Implications on accretion flow dynamics from spectral study of Swift~J1357.2-0933
• The X-ray view of black-hole candidate Swift J1842.5-1124 during its 2008 outburst
• Active galactic nuclei feedback at parsec scale
• Recurrent outbursts revealed in 3XMM J031820.8-663034