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GRB 120422A is a low-luminosity Gamma-ray burst (GRB) associated with abright supernova, which distinguishes itself by its relatively short T90 ~ 5 sand an energetic X-ray tail. We analyze the Swift BAT and XRT data and discussthe physical implications. We show that the early steep decline in the X-raylight curve can be interpreted as the curvature tail of a late emission episodearound 58-86 s, with a curved instantaneous spectrum at the end of the emissionepisode. Together with the main activity in the first ~ 20 s and the weakemission from 40 s to 60 s, the prompt emission is variable, which pointstowards a central engine origin, in contrast to the shock breakout origin asinvoked to interpret some other nearby low-luminosity supernova GRBs. Thecurvature effect interpretation and interpreting the early shallow decay as thecoasting external forward shock emission in a wind medium both give aconstraint on the bulk Lorentz factor \Gamma to be around several. Comparingthe properties of GRB 120422A and other supernova GRBs, we found that the maincriterion to distinguish engine-driven GRBs from the shock breakout GRBs is thetime-averaged luminosity, with a separation line of about ~ 10^48 erg s^-1.

GRB 120422A: A Low-luminosity Gamma-ray Burst Driven by Central Engine