GRB 120422A: A low-luminosity gamma-ray burst driven by a central engine

GRB 120422A is a low-luminosity gamma-ray burst (GRB) associated with a bright supernova, which distinguishes itself by its relatively short T ₉₀ (∼5 s) and an energetic and steep-decaying X-ray tail. We analyze the Swift Burst Alert Telescope and X-ray Telescope data and discuss the physical implications. We show that the steep decline early in the X-ray light curve can be interpreted as the curvature tail of a late emission episode around 58-86s, with a curved instantaneous spectrum at the end of the emission episode. Together with the main activity in the first ∼20 s and the weak emission from 40s to 60s, the prompt emission is variable, which points to a central engine origin in contrast to a shock-breakout origin, which is used to interpret some other nearby low-luminosity supernova GRBs. Both the curvature effect model and interpreting the early shallow decay as the coasting external forward shock emission in a wind medium provide a constraint on the bulk Lorentz factor Γ to be around several. Comparing the properties of GRB 120422A and other supernova GRBs, we find that the main criterion to distinguish engine-driven GRBs from shock-breakout GRBs is the time-averaged γ-ray luminosity. Engine-driven GRBs likely have a luminosity above ∼10 ⁴⁸ erg s^-1.