GRB 120422A: A Low-luminosity Gamma-Ray Burst Driven by a Central Engine
Abstract
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 58-86 s, 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 40 s to 60 s, 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 ~1048 erg s-1.
- Publication:
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The Astrophysical Journal
- Pub Date:
- September 2012
- DOI:
- 10.1088/0004-637X/756/2/190
- arXiv:
- arXiv:1206.0298
- Bibcode:
- 2012ApJ...756..190Z
- Keywords:
-
- gamma-ray burst: general;
- X-rays: general;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- ApJ accepted version