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

TL;DR

This paper analyzes GRB 120422A, a low-luminosity gamma-ray burst linked to a supernova, demonstrating it is driven by a central engine rather than shock breakout, based on detailed X-ray and gamma-ray data analysis.

Contribution

It provides evidence that low-luminosity GRBs like 120422A are powered by central engines, not shock breakout, based on spectral and temporal analysis of Swift data.

Findings

Early steep X-ray decline is due to curvature tail of late emission.

Prompt emission shows variability indicating a central engine origin.

Main criterion to distinguish engine-driven from shock breakout GRBs is luminosity (~10^48 erg s^-1).

Abstract

GRB 120422A is a low-luminosity Gamma-ray burst (GRB) associated with a bright supernova, which distinguishes itself by its relatively short T90 ~ 5 s and an energetic X-ray tail. We analyze the Swift BAT and XRT data and discuss the physical implications. We show that the early steep decline 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 towards a central engine origin, in contrast to the shock breakout origin as invoked to interpret some other nearby low-luminosity supernova GRBs. The curvature effect interpretation and interpreting the early shallow decay as the coasting external forward shock emission in a wind medium both give a constraint on the bulk Lorentz factor \Gamma to be around several. Comparing the properties of GRB 120422A and other supernova GRBs, we found that the main criterion to distinguish engine-driven GRBs from the shock breakout GRBs is the time-averaged luminosity, with a separation line of about ~ 10^48 erg s^-1.