# Probing the Structure of Gamma-Ray Burst Jets with Steep Decay Phase of their Early X-ray Afterglows

## TL;DR

This study uses the steep decay phase of early X-ray afterglows to investigate gamma-ray burst jet structures, revealing that decay index distributions can distinguish between different jet models.

## Contribution

It introduces a new method to analyze the decay index distribution, providing a way to differentiate jet structures in gamma-ray bursts using Swift X-ray data.

## Key findings

- Decay index distribution varies with jet structure.
- Wide scatter and multiple peaks indicate core-envelope jets.
- Decay index less than 2 challenges uniform and Gaussian jet models.

## Abstract

We show that the jet structure of gamma-ray bursts (GRBs) can be investigated with the tail emission of the prompt GRB. The tail emission which we consider is identified as a steep-decay component of the early X-ray afterglow observed by the X-ray Telescope onboard Swift. Using a Monte Carlo method, we derive, for the first time, the distribution of the decay index of the GRB tail emission for various jet models. The new definitions of the zero of time and the time interval of a fitting region are proposed. These definitions for fitting the light curve lead us an unique definition of the decay index, which is useful to investigate the structure of the GRB jet. We find that if the GRB jet has a core-envelope structure, the predicted distribution of the decay index of the tail has a wide scatter and has multiple peaks, which cannot be seen for the case of the uniform and the Gaussian jet. Therefore, the decay index distribution tells us the information on the jet structure. Especially, if we observe events whose decay index is less than about 2, both the uniform and the Gaussian jet models will be disfavored according to our simulation study.