Gamma-Ray Bursts: Evidence for a Common Origin of X-ray Plateaus with Diverse Temporal Decay Index

TL;DR

This study analyzes a large sample of gamma-ray bursts with X-ray plateaus, finding that variations in the early plateau decay slope do not indicate different subclasses but are part of a single, unified population.

Contribution

It demonstrates that the diversity in plateau slopes among GRBs does not correspond to distinct populations, supporting a common origin for these phenomena.

Findings

No significant differences in luminosity functions across groups

Redshift distributions are statistically similar

Event rates are consistent among the groups

Abstract

A significant fraction of gamma-ray bursts (GRBs) exhibit a plateau in the early X-ray afterglow light curve, whose mechanism remains uncertain. While the post-plateau normal decay index ($\alpha_2$) is commonly used to constrain the afterglow dynamics, the shallow-decay slope of the plateau itself ($\alpha_1$) has received comparatively little attention. Recent observations, however, reveal substantial dispersion in $\alpha_1$, raising the question of whether GRBs with rising, flat and mildly decaying plateaus represent intrinsically distinct populations. To address this question, we collect a uniform sample of 185 $\textit{Swift}$ GRBs with a well-defined plateau and divide them into three groups based on $\alpha_1$. Using a non-parametric approach, we reconstruct their X-ray luminosity functions, redshift distributions and event rates. It is found that the three groups exhibit statistically consistent properties across all diagnostics, with no evidence for group-specific features. Monte Carlo perturbation tests further show that these results are insensitive to the adopted classification boundaries of $\alpha_1$. Our results indicate that variations in the plateau slope $\alpha_1$ do not define distinct GRB subclasses, but instead the sample constitutes a statistically uniform population governed by a common framework.