Self-organized criticality in type I X-ray bursts

TL;DR

This paper suggests that type I X-ray bursts exhibit self-organized criticality, with power-law distributions in their properties, and introduces a new physical model explaining their waiting times and size distributions.

Contribution

It proposes a novel SOC-based physical model for X-ray burst occurrence rates, explaining properties not accounted for by previous models.

Findings

Power-law distributions in burst properties like fluence and duration.

Waiting time distribution with a power-law index around -1.

The burst rate model with inverse proportionality to time explains observed distributions.

Abstract

Type I X-ray bursts in a low-mass X-ray binary (LMXB) are caused by unstable nuclear burning of accreted materials. Semi-analytical and numerical studies of unstable nuclear burning have successfully reproduced partial properties of this kind of burst. However, some other properties (e.g. the waiting time) are not well explained. In this paper, we find that the probability distributions of fluence, peak count, rise time, duration and waiting time can be described as power-law-like distributions. This indicates that type I X-ray bursts may be governed by a self-organized criticality (SOC) process. The power-law index of waiting time distribution (WTD) is around $-1$, which is not predicted by any current waiting time model. We propose a physical burst rate model, in which the mean occurrence rate is inversely proportional to time $\lambda\propto t^{-1}$. In this case, the WTD is well explained by a non-stationary Poisson process within the SOC theory. In this theory, the burst size is also predicted to follow a power-law distribution, which requires that the emission area possesses only part of the neutron star surface. Furthermore, we find that the WTDs of some astrophysical phenomena can also be described by similar occurrence rate models.