A population study of type II bursts in the Rapid Burster

TL;DR

This study analyzes all available Rossi X-ray Timing Explorer data of type II bursts from the Rapid Burster, revealing their properties, energetics, and complex feedback mechanisms, and evaluates models to explain their origin.

Contribution

It provides a comprehensive observational analysis of type II bursts from the Rapid Burster and assesses existing models, highlighting the potential role of magnetic gating in their mechanism.

Findings

Type II bursts are Eddington-limited in flux.

Bursts can be as short as 0.130 seconds.

Recurrence times drop below 15-18 seconds.

Abstract

Type II bursts are thought to arise from instabilities in the accretion flow onto a neutron star in an X-ray binary. Despite having been known for almost 40 years, no model can yet satisfactorily account for all their properties. To shed light on the nature of this phenomenon and provide a reference for future theoretical work, we study the entire sample of Rossi X-ray Timing Explorer data of type II bursts from the Rapid Burster (MXB 1730-335). We find that type II bursts are Eddington-limited in flux, that a larger amount of energy goes in the bursts than in the persistent emission, that type II bursts can be as short as 0.130 s, and that the distribution of recurrence times drops abruptly below 15-18 s. We highlight the complicated feedback between type II bursts and the NS surface thermonuclear explosions known as type I bursts, and between type II bursts and the persistent emission. We review a number of models for type II bursts. While no model can reproduce all the observed burst properties and explain the source uniqueness, models involving a gating role for the magnetic field come closest to matching the properties of our sample. The uniqueness of the source may be explained by a special combination of magnetic field strength, stellar spin period and alignment between the magnetic field and the spin axis.