Ignition latitude and the shape of Type I X-ray bursts

TL;DR

This paper presents a phenomenological model linking burst lightcurve shapes to ignition latitude and burning regimes in Type I X-ray bursts, with application to observational data revealing off-equatorial ignition in many cases.

Contribution

The study introduces a simple diagnostic model for burst morphology that infers ignition latitude and burning regimes from lightcurve analysis, supported by application to real burst data.

Findings

Evidence for off-equatorial ignition in many bursts from 4U 1636-536

Model explains variations in burst oscillation detectability

Transition from hydrogen to helium ignition linked to ignition latitude

Abstract

The shape of the lightcurve during the rising phase of Type I X-ray bursts is determined by many factors including the ignition latitude, the accretion rate, and the rotation rate of the star. We develop a phenomenological model of the burst rise process and show that simple measures of the burst morphology can be robust diagnostics of ignition latitude and burning regime. We apply our results to the large sample of bursts from the Low Mass X-ray Binary 4U 1636-536, and find evidence for off-equatorial ignition for many of the bursts. We argue that such behaviour may be associated with the transition from hydrogen to helium ignition at accretion rates a few percent of Eddington. We show that this model can also explain variations in the detectability of burst oscillations, and discuss the implications for other burst sources.