The periodic bursters XB1323-619 and GS1826-24: longterm evolution of the nuclear burning regime and comparison with theory

TL;DR

This study investigates two X-ray bursters over 18 years, analyzing their burst rates and nuclear burning processes, revealing differences in emitting areas that influence burst behavior and challenge existing theories.

Contribution

It provides a detailed long-term analysis of two specific bursters, highlighting the role of emitting area differences in burst rate variations and comparing observations with theoretical models.

Findings

Burst rate matches theory for each source

Significant difference in burst rate at same luminosity between sources

Emitting area differences explain inverse burst rate-luminosity relation

Abstract

The majority of X-ray burst sources do not display a burst rate that increases with luminosity as expected, but this is seen in the two clocked bursters XB1323-619 and GS1826-24. We present a detailed investigation of these two sources which in the case of the first source, spans 18 years. Based on measurements of the burst rate, X-ray luminosity, the alpha-parameter and the two time constants generally present in the burst decays, we demonstrate the importance of the rp nuclear burning process. A detailed comparison with theory shows that although the burst rate in each source agrees well with the theoretical value, there is a difference of more than a factor of 5 in the burst rate at a given luminosity between the sources. We show that the main reason for this is that the two sources have substantially different emitting areas on the neutron star in non-burst emission, a factor often neglected. Variation of this area may explain the inverse relation of burst rate with luminosity in the majority of burst sources.