On the relationship between circumstellar disc size and X-ray outbursts in Be/X-ray binaries

TL;DR

This study investigates the link between Be star disc sizes and giant X-ray outbursts in binary systems, finding no direct correlation and suggesting other disc behaviors influence outburst occurrence.

Contribution

It provides long-term observational data on Be disc size variations and challenges the simple size-outburst correlation predicted by existing models.

Findings

Type II outbursts occur at various disc sizes, not correlated with size.

Disc size estimates are limited and may not fully capture disc behavior.

Alternative disc phenomena like precession and density may influence outbursts.

Abstract

We present long term H$\alpha$ monitoring results of five Be/X-ray binaries to study the Be disc size variations and their influence on Type II (giant) X-ray outbursts. The work is done in the context of the viscous decretion disc model which predicts that Be discs in binary systems are truncated by resonant torques induced by the neutron star in its orbit. Our observations show that type II outbursts are not correlated(nor anti-correlated) with the disc size, as they are seen to occur both at relatively small and large Be disc radii. We discuss these observations in context of alternate interpretation of Be disc behaviour, such as precession, elongation and density effects, and with cognisance of the limitations of our disc size estimates.