Disk Instabilities Caused the 2018 Outburst of AG Draconis

TL;DR

The 2018 outburst of AG Draconis was caused by disk instabilities, marking a deviation from its typical outburst cycle, with detailed analysis of its brightness, temperature, and activity levels.

Contribution

This study identifies disk instabilities as the cause of the 2018 outburst, providing new insights into the system's unusual activity pattern.

Findings

The 2018 outburst was driven by disk instability mechanisms.

AG Dra's activity pattern deviated from its typical cycle before the outburst.

Analysis of brightness and temperature evolution supported the disk instability model.

Abstract

Symbiotic binary AG~Draconis (AG~Dra) has an well-established outburst behavior based on an extensive observational history. Usually, the system undergoes a 9--15~yr period of quiescence with a constant average energy emitted, during which the system's orbital period of $\sim$550~d can be seen at shorter wavelengths (particularly in the U-band) as well as a shorter period of $\sim$355~d thought to be due to pulsations of the cool component. After a quiescent period, the marker of an active period is usually a major (cool) outburst of up to $\textrm{V}=8.4$~mag, followed by a series of minor (hot) outbursts repeating at a period of approximately 1~yr. However, in 2016 April after a 9-year period of quiescence AG~Dra exhibited unusual behavior: it began an active phase with a minor outburst followed by two more minor outbursts repeating at an interval of $\sim$1~yr. We present R-band observations of AG~Dra's 2018 April minor outburst and an analysis of the outburst mechanism and reports on the system's activity levels following the time of its next expected outburst. By considering the brightening and cooling times, the scale of the outburst, and its temperature evolution we have determined that this outburst was of disk instability nature.