Recent outburst activity of the super-soft X-ray binary AG Draconis

TL;DR

This paper analyzes the recent outburst activity of the symbiotic binary AG Draconis, highlighting its unusual behavior and the characteristics of its minor outbursts, with implications for future activity predictions.

Contribution

It provides a detailed observational analysis of AG Dra's recent outbursts, revealing atypical activity patterns and the mixed nature of hot and cool outbursts.

Findings

Recent outbursts in 2015-2016 were minor and atypical.

The 2016 outburst exhibited characteristics of both hot and cool outbursts.

The behavior of Raman scattered lines suggests complex outburst mechanisms.

Abstract

AG Draconis is a bright symbiotic binary consisting of a white dwarf and a pulsating cool giant. Moreover, it is the most intense X-ray source among symbiotic stars, and one of the best representatives of the super-soft X-ray objects. The system undergoes characteristic symbiotic activity with alternating quiescent and active stages. The active ones consist of several outbursts repeating at about a one-year interval. The recent activity stage of AG Dra began with the weak pre-outburst in 2015 followed by a more prominent outburst in 2016. According to photometric and some spectroscopic observations, both brightenings belong to the minor (hot) outbursts of AG Dra. Such behavior of the active stage is quite unusual because more often, the activity of AG Dra starts with a major (cool) outburst. Moreover, the behavior of Raman scattered OVI lines at {\lambda} 6825 {\AA} and {\lambda} 7082 {\AA} suggest that the minor outburst of AG Dra in April 2016 has the characteristics of both the hot and cool outbursts. Based on the above, an open question is the next evolution of activity of the symbiotic binary AG Dra in 2017 and beyond.