The Fourth Outburst during the Ongoing Active Stage of AG Draconis has Finished

AG Draconis (AG Dra) is a well-known symbiotic system which consists of a metal-poor red giant of spectral type K3 III (Shenavrin et al. 2011) and a white dwarf (WD). Due to the red giant's wind, the binary is surrounded by an extended circumbinary nebula, partially ionized by the WD. The system shows characteristic symbiotic activity of alternating quiescent and active stages. There are two types of outbursts in AG Dra, cool or hot (González-Riestra et al. 1999). They can be clearly distinguished by the behavior of prominent emission lines in optical spectra Leedjärv et al. (2016).

We closely monitored activity of this symbiotic system based on our own photometric observations and data obtained from AAVSO International Database (Kafka 2018). AG Dra had been quiescent since its 2006-08 major outbursts. In 2015 May, brightness began to increase, and the first, less prominent outburst (G0, Figure 1) during the new active stage of AG Dra was observed (Munari et al. 2015). Maximum brightness was achieved around JD 2,457,166 (10.7 and 9.6 mag in the B and V filters, respectively), ranking it among the minor, hot outbursts of AG Dra. Such classification was also supported by the observed increase in equivalent widths of prominent emission lines, typical for hot outbursts. Minor brightening at the beginning of active stages was observed as a precursor to more prominent cool outbursts, at least for the active stages B, E and probably C, see Figure 1 in Merc et al. (2017).

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The following outburst (G1) was observed in 2016 April (Munari & Righetti 2016). Around JD 2,457,517, brightness reached at maximum of 9.9 and 9.1 mag in the B and V filters, respectively, making it a minor outburst of AG Dra. Although this outburst was significantly brighter than the previous one, an even more prominent (major) outburst was expected, since the pre-outburst of AG Dra was usually followed by major outbursts of around 8.8 and 8.4 mag in the B and V filters, respectively. Minor outbursts were probably observed only during activity stage C in 1963-66, which was the shortest one in the almost 130 year photometric history of this symbiotic system. During the outburst G1, we observed an extreme temperature increase (Merc et al. 2018), which ranks this brightening among the hot outbursts of AG Dra.

AG Dra's third outburst (G2) during the ongoing stage of activity in 2017 May (Munari et al. 2017) was a very sharp, short-lasting, hot outburst. Maximal brightness of the symbiotic system at JD 2,457,890 (10.7 and 9.5 mag in the B and V filters, respectively) was similar to the case of G0.

So far the most recent observed outburst (G3) of AG Dra started in 2018 April (Gális et al. 2018). A smooth increase in brightness was recognized at JD 2,458,211 (B = 11.0 mag and V = 9.7 mag), and the onset of this new outburst was photometrically confirmed 11 days later (JD 2,458,222, B = 10.9 mag and V = 9.6 mag). The maximum was reached at JD 2,458,243 with brightness of 10.2 and 9.5 mag in the B and V filters, respectively. The increase of the He ii/H_β ratio as well as equivalent widths of prominent emission lines suggest that this outburst was also of hot type, like the previous three outbursts in the present series. At the end of 2018 July, the brightness of AG Dra returned to values almost typical for quiescence (11.4, 11.1 and 9.8 mag in U, B and V filter, respectively), indicating that the fourth outburst of the ongoing active stage of AG Dra has probably finished.

To sum it up, photometric as well as spectroscopic behavior suggests that the last four outbursts of AG Dra belong to the minor, hot type. Such evolution of the active stage is very unusual. On the other hand, some specific effects observed during the outburst G1 (e.g., the near disappearance of the Raman scattered lines of O vi) are more typical for cool outbursts, despite the fact that the WD's temperature was very high during this event. A detailed analysis of this active stage will be presented in our forthcoming paper.

The next evolution of AG Dra is an open question. Can we expect (finally) a major cool or (again) minor hot outburst? Another possibility is that the symbiotic system will return to quiescence. According to our statistical analysis of photometric observations, we determined that the time interval between outbursts of AG Dra vary from 300 to 400 days (without an apparent long-term trend), with a median around 360 days. If there will be an outburst during the spring of 2019, we can expect it in the interval from JD 2,458,581 (2019 April 7) to JD 2,458,625 (2019 May 21).