The fourth outburst during the present active stage of symbiotic binary AG Dra

ATel #11559; R. Galis, J. Merc (UPJS in Kosice), M. Vrastak (CAS), F. Teyssier, T. Lester, D. Boyd, W. Sims (ARAS Group), L. Leedjarv (Tartu Observatory)
on 22 Apr 2018; 14:23 UT
Credential Certification: Rudolf Galis (rudolf.galis@upjs.sk)

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The symbiotic system AG Dra regularly undergoes quiescent and active stages which consist of several outbursts repeating at about 360d interval (Galis et al. 2017, OEJV 180, 24). After seven years of flat quiescence following the 2006-08 major outbursts, in the late spring of 2015, AG Dra began rising again in brightness toward what appeared to be a new minor outburst (ATel #7582). The recent outburst activity of AG Dra was definitely confirmed by the outbursts in April 2016 and May 2017 (ATel #8975; ATel #10390). The maximal magnitudes (10.7, 9.9 and 10.7 in B filter) rank all these brightenings to the minor outbursts of symbiotic binary AG Dra. Such photometric behavior of the active stage is very unusual. More often, the activity of AG Dra starts with a major outburst, during which the brightness can reach the 8.8 and 8.4 mag in B and V band, respectively. Recent outburst activity resembles the weak activity stage 1963-66 (Hric et al. 2014, MNRAS 443, 1103).

UV and X-ray observations showed that there are two types of outbursts: cool and hot ones (Gonzalez-Riestra et al. 1999, A&A 347, 478). Leedjarv et al. (2016, MNRAS 456, 2558) demonstrated that the outbursts of AG Dra can be clearly distinguished also according to the behavior of the prominent emission lines in optical spectra. High-dispersion spectroscopy confirmed significant increase of equivalent widths (EWs) of the emission lines Hbeta, He II (4686), Halpha, He I (6678), and Raman-scattered O VI (6825) during all three recent outbursts of AG Dra. At the same times, the blue-wing absorption component observed in the profiles of the emission lines He I (6678), H alpha and H beta completely disappeared and HeII/Hbeta flux ratio was higher than 1. Such spectroscopic behavior is typical for the hot outbursts of AG Dra (Munari et a. 2009, PASP 121, 1070; Leedjarv et al. 2016, MNRAS 456, 2558). Here, we report about the onset of a new outburst, the fourth one during the present active stage of AG Dra.

After sharp and short-lasting hot outburst in May 2017 during which magnitudes of AG Dra reached U=10.29, B=10.723, V=9.528, Rc=8.728, and I=8.148 on May 16, 2017 (JD 2457890.439; ATel #10390), the symbiotic binary returned to brightness typical for its quiescence stages (April 2, 2018, JD 2458211.360; U=11.07, B=11.01, V=9.68, Rc=8.80, Ic=8.15, +/- 0.05 in U, 0.03 in B and 0.02 in VRI). Since then, AG Dra has begun a rise in brightness, reaching U=10.82, B=10.95, V=9.67, Rc=8.77, and I=8.13 on April 14, 2018 (JD 2458223.300) and B=10.57, V=9.45, on April 20, 2018 (JD 2458229.329).

As part of ARAS spectroscopic monitoring (link to database below), a mid-resolution spectrum of AG Dra (resolving power 13000, range 4000-7950 Ang) has been obtained with the 31cm telescope on Mill Ridge Observatory, Canada on February 26, 2018 (JD 2458175.883). The ratio HeII/Hbeta was 0.67, EWs were 18.91, 28.11, 97.19, 2.03 and 7.37 Ang for He II (4686), Hbeta, Halpha, He I (6678) and O VI (6825), respectively. For reference, values typical for quiescence of AG Dra are HeII/Hbeta = 0.70, EWs He II 16.5, Hbeta 22.2, Halpha 81.0, He I 1.5 and O VI 6.3 Ang. Low-resolution spectra of AG Dra obtained on April 15, 2018 (JD 2458223.734) and April 18, 2018 (JD 2458227.388) suggested an increase in the ratio of He II/Hbeta. A mid-resolution Echelle spectrum of AG Dra (resolving power 11000, range 4200-7400 Ang) has been obtained on April 20, 2018 (JD 2458229.363) with the 35cm telescope with eShel spectrograph in Rouen, France. The values of the HeII/Hbeta ratio (1.16), as well as EWs of prominent emission lines (He II 37.21, Hbeta 32.20, Halpha 87.97, He I 1.94, O VI 8.72 Ang), confirm that the fourth hot outburst in the present active stage of symbiotic binary AG Dra is in onset.

ARAS Spectral Database