New outburst of the symbiotic nova AG Pegasi after 165 yr

Abstract

Context. AG Peg is known as the slowest symbiotic nova, which experienced its nova-like outburst around 1850. After 165 years, during June of 2015, it erupted again showing characteristics of the Z And-type outburst. Aims. The primary objective is to determine basic characteristics, the nature and type of the 2015 outburst of AG Peg. Methods. We achieved this aim by modelling the spectral energy distribution using low-resolution spectroscopy (330-750 nm; R = 500-1000), medium-resolution spectroscopy (420-720 nm; R ∼ 11000), and U BVR C I C photometry covering the 2015 outburst with a high cadence. Optical observations were complemented with the archival HST and FUSE spectra from the preceding quiescence. Results. During the outburst, the luminosity of the hot component was in the range of 2-11×10 37 (d/0.8 kpc) 2 erg s −1 , being in correlation with the light curve (LC) profile. To generate the maximum luminosity by the hydrogen burning, the white dwarf (WD) had to accrete at ∼ 3 × 10 −7 M ⊙ yr −1 , which exceeds the stable-burning limit and thus led to blowing optically thick wind from the WD. We determined its mass-loss rate to a few ×10 −6 M ⊙ yr −1 . At the high temperature of the ionising source, 1.5 − 2.3 × 10 5 K, the wind converted a fraction of the WD's photospheric radiation into the nebular emission that dominated the optical. A one order of magnitude increase of the emission measure, from a few ×10 59 (d/0.8 kpc) 2 cm −3 during quiescence, to a few ×10 60 (d/0.8 kpc) 2 cm −3 during the outburst, caused a 2 mag brightening in the LC, which is classified as the Z And-type of the outburst. Conclusions. The very high nebular emission and the presence of a disk-like H i region encompassing the WD, as indicated by a significant broadening and high flux of the Raman-scattered O vi 6825 Å line during the outburst, is consistent with the ionisation structure of hot components in symbiotic stars during active phases.

The path to Z And-type outbursts: The case of V426 Sagittae (HBHA 1704-05)

“…However, in some cases we observe this ratio to be around 6 (Birriel et al 2000) and probably even larger for AG Peg (see Fig. 10 of Skopal et al 2017). A very high ratio in the range of F(1032)/F(1038) ∼ 7 − 20 was reported by Birriel et al (1998) and Schmid et al (1999), who explained this anomaly by a significant attenuation of the O vi 1038 Å line by the interstellar H 2 absorption.…”

“…Paczyński & Rudak 1980) and/or blowing optically thick wind from the WD (Hachisu et al 1996). The latter was recently demonstrated for the 2015 outburst of AG Peg (Skopal et al 2017). This type of outburst shows 1-3 mag brightening in the optical evolving on a timescale of weeks to years, often prolonged with multiple re-brightenings (see e.g.…”

“…In this section we determine physical parameters of V426 Sge and describe their temporal evolution throughout the 2018 outburst. Our analysis is similar to the one we used for the 2015 outburst of AG Peg, because of their close similarity (see Skopal et al 2017, and Sect. 4.1 here).…”

“…3), β ∼ 1.7, a = 50 km s −1 and assuming R in = R eff WD (see Sect. 3.2.6. of Skopal et al 2017, and references therein). Because a significant fraction of the wind emission is produced within its densest innermost part, the corre-spondingṀ WD strongly depends on the wind origin, R 0 , which, however, cannot be derived from observations.…”

“…Optically thick conditions where the nebula is not transparent to ionising radiation are indicated by the presence of elements at very different ionisation states in the spectrum, such as during the outburst of AG Peg (see Skopal et al 2017, and references therein) and the increase of particle density around the WD due to the increase of the mass-loss rate from the WD (see Sect. 3.2.4).…”