New outburst of the symbiotic nova AG Pegasi after 165 yr ^⋆

¹ Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
² Sternberg Astronomical Institute, Moscow State University, Universitetskij pr., 13, 119991 Moscow, Russia
³ Astronomical Institute, Charles University Prague, CZ-180 00 Praha 8, V Holešovičkách 2, The Czech Republic
⁴ Crimean Astrophysical Observatory, 298409 Nauchny, Crimea
⁵ 67 rue Jacques Daviel, 76100 Rouen, France
⁶ Fujii Kurosaki Observatory, 4500 Kurosaki, Tamashima, Kurashiki, 713-8126 Okayama, Japan
⁷ Balmes 2, 08784 PIERA, Barcelona, Spain
⁸ Observatoire de la Tourbière, 38690 Chabons, France
⁹ 23746 Schoolhouse Road, Manhattan, IL 60442, USA
¹⁰ 1178 Mill Ridge Road, Arnprior, ON, K7S3G8, Canada
¹¹ 21 rue de Guémar, 68000 Colmar, France
¹² Chelles Observatory, 23 avenue Hénin, 77500 Chelles, France
¹³ Via dei Malatesta 10, 67100 L’ Aquila, Italy
¹⁴ 30 rue de la Boulais, 35000 Rennes, France
¹⁵ West Challow Observatory, Oxfordshire OX12 9TX, UK

Received: 26 August 2016
Accepted: 23 April 2017

Abstract

Context. AG Peg is known as the slowest symbiotic nova, which experienced its nova-like outburst around 1850. After 165 yr, 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 ~ 11 000), and UBVR_CI_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 × 10³⁷ (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 × 10⁵ 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 × 10⁵⁹ (d/ 0.8 kpc)² cm^-3 during quiescence, to a few × 10⁶⁰ (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.