Title: New outburst of the symbiotic nova AG Pegasi after 165 years
Authors: A. Skopal, S. Shugarov, M. Sekeras, M. Wolf, T.N. Tarasova, F. Teyssier, M. Fujii, J. Guarro, O. Garde, K. Graham, T. Lester, V. Bouttard, T. Lemoult, U. Sollecchia, J. Montier, D. Boyd
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Description: 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 exploded again showing characteristics of the so-called Z And-type outburst. We investigated the new outburst using archival `Hubble Space Telescope' spectra and historical photometric measurements prior to the 2015 event, and new observations in the optical we acquired during the outburst with a high cadence. Modelling the spectral energy distribution we found a dramatic increase of the nebular radiation during the outburst, which indicates the presence of a very strong source of ionizing photons, which radiates at the temperature of ~200000 K and generates the luminosity of around 20000 Solar units. These quantities suggest that the outburst was caused by an abrupt accretion onto the white dwarf (WD) at a rate of ~ 3 x 10-7 solar masses per year, which exceeds the limit for the stable thermonuclear burning of hydrogen on its surface. Such the nuclearly burning WD blows a strong stellar wind and generates the luminosity around the so-called Eddington limit, at which the light energy balances the gravitational force of the WD. We confirmed this case by determining the mass-loss rate via the wind to 1-5 x 10-6 solar masses per year. Our explanation of the nature of the new Z And-type outburst by AG Peg agrees well with theoretical models elaborated during 70's and 80's decades of the last century.
Reference: Astronomy & Astrophysics 604, article no. A48, p. 1-19 (2017)
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