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
Image & caption:
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Image caption::
Figure. Left panels demonstrate a significant increase of the nebular radiation (green line) during the 2015 outburst (bottom panel) with respect to quiescence (top panel). Its amount corresponds to the luminosity of the nuclearly burning WD around the Eddington limit. Right panels show significant broadening of spectral lines observed during the outburst (red line) with respect to quiescence (blue line), which indicates a strong stellar wind blowing from the WD.
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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