New outburst of the symbiotic nova AG Pegasi after 165 yr

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.

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