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 similar to 11 000), and UBVRCIC 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 x 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 similar to 3 x 10(-7) M-circle dot 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 x10(-6) M-circle dot yr(-1).
At the high temperature of the ionising source, 1.5-2.3 x 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 x10(59) (d/0.8 kpc)(2) cm(-3) during quiescence, to a few x10(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 angstrom line during the outburst, is consistent with the ionisation structure of hot components in symbiotic stars during active phases.