The 2015 super-active state of recurrent nova T CrB and the long term evolution after the 1946 outburst

•In 2015 T CrB has entered a phase of unprecedented high activity.•Similar to what the star underwent in 1938, a few years before its 1946 nova outburst.•High ionization conditions, [NeV], OIV and a strong HeII in emission.•Wind of the M3III companion almost fully ionized, enhanced irradiation of its side facing the WD.•Disappearance of the orbital modulation from B-band lightcurve, now fully dominated by nebular emission.

The recurrent nova T CrB has entered in 2015 a phase of unprecedented high activity. To trace something equivalent, it is necessary to go back to 1938, before the last nova eruption in 1946. The 2015 super-active state is characterized by: a large increase in the mean brightness (ΔB=0.72 mag over the underlying secular trend), vanishing of the orbital modulation from the B-band lightcurve, and appearance of strong and high ionization emission lines, on top of a nebular continuum that overwhelms at optical wavelengths the absorption spectrum of the M giant. Among the emission lines, HeII 4686 attains a flux in excess of Hγ, the full set of OIII and NIII lines involved in the Bowen fluorescence mechanism are strong and varying in intensity in phase with HeII 4686, and OIV and [NeV] are present. A large increase in the radiation output from the hot source is responsible for a large expansion in the ionized fraction of the M giant wind. The wind is completely ionized in the direction to the observer. A high electron density is supported by the weakness of forbidden lines and by the large amplitude and short time scale of the reprocessing by the nebular material of the highly variable photo-ionization input from the hot source. During the super-active state the nebula is varying to and from ionization-bounded and density-bounded conditions, and the augmented irradiation of the cool giant has changed the spectral type of its side facing the WD from M3III to M2III, i.e. an increase of ∼80 K in effective temperature.