Atlas of photometric and spectroscopic daily monitoring of the very slow Nova Cas 2021 (=V1405 Cas) during the first 660 days of its outburst

Nova Cas 2021 erupted on 18 March 2021, reaching naked-eye brightness when passing through photometric maximum 53 days later. We describe our ~daily monitoring of its evolution, covering the first 660 days since discovery. In all we obtained 574 highly accurate photometric runs simultaneously in the BVRI bands, and 110 Echelle high-resolution spectra. The multi-band photometric evolution of Nova Cas 2021 has been mapped in detail, and the strict similarities to the proto-type very-slow novae HR Del and V723 Cas are discussed. All three novae displayed multiple and short-lasting maxima while for months lingering around a bright plateau, leading eventually into the final decline and the nebular phase. The decline for all the novae proceeded at the same pace Flux(V)=(t-to)**alpha with alpha=-2.3 Prior to the primary maximum, the emission lines of Nova Cas 2021 were characterized by a slim Voigt profile, and after it all lines became much wider and characterized by a broad central component superimposed on a ever broader pedestal. This transition happened at the same time gamma-ray emission was detected for a few days. Along the 7-month plateau, the line profile displayed a reckless variability at all velocity scales in response to the ever changing brightness of the nova during the secondary maxima. Upon leaving the plateau on day +230, nebular lines appeared and the ionization degree quickly increased, passing from FeII/Balmer/HeI through HeII/Bowen and then to [CaV]/[NeV]/[FeVII]. After about day +550, the profiles stopped evolving, freezing their aspect. At later epochs, the profiles of all emission lines turned densely castellated, with all dents stable in radial velocity at 1 km/s over the last four months.