ASASSN-13dn: A luminous and double-peaked type II supernova

Aim. We present observations of ASASSN-13dn, one of the first supernovae discovered by ASAS-SN and a new member of the rare group of known type II luminous supernovae (LSNe). It was discovered near maximum light, reaching an absolute magnitude of M v ∼ −19 mag, placing this object between normal luminosity type II supernovae and superluminous supernovae. Methods. We performed a detailed analysis of the photometric and spectroscopic data of ASASSN-13dn. The spectra are characterized by broad lines, in particular the H α lines, where we measure expansion velocities ranging between 14 000 and 6000 km s −1 over the first 100 days. H α dominates the nebular spectra, and we detect a narrow P-Cygni absorption within the broader emission line with an expansion velocity of 1100 km s −1 . Photometrically, its light curve shows a re-brightening of ∼0.6 mag in the gri bands starting 25 ± 2 days after discovery, with a secondary peak at ∼73d followed by an abrupt and nearly linear decay of 0.09 mag d −1 for the next 35 days. At later times, after a drop of 4 magnitudes from the second maximum, the light curves of ASASSN-13dn show softer undulations from 125 to 175 days. Results. We compared ASASSN-13dn with other type II LSNe in the literature, finding no match to either the light curve or spectroscopic properties. We discuss the main powering mechanism and suggest that interaction between the ejecta and a dense circumstellar material produced by eruptions from a luminous blue variable-like progenitor could potentially explain the observations.