  Transient surveys have recently revealed the existence of H-rich
super-luminous supernovae (SLSN; e.g., iPTF14hls, OGLE-SN14-073) characterized
by an exceptionally large time-integrated bolometric luminosity, a sustained
blue optical color, and Doppler-broadened HI lines at all times. Here, I
investigate the effect that a magnetar (initial rotational energy of 4x10^50erg
and field strength of 7x10^13G) would have on the properties of a typical Type
II SN ejecta (mass of 13.35Msun, kinetic energy of 1.32x10^51erg, 0.077Msun of
56Ni) produced by the terminal explosion of an H-rich blue-supergiant star. I
present a non-LTE time-dependent radiative transfer simulation of the resulting
photometric and spectroscopic evolution from 1d until 600d after explosion.
With magnetar power, the model luminosity and brightness are enhanced, the
ejecta is everywhere hotter and more ionised, and the spectrum formation region
is much more extended. This magnetar-powered SN ejecta reproduces most of the
observed properties of SLSN iPTF14hls, including the sustained brightness of
-18mag in the R band, the blue optical color, and the broad HI lines for 600d.
The non-extreme magnetar properties, combined with the standard Type II SN
ejecta properties offer an interesting alternative to the pair-unstable
super-massive star model recently proposed, which involves a highly-energetic
and super-massive ejecta. Hence, such Type II SLSNe may differ from standard
Type II SNe exclusively through the influence of a magnetar.
