  We present optical photometric and spectroscopic coverage of the
superluminous supernova (SLSN) PS1-11ap, discovered with the Pan-STARRS1 Medium
Deep Survey at z = 0.524. This intrinsically blue transient rose slowly to
reach a peak magnitude of M_u = -21.4 mag and bolometric luminosity of 8 x
10^43 ergs^-1 before settling onto a relatively shallow gradient of decline.
The observed decline is significantly slower than those of the superluminous
type Ic SNe which have been the focus of much recent attention. Spectroscopic
similarities with the lower redshift SN2007bi and a decline rate similar to
56Co decay timescale initially indicated that this transient could be a
candidate for a pair instability supernova (PISN) explosion. Overall the
transient appears quite similar to SN2007bi and the lower redshift object
PTF12dam. The extensive data set, from 30 days before peak to 230 days after,
allows a detailed and quantitative comparison with published models of PISN
explosions. We find that the PS1-11ap data do not match these model explosion
parameters well, supporting the recent claim that these SNe are not pair
instability explosions. We show that PS1-11ap has many features in common with
the faster declining superluminous Ic supernovae and the lightcurve evolution
can also be quantitatively explained by the magnetar spin down model. At a
redshift of z = 0.524 the observer frame optical coverage provides
comprehensive restframe UV data and allows us to compare it with the
superluminous SNe recently found at high redshifts between z = 2-4. While these
high-z explosions are still plausible PISN candidates, they match the
photometric evolution of PS1-11ap and hence could be counterparts to this lower
redshift transient.
