  We present the Pan-STARRS1 discovery of the long-lived and blue transient
PS1-11af, which was also detected by GALEX with coordinated observations in the
near-ultraviolet (NUV) band. PS1-11af is associated with the nucleus of an
early-type galaxy at redshift z=0.4046 that exhibits no evidence for star
formation or AGN activity. Four epochs of spectroscopy reveal a pair of
transient broad absorption features in the UV on otherwise featureless spectra.
Despite the superficial similarity of these features to P-Cygni absorptions of
supernovae (SNe), we conclude that PS1-11af is not consistent with the
properties of known types of SNe. Blackbody fits to the spectral energy
distribution are inconsistent with the cooling, expanding ejecta of a SN, and
the velocities of the absorption features are too high to represent material in
homologous expansion near a SN photosphere. However, the constant blue colors
and slow evolution of the luminosity are similar to previous optically-selected
tidal disruption events (TDEs). The shape of the optical light curve is
consistent with models for TDEs, but the minimum accreted mass necessary to
power the observed luminosity is only ~0.002M_sun, which points to a partial
disruption model. A full disruption model predicts higher bolometric
luminosities, which would require most of the radiation to be emitted in a
separate component at high energies where we lack observations. In addition,
the observed temperature is lower than that predicted by pure accretion disk
models for TDEs and requires reprocessing to a constant, lower temperature.
Three deep non-detections in the radio with the VLA over the first two years
after the event set strict limits on the production of any relativistic outflow
comparable to Swift J1644+57, even if off-axis.
