Type-IIn supernovae (SNe), which are characterized by strong interaction
of their ejecta with the surrounding circumstellar matter (CSM), provide
a unique opportunity to study the mass-loss history of massive stars
shortly before their explosive death. We present the discovery and
follow-up observations of a Type IIn SN, PTF 09uj, detected by the
Palomar Transient Factory (PTF). Serendipitous observations by GALEX at
ultraviolet (UV) wavelengths detected the rise of the SN light curve
prior to the PTF discovery. The UV light curve of the SN rose fast, with
a time scale of a few days, to a UV absolute AB magnitude of about
-19.5. Modeling our observations, we suggest that the fast rise of the
UV light curve is due to the breakout of the SN shock through the dense
CSM (n~10^10 cm^-3). Furthermore, we find that prior to the explosion
the progenitor went through a phase of high mass-loss rate (~0.1 solar
mass per year) that lasted for a few years. The decay rate of this SN
was fast relative to that of other SNe IIn.
