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The Unusual Temporal and Spectral Evolution of the Type IIn Supernova 2011ht

We present very early UV to optical photometric and spectroscopic observations of the peculiar Type IIn supernova (SN) 2011ht in UGC 5460. The UV observations of the rise to peak are only the second ever recorded for a Type IIn SN and are by far the most complete. The SN, first classified as a SN impostor, slowly rose to a peak of M_V \sim -17 in \sim55 days. In contrast to the \sim2 magnitude increase in the v-band light curve from the first observation until peak, the UV flux increased by >7 magnitudes. The optical spectra are dominated by strong, Balmer emission with narrow peaks (FWHM\sim600 km/s), very broad asymmetric wings (FWHM\sim4200 km/s), and blue shifted absorption (\sim300 km/s) superposed on a strong blue continuum. The UV spectra are dominated by FeII, MgII, SiII, and SiIII absorption lines broadened by \sim1500 km/s. Merged X-ray observations reveal a L_(0.2-10)=(1.0+/-0.2)x10^(39) erg/s. Some properties of SN 2011ht are similar to SN impostors, while others are comparable to Type IIn SNe. Early spectra showed features typical of luminous blue variables at maximum and during giant eruptions. However, the broad emission profiles coupled with the strong UV flux have not been observed in previous SN impostors. The absolute magnitude and energetics (~2.5x10^(49) ergs in the first 112 days) are reminiscent of normal Type IIn SN, but the spectra are of a dense wind. We suggest that the mechanism for creating this unusual profile could be a shock interacting with a shell of material that was ejected a year before the discovery of the SN.