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.
