The unusual temporal and spectral evolution of the typeiin supernova 2011ht

We present very early UV to optical photometric and spectroscopic observations of the peculiar TypeIIn supernova (SN) 2011ht in UGC5460. The UV observations of the rise to peak are only the second ever recorded for a TypeIIn SN and are by far the most complete. The SN, first classified as an SN impostor, slowly rose to a peak of M_V -17 in 55days. In contrast to the 2mag increase in the v-band light curve from the first observation until peak, the UV flux increased by >7mag. The optical spectra are dominated by strong, Balmer emission with narrow peaks (FWHM 600kms^-1), very broad asymmetric wings (FWHM 4200kms^-1), and blueshifted absorption (300kms^-1) superposed on a strong blue continuum. The UV spectra are dominated by Fe II, Mg II, Si II, and Si III absorption lines broadened by 1500kms^-1. Merged X-ray observations reveal a L _0.2-10 = (1.0 ± 0.2) × 10³⁹ergs^-1. Some properties of SN 2011ht are similar to SN impostors, while others are comparable to TypeIIn 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 (2.5 × 10⁴⁹erg in the first 112days) are reminiscent of normal TypeIIn 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.