THE UNUSUAL TEMPORAL AND SPECTRAL EVOLUTION OF THE TYPE IIn SUPERNOVA 2011ht

Abstract

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