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K. Hatano (University of Tokyo), D. Branch (University of Oklahoma), K. Nomoto, J. S. Deng, K. Maeda (University of Tokyo), P. Nugent, G. Aldering (Lawrence Berkeley National Laboratory)
SN~1999as was discovered on February 18, 1999 during the LBNL 1999 nearby supernova campaign, in an anonymous galaxy having a redshift of 0.127. The absolute magnitude was exceptionally bright, at least -21.4 --- at least nine time brighter than the hypernova SN~1998bw, which apparently was associated with a gamma--ray burst.
The light curve and the photospheric velocities have been computed with a one-dimensional spherically symmetric radiative transfer code. From fitting the observed light curve, we have obtained the following constraints on the explosion model: the ejected mass M\rm ej {~ 10 - 20 M\odot, the kinetic energy of ejected matter E\rm K {~ 1052 - 1053 ergs, and the mass of ejected radioactive nickel M_{Ni} > 4 M_{\odot}. SN~1999as is spectroscopically classified as a type Ic supernova because its photospheric phase spectra show no conspicuous lines of hydrogen, He~I, or Si II \lambda6355. However, in addition to the usual supernova SN~Ic spectral lines such as Ca~II and O~I, some narrow (~~2000 km~s-1) but highly blueshifted (~ 11,000 km~s-1) lines of Fe~II are present. We present the results of a direct spectroscopic analysis using the parameterized supernova spectrum synthesis code, SYNOW, as well as a comparison with other type Ic hypernovae, SNe~1997ef and 1998bw.
We suggest that the 2--dimensional asymmetric hydrodynamical model of Maeda et al. (astro-ph/0011003) could represent this new class of hypernova. In the Maeda model, a clump of freshly synthesized 56Ni exists at high velocity (~~15,000 km~s-1) near the symmetry axis; such a clump could produce narrow but high--velocity absorption lines if the viewing angle with respect to the symmetry axis is small.
All of the observed spectroscopic and light curve data were obtained during the LBNL 1999 nearby supernova campaign.