DISCOVERY AND EARLY MULTI-WAVELENGTH MEASUREMENTS OF THE ENERGETIC TYPE IC SUPERNOVA PTF12GZK: A MASSIVE-STAR EXPLOSION IN A DWARF HOST GALAXY

We present the discovery and extensive early-time observations of the Type Ic\nsupernova (SN) PTF12gzk. Our finely sampled light curves show a rise of 0.8mag\nwithin 2.5hr. Power-law fits [f(t)\\sim(t-t_0)^n] to these data constrain the\nexplosion date to within one day. We cannot rule out the expected quadratic\nfireball model, but higher values of n are possible as well for larger areas in\nthe fit parameter space. Our bolometric light curve and a dense spectral\nsequence are used to estimate the physical parameters of the exploding star and\nof the explosion. We show that the photometric evolution of PTF12gzk is slower\nthan that of most SNe Ic, and its high ejecta velocities (~30,000km/s four days\nafter explosion) are closer to the observed velocities of broad-lined SNe Ic\nassociated with gamma-ray bursts (GRBs) than to the observed velocities in\nnormal Type Ic SNe. The high velocities are sustained through the SN early\nevolution, and are similar to those of GRB-SNe when the SN reach peak\nmagnitude. By comparison with the spectroscopically similar SN 2004aw, we\nsuggest that the observed properties of PTF12gzk indicate an initial progenitor\nmass of 25-35 solar mass and a large (5-10E51 erg) kinetic energy, close to the\nregime of GRB-SN properties. The host-galaxy characteristics are consistent\nwith GRB-SN hosts, and not with normal SN Ic hosts as well, yet this SN does\nnot show the broad lines over extended periods of time that are typical of\nbroad-line Type Ic SNe.\n