2009ApJ...707.1560I

We report the discovery of a Type IIn supernova (SN) in the low-metallicity dwarf galaxy J1320+2155, with an oxygen abundance 12 + log O/H = 8.0±0.2. This finding is based on Sloan Digital Sky Survey (SDSS; 2008 February) and 3.5 m Apache Point Observatory (2009 February) spectra taken one year apart, and on the observations that: the Hβ and Hα emission lines show broad components corresponding to gas expansion velocities of ∼1600 km/s; the Balmer decrement is exceedingly high: the Hα/Hβ flux ratio, being more than 30, implies a very dense environment (>10⁷/cm3); and the Hα broad luminosity decreases slowly, by only a factor of ∼1.8 over the course of a year, typical of the slow luminosity evolution of a Type IIn SN. Several weak coronal lines of [Fe VII] and [Fe X] are also seen in the SDSS spectrum, implying ionization of the pre-shock circumstellar medium by shock-induced X-ray emission. The galaxy J1320+2155 is the first dwarf system ever to be discovered with a Type IIn SN exhibiting coronal lines in its spectrum.