A type IIn supernova with coronal lines in the low-metallicity compact
  dwarf galaxy J1320+2155
  We report the discovery of a type IIn supernova in the low-metallicity dwarf
galaxy J1320+2155, with an oxygen abundance 12+logO/H = 8.0+/-0.2. This finding
is based on SDSS (February 2008) and 3.5m Apache Point Observatory (February
2009) spectra taken one year apart, and on the observations that: the Hbeta and
Halpha emission lines show broad components corresponding to gas expansion
velocities of ~1600 km/s; the Balmer decrement is exceeedingly high: the
Halpha/Hbeta flux ratio, being more than 30, implies a very dense environment
(>10^7 cm^-3); and the Halpha 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 supernova. 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 supernova
exhibiting coronal lines in its spectrum.
