A type IIn supernova with coronal lines in the low-metallicity compact dwarf galaxy J1320+2155

TL;DR

This paper reports the discovery of a type IIn supernova in a low-metallicity dwarf galaxy, characterized by broad emission lines, high-density environment, slow luminosity decline, and the presence of coronal lines indicating shock-induced ionization.

Contribution

First detection of a type IIn supernova with coronal lines in a low-metallicity dwarf galaxy, expanding understanding of supernova environments in such galaxies.

Findings

Broad emission lines indicating gas velocities of ~1600 km/s

High Balmer decrement implying dense environment (>10^7 cm^-3)

Presence of coronal lines [Fe VII] and [Fe X] indicating shock ionization

Abstract

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.