  A type Ia supernova is thought to be a thermonuclear explosion of either a
single carbon-oxygen white dwarf or of a pair of merging white dwarfs. The
explosion fuses a large amount of radioactive 56Ni. After the explosion, the
decay chain from 56Ni to 56Co to 56Fe generates gamma-ray photons, which are
reprocessed in the expanding ejecta and give rise to powerful optical emission.
Here we report the detection of 56Co lines at energies of 847 and 1238 keV and
a gamma-ray continuum in the 200-400 keV band from the type Ia supernova 2014J
in the nearby galaxy M82. The line fluxes suggest that about 0.6 +/- 0.1 solar
masses of radioactive 56Ni were synthesized during the explosion. The line
broadening gives a characteristic mass-weighted ejecta expansion velocity of
10000 +/- 3000 km/s. The observed gamma-ray properties are in broad agreement
with the canonical model of an explosion of a white dwarf just massive enough
to be unstable to gravitational collapse, but do not immediately exclude more
complicated merger scenarios, which fuse comparable amount of 56Ni.
