February 28, 2017

Discovery of 56Co gamma ray emission from a Type Ia supernova

Analysing the observations of the type Ia supernova SN2014J with the SPI spectrometer and the ISGRI / IBIS imager on the Integral space observatory (International Gamma-Ray Laboratory of the European Space Agency), a team of researchers, including researchers from France, discovered low-energy gamma-ray emission lines (photons between 100 keV and 10 MeV). This is the first time that such radiation has been observed from this type of supernova.

 KAIT/LOSS - W. Zheng and A. Filippenko, UC Berkeley
Images of the M82 galaxy taken between 6 and 23 January 2014.
The supernova is visible above the arrow from 18 January.
Credits: KAIT/LOSS - W. Zheng and A. Filippenko, UC Berkeley

The supernova was optically discovered 21 January 2014 in the M82 galaxy, at a distance of 3.5 Megaparsec. It was quickly identified as a Type Ia supernova, which results from the thermonuclear explosion of a white dwarf in a close binary system. This is the closest type Ia supernova detected in almost 40 years.

By analysing data from the Integral observatory, acquired between 50 and 100 days after the explosion, the researchers observed nuclear gamma-ray lines at 847 keV and 1,238 keV which are the signature of the radioactive decay of 56Co nuclei whose half-life is 77 days. This observation is direct evidence of the nucleosynthesis of a significant amount of 56Ni nuclei during the explosion of the star. The 56Ni decreases rapidly with a half-life of 6 days to 56Co, which in turn decreases to iron nuclei (56Fe) while emitting the observed nuclear gamma-ray lines. The Integral observatory also detected continuum gamma-ray emission due to Compton scattering (interaction between an incident photon and atom, resulting in the deviation and modification of the photon energy, much like an elastic collision) of these gamma rays in the expanding matter of the supernova.

SN2014J supernova spectrum measured by the instruments SPI (red) and ISGRI / IBIS (blue) onboard the space observatory INTEGRAL. The black curve presents a theoretical model fitted to the measured spectrum.
SN2014J supernova spectrum measured by the SPI (red)
and ISGRI / IBIS (blue) instruments onboard the Integral space observatory.
The black curve presents a theoretical model fitted to the measured spectrum.

The Integral observations suggest that 0.6 solar mass of radioactive 56Ni was synthesized during the explosion. Gamma-ray spectral analysis provides an estimate of the ejecta speed of about 10,000 km/s. The properties of the observed radiation are consistent with a scenario in which a massive white dwarf explodes after enough matter has been accreted from a companion star to become gravitationally unstable. It is, however, not possible to exclude scenarios in which the supernova is due to the merger of two white dwarfs.

According to nucleosynthesis models, supernova explosions are the main source of iron in the Universe. The Integral observations have now confirmed this hypothesis. They also provide valuable information to understand the physics of the explosion of Type Ia supernovae, these very singular objects, particularly used as "standard candles" to measure distances in the Universe.

Reference:

  • "Cobalt-56 gamma-ray emission lines from the type Ia supernova 2014J", Churazov et al., Nature, August 28, 2014 doi:10.1038/nature13672.