An international team of scientists, including Dr. Jonathan Granot from the University of Hertfordshire in the UK, has studied a supernova explosion (known as SN 2007gr) of a star and have found long-sought evidence that certain types of supernovae produce "relativistic" jets of particles, travelling at more than half the speed of light.
This discovery, which is to be reported in a paper to be published in the 28 January 2010 issue of Nature was made by a team, led by Dr. Zsolt Paragi from the Joint Institute for VLBI in Europe (JIVE) in the Netherlands.
The team reports for the first time ever, detection of a relativistic outflow in a Type Ic supernova without a gamma-ray burst. Previously, the only supernova explosions in which such relativistic jets were observed were of the same kind (Type Ic), but produced gamma-ray bursts -- the universe's most luminous explosions, and the supernovae were detected only after and thanks to the gamma-ray bursts.
Supernovae are very distant sources, and their radio emission fades quickly. Therefore, the highest angular resolution imaging technique, called Very Long Baseline Interferometry (VLBI), is required to receive the extremely faint emission and reveal the details of the explosion aftermath. The electronic VLBI (e-VLBI) capabilities of the European VLBI Network (EVN) enabled an early first observation of SN 2007gr with this sensitive array, and allowed the team to timely perform more observations, which provided direct evidence for expansion of the radio image, and determined an apparent expansion velocity of at least 60 percent of the speed of light.
The relativistic radio emitting material carried only about 0.01 percent of the total energy in the supernova explosion -- much lower than in the relativistic jets of gamma-ray bursts, suggesting that SN 2007gr did not produce a gamma-ray burst.
These new observations suggest a broad continuous distribution in the properties of such core collapse supernovae. "The intrinsically dim radio emission of SN 2007gr and its small distance from us imply that such events, with mildly relativistic jets carrying a very small fraction of the total explosion energy, may represent most of the population" said Dr. Granot at the University of Hertfordshire's School of Physics Astronomy and Mathematics.
"The radio luminosity of some nearby, dim gamma-ray bursts is somewhat larger than that of SN 2007gr, suggesting a gradual increase in the energy and speed of the radio emitting jets, as their fraction of the total population decreases," he added.
The most extreme (and correspondingly rare -- about one out of a few thousand such core collapse supernovae) events in this continuum are the bright gamma-ray bursts, which are typically observed from billions of light-years away, and whose jets move at more than 99.995 percent of the speed of light and have energies similar to or even larger than that of typical supernovae.
- Paragi et al. A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr. Nature, 2010; 463 (7280): 516 DOI: 10.1038/nature08713
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