For the first time, astronomers have uncovered two supernovae explosions with properties similar to a gamma-ray burst but without the gamma rays, leading them to suspect that newborn black holes are providing the extra boost.

High-speed jets, traveling at nearly the speed of light, are usually only associated with gamma-ray bursts, resulting from the dramatic finale of a massive star's life. Once the star has run out fuel it collapses into a neutron star or black hole, blasting the rest of its material out into space in a supernova explosion. Around one in one hundred core-collapse supernovae produce gamma-ray bursts, but the most common type of supernovae blasts the star’s material outward in a roughly-spherical pattern at speeds of just three percent of the speed of light.

“In every respect, these objects look like gamma-ray bursts, except that they produced no gamma rays,” says Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics. Soderberg's team studied supernova explosion SN 2009bb, which was discovered in March last year in the spiral galaxy NGC 3278, located about 130 million light-years away. The group found that their radio observations showed material racing away from the heart of the explosion at around 85 percent the speed of light.

Another supernova, SN 2007gr, was discovered in August 2007 in the 35 million light year distant galaxy NGC 1058, and studied by a team of astronomers including Chryssa Kouveliotou, Alexander van der Horst and Zsolt Paragi. The fastest outflows from this supernova reached around 60 percent that of the speed of light, but yet searching through archival data for possible gamma-ray signals revealed none.

It is very unusual that such low-energy radiation – radio waves – can signal a very high-energy event, and the only way to explain the observations, says Soderberg, is by a central engine powering the high speed jets. In this scenario, material falling toward the core enters a swirling disc surrounding the new neutron star or black hole, which produces jets of material accelerated from the poles of the disc.

Until now, no such engine-driven supernova had been found any way other than by detecting gamma rays emitted by it. “Discovering such a supernova by observing its radio emission, rather than through gamma rays, is a breakthrough,” says Soderberg. With the new capabilities of the Expanded VLA coming soon, the astronomers believe they will be able to find more examples in the future through radio observations than with gamma-ray satellites.

The absence of gamma-rays in these explosions is still a mystery. “We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen,” suggests Soderberg. Another possibility is that the gamma rays were ‘smothered’ as they tried to escape the star. “This is perhaps the more exciting possibility since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites.

The scientists hope that future observations will find what causes the difference between the “ordinary” and the “engine-driven” core-collapse supernovae. “There must be some rare physical property that separates the stars that produce the ‘engine-driven’ blasts from their more-normal cousins,” says Soderberg. “We’d like to find out what that property is.”

One popular idea is that such stars have an unusually low concentration of elements heavier than hydrogen, but this does not appear to be the case in this study. “We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” comments Kouveliotou. “These likely represent most of the population.”

The observations were made using the National Science Foundation’s Very Large Array (VLA) radio telescope, the Robert C. Byrd Green Bank Telescope in West Virginia, the European Very Long Baseline Interferometry Network and NASA's space-based Swift observatory, and the results published in the current issue of the journal Nature.

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