Newborn Black Holes Boost Explosive Power of Supernovae

An international team of scientists, including two astronomers from NASA's Marshall Space Flight Center in Huntsville, Ala., have observed a supernova with peculiar radio emission. In the Jan. 28 issue of Nature, the team -- led by Zsolt Paragi of the Joint Institute for Very Long Baseline Interferometry in Europe, or JIVE -- reveals new details of these highly energetic explosions.

Supernova SN 2007gr was discovered less than five days after its explosion at a distance of just 35 million light years away -- one of the closest Type Ic supernovae ever seen in radio waves from Earth. Supernovae mark the violent deaths of massive stars via core collapse, followed by a gigantic explosion which expels their outer layers in an expanding fireball. As the explosion ejecta expand, they cool and slow down, and their emission progressively move to longer wavelengths, from X-rays to radio waves. SN 2007gr was close enough and found early enough to be a great candidate for extensive follow-up observations.

"The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent of the speed of light," said Chryssa Kouveliotou, an astrophysicist at the Marshall Center who co-authored the new study. "Yet in this new object we're tracking gas moving some 20 times faster than this."

Type Ic supernovae also have been associated with another very energetic phenomenon in high energy astrophysics: Gamma Ray Bursts. Though most of the energy in these bursts is released by gamma-ray jets traveling almost at the speed of light no such fast expansion evidence has yet been observed from other Type Ic supernovae.

Kouveliotou quickly put together an international team of radio astronomers. Led by Paragi and JIVE, the team included 14 members from 12 institutions spread over seven countries -- including the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.

The team proposed to use the highest-resolution imaging technique -- Very Long Baseline Interferometry, or VLBI -- to collect the extremely faint emission from SN 2007gr and reveal details of the explosion process. With the VLBI technique, multiple radio telescopes thousands of kilometers from one another carry out measurements simultaneously. The astronomers exploited the electronic VLBI capabilities of the European VLBI Network, by which the data are streamed in real time from the telescopes to the joint institute's central data processor in the Netherlands.

A rapid analysis of the SN 2007gr data, obtained 22 days after initial discovery, showed that the source was still visible in the radio. Based on this result, the team carried out further observations with the European VLBI Network and the Green Bank Telescope in Pocahontas County, W.V. For the first time ever, scientists measured mildly relativistic expansion in such a source.

One other instrument, the Westerbork Synthesis Array Telescope played an important role in obtaining this result due to its large collecting area, which significantly improved the sensitivity of the VLBI observations. Westerbork telescope observations, combined with radio data from the Very Large Array in Socorro, N.M., provided an independent measurement of the total flux density, or brightness, of the source.

In the second VLBI observation, the source was less bright when measured in very high resolution with the global VLBI network of radio telescopes than it was with the Westerbork telescope. The team of radio astronomers concluded that the latter had captured all the radio waves coming from the supernova, while the VLBI zoomed in so much that it only observed part of the source.

"It was the synergy between these radio observatories that led to our discovery," said Alexander van der Horst, a NASA postdoctoral program fellow in Huntsville and a co-author on the Nature article. "Zooming in and out on the supernova quickly led us to the conclusion that the ejecta had to be expanding very fast."

Although it showed peculiar radio properties, SN 2007gr was otherwise a normal type Ic supernova. It appears that only a small fraction of the matter ejected in the explosion reached a velocity of at least half the speed of light -- what scientists call mildly relativistic speed. According to the emerging picture, this mildly relativistic matter was beamed into a bipolar narrow cone, or jet. The team concluded that it is possible that most or all type Ic supernovae produce bipolar jets, but the energy content of these outflows varies dramatically, whereas the total energy of the explosions is much more standard.

"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," Kouveliotou said. "These likely represent most of the population."

These observations showcase how the new electronic capabilities of the European VLBI Network empower astronomers to react quickly when transient events occur.

"Using the electronic VLBI technique eliminates some of the major issues," said Huib Jan van Langevelde, the director of JIVE "Moreover it allows us to produce immediate results necessary for the planning of additional measurements."

"This is a fantastic new facility that has proven to be extremely valuable for studies of supernovae and other transient sources in the radio sky," said Van der Horst.

The scientific outcome from the SN 2007gr observations demonstrates the impact of new technological development in radio astronomy in recent years, enabling highly efficient international collaboration between radio telescopes in the United States, Europe and elsewhere across the globe.