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.
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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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