January 27, 2010
Astronomers Catch Supernova, Observe Relativistic Expansion
An international team of scientists, including several astronomers from the Joint Institute for VLBI in Europe (JIVE) and the Netherlands Institute for Radio Astronomy (ASTRON), both located in Dwingeloo, have observed a supernova with peculiar radio emission. In a paper to be published in the January 28, 2010 issue of Nature, the team led by JIVE's Zsolt Paragi reports, for the first time ever, detection of a relativistic outflow in a Type Ic supernova, thus supporting the link with the even more energetic Gamma Ray Bursts, some of the most energetic explosions in the Universe.
At the end of its life, the central region of a massive star collapses while its outer layer is expelled in a gigantic explosion. This phenomenon is known as a supernova. Supernova SN 2007gr was discovered in 2007 with the Katzman Automatic Imaging Telescope in California, USA. Optical observations showed that it was Type Ic, known to result from the most massive stars. Supernovae are very distant sources, and the radio emissions they produce fade quickly. Therefore, the highest resolution imaging technique, called Very Long Baseline Interferometry (VLBI), is required to receive the extremely faint emission and reveal the details of the explosion process. Because SN 2007gr was located in a relatively nearby galaxy, closer than any other Type Ic supernovae detected in the radio spectrum, it offered a unique opportunity to study this phenomenon.With the VLBI technique, radio telescopes located up to thousands of kilometers from each other carry out measurements synchronously. Paragi's team exploited the electronic, real-time VLBI (e-VLBI) capabilities of the European VLBI Network (EVN), by which the data are streamed in real-time from the telescopes to the central data processor at JIVE. Rapid analysis of the SN 2007gr data, obtained 22 days after the initial discovery, showed that the source was still visible in the radio spectrum, and confirmed the technical feasibility of radio observations. Based on this result, the team carried out further observations with the EVN and the Green Bank Telescope in West Virginia, USA. For the first time ever, they were able to show mildly relativistic expansion in such a source.
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 that was ejected in the explosion reached a velocity at least half the speed of light. According to the emerging picture, this mildly relativistic matter was collimated into a bipolar narrow cone, or jet. The team concludes that it is possible that all, or at least most, Type Ic supernovae produce bipolar jets, but the energy content of these mildly-relativistic outflows varies dramatically, while the total energy of the explosions is much more standard.
"At least a fraction of Type Ic supernovae have been thought for a long time to produce highly collimated relativistic jets," says Paragi. "Our observations support this and provide new clues for the understanding of how supernovae explode, and how some of them may be related to the even more energetic gamma ray bursts."
The Westerbork Synthesis Array Telescope, operated by ASTRON, played an important role in obtaining this result due to its large collecting area, which significantly improved the sensitivity of the VLBI observations. Moreover, it provided an independent measurement of the total flux density, or brightness, of the source.
These observations also showcase how the new e-VLBI services of the EVN empower astronomers to react quickly when transient events occur. "Organizing VLBI observations on a short timescale with the most sensitive radio telescopes on Earth is a challenging task," notes JIVE director Huib Jan van Langevelde. "Using the electronic-VLBI technique eliminates some of the major issues. Moreover, it allows us to produce immediate results necessary for the planning of additional measurements. The scientific outcome from the SN 2007gr observations shows the impact of the technological development in our field in the last few years, which allows highly efficient collaboration between radio telescopes within and even outside of Europe."
Image 1: Initial e-VLBI detection of SN 2007gr with the EVN on September 6-7, 2007 (colors). The EVN and Green Bank Telescope VLBI image obtained on November 5-6, 2007 is overlaid (contour representation). By the time of the second observation the source had expanded and was no longer consistent with an unresolved object as bright as indicated by the independent WSRT measurements. At the distance of the supernova this is consistent with an expansion velocity higher than half of the speed of light.
Image 2: Telescopes that participated in the e-VLBI observations of SN2007gr, and the network paths by which data is streamed to the correlator at JIVE.
Image 3: Westerbork Synthesis Radio Telescope, located near Westerbork, the Netherlands.
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