A composite image of the supernova remnant SN 1006 combining data from different wavelengths: Radio (red), X-ray (blue) and optical (yellow, orange and light blue). The region of the shock front studied in detail by Nikolic et al. is inside the green rectangle.
Credit: X-ray: NASA/CXC/Rutgers/G.Cassam-Chenaï, J.Hughes et al.; Radio: NRAO/AUI/NSF/GBT/VLA/Dyer, Maddalena & Cornwell; Optical: Middlebury College/F. Winkler, NOAO/AURA/NSF/CTIO Schmidt & DSS
Such remnants have long been thought to be the source of cosmic ray particles hitting Earth. The observations show, for the first time, the presence of "seed particles", possible precursors of such cosmic rays. The novel approach used by the astronomers promises further insights as to how supernovae remnants act as cosmic particle accelerators. The results will be published on February 14, 2013 in the journal Science.
When Victor Hess first discovered cosmic ray particles hitting Earth almost exactly a hundred years ago, he had little notion about their origin. Since then, ever more sensitive observations of these particles have turned up a number of sources. Among them are supernova remnants – cosmic blast waves launched by stellar explosions; expanding gas shells flung into space when certain stars end their life in a supernova.
Where such a blast wave meets the surrounding interstellar medium, there is an abrupt change in density and temperature: a shock front similar to the sonic boom produced by an aircraft going supersonic. This expanding, high-velocity shock front is a natural candidate for a cosmic particle accelerator. Now, for the first time, astronomers have found observational evidence of accelerated protons in these shock regions. While these are not the sought-for high-energy cosmic rays themselves, they could be the necessary "seed particles”, which the go on to interact with the shock to reach the extremely high energies required and fly off into space as cosmic ray particles.
Nikoliæ explains: "This is the first time we were able to take a detailed look at the microphysics in and around the shock region. We found evidence for a precursor region directly in front of the shock, which is thought to be a prerequisite of cosmic ray production. Also, the precursor region is being heated in just the way one would expect if there were protons carrying away energy from the region directly behind the shock."
The new evidence emerged during careful observational analysis by the Serbian astronomer Sladjana Nikoliæ (Max Planck Institute for Astronomy) as part of her work towards a doctoral degree at the University of Heidelberg. The key to the new result is a modern technique used by Nikoliæ and her colleagues, known as integral field spectroscopy, which allows astronomers to simultaneously probe the composition of light received from numerous locations within their telescope's field of view. This is the first time the technique has been applied successfully to a supernova remnant.
Nikoliæ and her colleagues used the spectrograph VIMOS at the European Southern Observatory's Very Large Telescope in Chile to map a small part of the shock front of the supernova SN 1006, simultaneously analyzing light ("taking spectra") from more than 100 locations within that region. Analysis of the data – one and a half years' worth of hard work – provided detailed information about the way that hydrogen atoms in the region are being excited in and around the shock front, and of the temperatures in front of and behind the shock.
Kevin Heng of the University of Bern, one of the supervisors of Nikoliæ's doctoral work, says: "We are particularly proud of the fact that we managed to use integral field spectroscopy in a rather unorthodox way, since it is usually used for the study of high-redshift galaxies. In doing so, we achieved a level of precision that far exceeds all previous studies."
The study is also important because it opens up avenues for further research. Nikoliæ explains: "This was a pilot project. The emissions we observed from the supernova remnant are very, very faint compared to the usual target objects for this type of instrument. Now that we know what's possible, it's really exciting to think about follow-up projects." Glenn van de Ven of the Max Planck Institute for Astronomy, Nikoliæ's other co-supervisor and an expert in integral field spectroscopy, adds: "This kind of novel observational approach could well be the key to solving the puzzle of how cosmic rays are produced in supernova remnants."
Contact informationSladjana Nikoliæ (lead author)
Dr. Markus Pössel | Max-Planck-Institut
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