Galactic cosmic rays are immensely high-energy protons and atomic nuclei that impact our atmosphere, originating from outside of our solar system. Where do they come from, and how are they accelerated? These are both open topics of research.
One of the leading theories is that cosmic-ray source material is primarily a mixture of material that has been ejected from massive stars — either from supernovae or in stellar wind outflows — and normal interstellar medium (ISM). This material is then accelerated to cosmic-ray energies by supernova shocks.How can we test this model? An important step is understanding the composition of galactic cosmic rays: what elemental nuclei are they made up of? If abundances are similar to solar-system abundances, then the material is likely mostly ISM. If the abundances of rarer heavy elements are high, however, then the material is more likely to have come from massive stars in star-forming regions.
Enter SuperTIGER, an experiment designed to collect cosmic rays and measure the abundances of the rare heavy elements — those with atomic number between iron (Z=26) and zirconium (Z=40).To gather galactic cosmic rays, the detector must be above the Earth’s atmosphere; interactions with the atmosphere cause these particles to decay into lower-energy secondary particles upon impact. In addition, the detector must operate for a long time in order to make meaningful abundance measurements: millions of cosmic-ray detections can result in just a few hundreds of detections of heavy-element nuclei.
The SuperTIGER team solved these problems by flying their instrument on a high-altitude scientific balloon at 127,000 feet. The project launched in Antarctica, taking advantage of the wind patterns circulating around the pole to maximize the flight time. In the end, SuperTIGER flew for 55 days, shattering the record for longest flight of a heavy-lift scientific balloon (which was previously 42 days). During this time, the experiment detected a whopping 50 million cosmic rays.
Pinning Down AbundancesNow, in a publication led by Ryan Murphy (Washington University in St. Louis), the team has detailed the results of SuperTIGER’s observations. Their measurements represent the tightest constraints on the abundances of galactic cosmic ray nuclei made in this charge and energy range, and the abundances are consistent with a model in which source material consists of a mixture of 19% material from massive stars and 81% normal ISM.
These results support the idea that a significant fraction of cosmic-ray material originates in associations of young, massive stars, and that the remainder is made up of solar-system-abundance ISM that’s injected and accelerated — possibly by supernova shocks — into cosmic rays.
R. P. Murphy et al 2016 ApJ 831 148. doi:10.3847/0004-637X/831/2/148