Hundred years after Victor Hess discovered cosmic rays in 1912, astronomers have finally found proof of what they had always suspected — but could not find direct evidence — that these highly energetic particles, which are constantly bombarding the Earth’s atmosphere from all directions, originate in the aftermath of exploding stars, or supernovae as they are called, which are the most energetic events in the galaxy.
Importantly, the proof is doubly strengthened as the evidence has come from two different experiments — one space-based and the other ground-based — which have looked at different supernova remnants (SNRs) in different parts of the sky and interpreted their observations from different physical perspectives. The results of both experiments have been published in the latest issue of Science.
The first is based on years of data from two 10,000-year-old SNRs IC443 (which is 5,000 light years away in the constellation Gemini) and W44 (which is 10,000 light years away in the constellation Aquila) gathered by NASA’s Fermi Gamma Ray Telescope (launched in June 2008). The experiment is led by scientists of the Kavli Institute for Particle Physics and Cosmology at the SLAC National Accelerator laboratory.
The second is based on detailed observations of the remnants of a 1,000-year-old supernova SN1006 with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. The research was carried out by astronomers from the Max-Planck Institute for Astronomy, Heidelberg.
Supernovae occur in our galaxy 2 to 3 times a century when a massive star explodes. While the star’s core remains as a neutron star, or a black hole, the rest is ejected into the space in the form of rapidly expanding debris behind a powerful shockwave. As the remnant expands, it gathers the low density interstellar gas (about one particle/cm3) and gradually decelerates but the imprint remains in the sky for thousands of years which is what astronomers study to solve the mystery of cosmic rays.
Cosmic rays mostly comprise protons (about 90%), electrons and other nuclei. The two experiments claim to have found tell-tale evidence of the protons being accelerated to nearly the speed of light by the shockwave that precedes the ejecta from the supernovae. This acceleration occurs due to a mechanism proposed in 1949 by Enrico Fermi — after whom the space telescope has been named — in which protons are trapped in the fast-moving shock region by magnetic fields that travel with the shock front and are boosted to high speeds when they are repeatedly reflected in the magnetic field.
With each round trip, protons gain energy by about 1 per cent. After several tens to hundreds of such round trips, the protons are travelling at nearly the speed of light when they are able to break free from the shockwave front. But because protons get deflected by any magnetic field they encounter on the way to the Earth, and their paths are totally scrambled, tracing them back to their source becomes impossible. It is for this reason that one could not unambiguously say cosmic rays are produced in the SNRs.
So scientists look for indirect signatures of this acceleration. One such signature is the production of gamma rays (high-energy photons), when these speeding protons meet slow-moving protons in the surrounding clouds of gas or dust in the debris. These proton-proton collisions create chargeless particles called neutral pions. The pions, in turn, decay quickly into two gamma ray photons. Unlike protons, photons, being neutral, are unaffected by intervening stellar magnetic fields and travel in straight lines, which can be traced back to the source.
The problem is that there are different processes in the universe that produce gamma rays. But the energy of gamma rays from pion decays have a characteristic energy range arising from the fact that the pion has a rest mass of 135 MeV (in energy units through E=mc 2 relation) which is divided between the two photons. Accordingly, the gamma ray spectrum declines steeply toward lower energies. Detecting the lower-end cut-off would be clear proof that the gamma rays observed by the Fermi Telescope are from the decaying pions formed by protons accelerated within the SNRs.
The SLAC team analysed four years of data collected by the Fermi observatory’s Large Area Telescope (LAT), whose efficiency is just right to look at gamma rays in this distinctive energy band, from the two SNRs, IC 443 and W44 and, by measuring photons down up to 60 MeV, found that the gamma ray photons did have the low energy cut-off.
But such gamma rays can also come from another competing source: ‘electron bremsstrahlung’ where high energy electrons emit gamma rays because of deceleration when they pass atomic nuclei, which can occur within the SNRs.
But the authors, Stefan Funk and colleagues, have modelled the observed gamma ray spectrum based on both the possible processes but find that while the pion decay mechanism fits the spectrum without any further assumptions, electron bremsstrahlung requires an ad hoc assumption to be made with regard to the electron energy spectrum in a way that is not consistent with the radio astronomy observations on the synchrotron radiation from such electrons. This has led them to conclude that the observed gamma ray spectra provide direct evidence for the acceleration of protons in the SNRs.
“The arguments are persuasive, but not yet conclusive,” said Professor Sunil Gupta, a cosmic ray scientist at the Tata Institute of Fundamental research (TIFR).
The other ground-based experiment using the VLT has attempted to actually see the presence of fast-moving protons in the shock region through a different signature. The team, led by Sladjana Nikoloc, has used the instrument called Visible Multi Object Spectrograph (VIMOS) on the VLT to look at the remnant of SN1006 — designated so because it was seen in the south-western skies in the year 1006 in various parts of the world — in more detail than ever before. The scientists wanted to study what exactly happens where the high-speed ejecta is ploughing into the expanding high-velocity shock front. For the first time, the team has not just obtained information about the shock front but also built up a map of the properties of the gas, and how these properties change across the shock front.
In particular, they have looked at the characteristics of a particular emission line of hydrogen (protons are nuclei of hydrogen atoms) called H-alpha Balmer line, which dominate the spectra of hydrogen in the environments of shocks surrounding SNRs. This directly probes the proton populations in the pre-shock and post-shock phases and the broadening of the spectral line is a measure of the velocity distribution of the protons.
Spectroscopic techniques, hitherto used for such observations, have provided rich spectral information, but very little spatial information. Looking at the north-western rim of the SN 1006, the present experiment finds a very high post-shock population of speeding protons (suprathermal protons), which would be the necessary “seed particles” to interact with the shock front material a la Fermi acceleration to reach extremely high energies and fly off as cosmic rays.
Thus, the observations actually tell us that pre-shock hydrogen atoms pass through the shock front and get ionized into protons, and these act as precursors to the high-energy cosmic rays that we receive on the Earth.
