Particles with TeV energies, like those produced at the LHC, seem exotic. But once outside the protection of our atmosphere, these “cosmic rays” (CR) become exceedingly common. The Fermi Telescope, for example, encounters a hundred thousand CR for every gamma ray it detects. These particles have an impressive scope of local effects, from damaging electronics and inhibiting manned space travel to possibly triggering lightning strikes. And although we have been aware of their existence since the early 1900ʼs, their exact origins remained unclear. Now a study by
the Fermi Collaboration claims to have solved the mystery.
The list of mechanisms capable of accelerating so many particles to high
energies was already a short one. Strong electromagnetic fields, like those surrounding pulsars, could do the job. So could the process for which the gamma-ray telescope got its name – Fermi acceleration. Less intuitive, it amounts to trapping particles in a region where they are repeatedly reflected by inhomogeneous magnetic fields. Each reflection supplies them with more energy, and the series of small enhancements adds up. One place there such conditions exist is at the boundary between slow and fast moving groups of particles, sometimes known as a “shock.” Supernova remnants (SNR), the
aftermath of massive astrophysical explosions, are expected to shock where the expanding sphere of stellar material encounters the surrounding gas.
Discerning which (if either) of these objects produce CR is a difficult task. Except at the very highest energies, CR do not travel in straight lines through the galaxy. This means they reach Earth with no positional information about their origin. They can interact along their way, however, and so leave a trail that we might follow. Physicists found the start of such a trail last year (ref W44), where molecular clouds surrounding the SNR known as W44 displayed the neutral pion decay spectrum characteristic of proton-proton (i.e. CR-hydrogen) collisions.
Taking advantage of accumulating statistics and a re-vamped instrument model, the Fermi team took a closer look at W44 and a similar system known as IC 443. The detection of a pion-decay spectrum with sufficient intensity emanating from the innermost regions of these objects would prove that CR are in fact accelerated from within the SNR. In order to distinguish the spectrum from other possible signals, the study focuses on gamma rays below 200 MeV, where the pion spectrum falls off with characteristic sharpness. Here, they compared fits with a single power law and a smoothly broken one to measure this feature in the data.
The results are clear: the spectrum emanating from the central regions of both SNR strongly favor the broken power law. This means there is indeed a sharp drop below 200 MeV, just as one would expect from CR-gas interaction. While not the only possible explanation of the signal*, it is the simplest. Caveats aside, this is the best direct evidence yet regarding the origins of these exotic-yet-commonplace particles.
* Leptonic (e.g. electrons and positrons) particles could also interact with the clouds inside the SNR to produce the observed spectrum. For this to work, however, the initial lepton population would need to be configured in an ad-hoc manner for which we have no physical justification.
– Brandon Anderson – OKC fellow