The origin of Galactic cosmic-ray ions has remained an enigma for almost a century. Although it has generally been thought that they are accelerated in the shock waves associated with powerful supernova explosions-for which there have been recent cla
ims of evidence-the mystery is far from resolved. In fact, we may be on the wrong track altogether in looking for isolated regions of cosmic-ray acceleration.
For more than fifty years, it has been believed that cosmic ray (CR) nuclei are accelerated to high energies in the rapidly expanding shockwaves created by powerful supernova explosions. Yet observational proof of this conjecture is still lacking. Re
cently, Uchiyama and collaborators reported the detection of small-scale X-ray flares in one such supernova remnant, dubbed RX J1713-3946 (a.k.a. G347.3-0.5), which also emits very energetic, TeV (10^12 eV) range, gamma-rays. They contend that the variability of these X-ray hotspots implies that the magnetic field in the remnant is about a hundred times larger than normally assumed; and this, they say, means that the detected TeV range photons were produced in energetic nuclear interactions, providing a strong argument for acceleration of protons and nuclei to energies of 1 PeV (10^15 eV) and beyond in young supernova remnants. We point out here that the existing multiwavelength data on this object certainly do not support such conclusions. Though intriguing, the small-scale X-ray flares are not the long sought-after smoking gun of nucleonic CR acceleration in SNRs.
I comment on the -- apparent -- diffuse X-ray emission reported by Horns et al. in their XMM observations of TeV J2032+4130
