Cosmic rays are very high-energy particles, mainly originating outside the Solar System. They may produce showers of secondary particles that penetrate and impact the Earth's atmosphere and sometimes even reach the surface. SOURCE OF COSMIC RAYS: Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and Zwicky suggesting cosmic rays originating from supernovae. A 1948 proposal by Horace W. Babcock suggested that magnetic variable stars could be a source of cosmic rays. Subsequently in 1951, Y. Sediko et al. identified the Crab Nebula as a source of cosmic rays. Since then, a wide variety of potential sources for cosmic rays began to surface, including supernovae, active galactic nuclei, quasars, and gamma-ray bursts. In 2009, a paper presented at the International Cosmic Ray Conference (ICRC) by scientists at the Pierre Auger Observatory showed ultra-high energy cosmic rays (UHECRs) originating from a location in the sky very close to the radio galaxy Centaurus A, although the authors specifically stated that further investigation would be required to confirm Cen A as a source of cosmic rays. However, no correlation was found between the incidence of gamma-ray bursts and cosmic rays, causing the authors to set a lower limit of 10−6 erg cm−2 on the flux of 1 GeV-1 TeV cosmic rays from gamma-ray bursts. In 2009, supernovae were said to have been "pinned down" as a source of cosmic rays, a discovery made by a group using data from the Very Large Telescope. This analysis, however, was disputed in 2011 with data from PAMELA, which revealed that "spectral shapes of [hydrogen and helium nuclei] are different and cannot be described well by a single power law", suggesting a more complex process of cosmic ray formation. In February 2013, though, research analyzing data from Fermi revealed through an observation of neutral pion decay that supernovae were indeed a source of cosmic rays, with each explosion producing roughly 3 × 1042 - 3 × 1043 J of cosmic rays. However, supernovae do not produce all cosmic rays, and the proportion of cosmic rays that they do produce is a question which cannot be answered without further study.