Editing Cosmic ray (section)

==Sources==
Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and [[Fritz Zwicky|Zwicky]] suggesting cosmic rays originated from supernovae.<ref>{{cite journal|last1=Baade|first1=W.|last2=Zwicky|first2=F.|year=1934|title=Cosmic rays from super-novae|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=20|issue=5|pages=259–263|jstor=86841|doi=10.1073/pnas.20.5.259|pmid=16587882|bibcode=1934PNAS...20..259B|pmc=1076396|doi-access=free}}</ref> A 1948 proposal by [[Horace W. Babcock]] suggested that magnetic variable stars could be a source of cosmic rays.<ref>{{cite journal|last1=Babcock|first1=H.|title=Magnetic variable stars as sources of cosmic rays|doi=10.1103/PhysRev.74.489|journal=Physical Review|volume=74|issue=4|page=489|year=1948|bibcode=1948PhRv...74..489B}}</ref> Subsequently, Sekido ''et al.'' (1951) identified the [[Crab Nebula]] as a source of cosmic rays.<ref>{{cite journal|last1=Sekido|first1=Y.|last2=Masuda|first2=T.|last3=Yoshida|first3=S.|last4=Wada|first4=M.|title=The Crab Nebula as an observed point source of cosmic rays|doi=10.1103/PhysRev.83.658.2|journal=Physical Review|volume=83|issue=3|pages=658–659|year=1951|bibcode=1951PhRv...83..658S}}</ref> Since then, a wide variety of potential sources for cosmic rays began to surface, including [[supernova]]e, active galactic nuclei, [[quasar]]s, and [[gamma-ray burst]]s.<ref>{{cite web|last=Gibb|first=Meredith|title=Cosmic rays|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html|website=Imagine the Universe|publisher=NASA Goddard Space Flight Center|access-date=17 March 2013|date=3 February 2010}}</ref>

[[File:PIA16938-RadiationSources-InterplanetarySpace.jpg|thumb|left|Sources of ionizing radiation in interplanetary space.]]
Later experiments have helped to identify the sources of cosmic rays with greater certainty. In 2009, a paper presented at the [[International Cosmic Ray Conference]] by scientists at the Pierre Auger Observatory in Argentina showed ultra-high energy cosmic rays 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 Centaurus&nbsp;A as a source of cosmic rays.<ref>{{cite conference|url=http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|title=Correlation of the Highest Energy Cosmic Rays with Nearby Extragalactic Objects in Pierre Auger Observatory Data|access-date=17 March 2013|author=Hague, J.D.|book-title=Proceedings of the 31st ICRC, Łódź 2009|date=July 2009|conference=International Cosmic Ray Conference|location=Łódź, Poland|pages=6–9|url-status=dead|archive-url=https://web.archive.org/web/20130528011624/http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|archive-date=28 May 2013}}</ref> However, no correlation was found between the incidence of gamma-ray bursts and cosmic rays, causing the authors to set upper limits as low as 3.4&nbsp;×&nbsp;10<sup>−6</sup>×&nbsp;[[erg]]·cm<sup>−2</sup> on the flux of {{nowrap|1 GeV – 1 TeV}} cosmic rays from gamma-ray bursts.<ref>{{cite journal|author=Hague, J.D.|title=Correlation of the highest energy cosmic rays with nearby extragalactic objects in Pierre Auger Observatory data|url=http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|journal=Proceedings of the 31st ICRC, Łódź, Poland 2009 – International Cosmic Ray Conference|date=July 2009|pages=36–39|access-date=17 March 2013|url-status=dead|archive-url=https://web.archive.org/web/20130528011624/http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|archive-date=28 May 2013}}</ref>

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]].<ref>{{cite web|url=http://www.space.com/6890-source-cosmic-rays-pinned.html|title=Source of cosmic rays pinned down|publisher=Tech Media Network|website=Space.com|date=25 June 2009|access-date=20 March 2013|author=Moskowitz, Clara|author-link= Clara Moskowitz}}</ref> This analysis, however, was disputed in 2011 with data from [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|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.<ref>{{cite journal|last1=Adriani|first1=O.|last2=Barbarino|first2=G.C.|last3=Bazilevskaya|first3=G.A.|last4=Bellotti|first4=R.|last5=Boezio|first5=M.|last6=Bogomolov|first6=E.A.|last7=Bonechi|first7=L.|last8=Bongi|first8=M.|last9=Bonvicini|first9=V.|last10=Borisov|doi=10.1126/science.1199172|first10=S.|last11=Bottai|first11=S.|last12=Bruno|first12=A.|last13=Cafagna|first13=F.|last14=Campana|first14=D.|last15=Carbone|first15=R.|last16=Carlson|first16=P.|last17=Casolino|first17=M.|last18=Castellini|first18=G.|last19=Consiglio|first19=L.|last20=de Pascale|first20=M.P.|last21=de Santis|first21=C.|last22=de Simone|first22=N.|last23=di Felice|first23=V.|last24=Galper|first24=A.M.|last25=Gillard|first25=W.|last26=Grishantseva|first26=L.|last27=Jerse|first27=G.|last28=Karelin|first28=A.V.|last29=Koldashov|first29=S.V.|last30=Krutkov|first30=S.Y.|title=PAMELA measurements of cosmic-ray proton and helium spectra|journal=Science|volume=332|issue=6025|pages=69–72|year=2011|pmid=21385721|arxiv=1103.4055|bibcode=2011Sci...332...69A|display-authors=6|hdl=2108/55474|s2cid=1234739}}</ref> 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&nbsp;×&nbsp;10<sup>42</sup> – 3&nbsp;×&nbsp;10<sup>43</sup>{{nbsp}}[[joule|J]] of cosmic rays.<ref name=ackermann-2013/><ref name=pinholster-2013/>

[[File:Shockfrontacceleration.svg|thumb|Shock front acceleration (theoretical model for supernovae and active galactic nuclei): Incident proton gets accelerated between two shock fronts up to energies of the high-energy component of cosmic rays.]]
Supernovae do not produce all cosmic rays, however, and the proportion of cosmic rays that they do produce is a question which cannot be answered without deeper investigation.<ref>{{cite news|url=https://www.theguardian.com/science/2013/feb/14/cosmic-ray-mystery-solved|title=Cosmic ray mystery solved|publisher=Guardian News and Media Ltd|newspaper=[[The Guardian]]|place=London, UK|date=14 February 2013|access-date=21 March 2013|author=Jha, Alok}}</ref> To explain the actual process in supernovae and active galactic nuclei that accelerates the stripped atoms, physicists use shock front acceleration as a plausibility argument (see picture at right).

In 2017, the [[Pierre Auger Collaboration]] published the observation of a weak [[anisotropy]] in the arrival directions of the highest energy cosmic rays.<ref>{{cite journal|collaboration=The Pierre Auger Collaboration|title=Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8×10{{sup|18&nbsp;}}eV|journal=Science|year=2017|pages=1266–1270|volume=357|issue=6357|arxiv=1709.07321|doi=10.1126/science.aan4338|pmid=28935800|author1=Pierre Auger Collaboration|last2=Aab|first2=A.|last3=Abreu|first3=P.|last4=Aglietta|first4=M.|last5=Al Samarai|first5=I.|last6=Albuquerque|first6=I. F. M.|last7=Allekotte|first7=I.|last8=Almela|first8=A.|last9=Alvarez Castillo|first9=J.|last10=Alvarez-Muñiz|first10=J.|last11=Anastasi|first11=G. A.|last12=Anchordoqui|first12=L.|last13=Andrada|first13=B.|last14=Andringa|first14=S.|last15=Aramo|first15=C.|last16=Arqueros|first16=F.|last17=Arsene|first17=N.|last18=Asorey|first18=H.|last19=Assis|first19=P.|last20=Aublin|first20=J.|last21=Avila|first21=G.|last22=Badescu|first22=A. M.|last23=Balaceanu|first23=A.|last24=Barbato|first24=F.|last25=Barreira Luz|first25=R. J.|last26=Beatty|first26=J. J.|last27=Becker|first27=K. H.|last28=Bellido|first28=J. A.|last29=Berat|first29=C.|last30=Bertaina|first30=M. E.|bibcode=2017Sci...357.1266P|s2cid=3679232|display-authors=29}}</ref> Since the Galactic Center is in the deficit region, this anisotropy can be interpreted as evidence for the extragalactic origin of cosmic rays at the highest energies. This implies that there must be a transition energy from galactic to extragalactic sources, and there may be different types of cosmic-ray sources contributing to different energy ranges.