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Unformatted text preview: DOI: 10.1126/science.1135375 , 773 (2006); 314 Science et al. Clifford B. Saper, Body Temperature BIOMEDICINE: Enhanced: Life, the Universe, and www.sciencemag.org (this information is current as of February 7, 2007 ): The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/314/5800/773 version of this article at: including high-resolution figures, can be found in the online Updated information and services, found at: can be related to this article A list of selected additional articles on the Science Web sites http://www.sciencemag.org/cgi/content/full/314/5800/773#related-content http://www.sciencemag.org/cgi/collection/medicine Medicine, Diseases http://www.sciencemag.org/cgi/collection/enhanced Enhanced Content : subject collections This article appears in the following http://www.sciencemag.org/help/about/permissions.dtl in whole or in part can be found at: this article permission to reproduce of this article or about obtaining reprints Information about obtaining registered trademark of AAAS. c 2006 by the American Association for the Advancement of Science; all rights reserved. The title SCIENCE is a Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on February 7, 2007 www.sciencemag.org Downloaded from The efficiency of diffusive shock acceler- ation increases rapidly with the ratio of the shock velocity to the initial sound speed, a quantity known as the Mach number. Although most of the energy of the cosmic structure formation is dissipated in the cen- ters of galaxy clusters, the shock waves in the outskirts and especially the accretion shocks have much higher Mach numbers and there- fore should be more efficient particle accel- erators, as can be seen in the figure ( 2 ). Electrons, which can be accelerated to energies of 10 4 to 10 5 times their rest mass, produce radio emission due to their gyro- motion in intergalactic magnetic fields. Such radio emission in galaxy clusters has been observed since the 1970s ( 3 ) and named cluster radio relics. However, only recently has the association with cluster merger shock waves been recognized ( 4 ). Bagchi et al. have found a pair of giant radio structures and propose that the double relic in galaxy Abell 3376 may be emission from the accretion shock of the cluster. This dual radio morphology may be caused by the stronger matter flow onto the cluster along an embedding galaxy filament. If this inter- pretation is correct, it would be a remarkable finding, because it would imply the presence of magnetic fields in the infalling gas, whereas magnetic fields have so far only been detected within galaxy clusters. Further- more, we would have the first observational identification of an accretion shock wave....
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