Galactic cosmic rays (CRs) are widely believed to be accelerated by shock waves associated with the expansion of supernova ejecta into the interstellar medium. A key issue in this long-standing conjecture is a theoretical prediction that the interstellar magnetic field can be substantially amplified at the shock of a young supernova remnant (SNR) through magnetohydrodynamic waves generated by cosmic rays. Here we report a discovery of the brightening and decay of X-ray hot spots in the shell of theSNRRXJ1713.723946 on a one-year timescale. This rapid variability shows that the X-rays are produced by ultrarelativistic electrons through a synchrotron process and that electron acceleration does indeed take place in a strongly magnetized environment, indicating amplification of the magnetic field by a factor of more than 100. The X-ray variability also implies that we have witnessed the ongoing shock-acceleration of electrons in real time. Independently, broadband X-ray spectrometric measurements of RXJ1713.723946 indicate that electron acceleration proceeds in the most effective ('Bohm-diffusion') regime. Taken together, these two results provide a strong argument for acceleration of protons and nuclei to energies of 1 PeV (1015 eV) and beyond in young supernova remnants.
Galactic cosmic rays (CRs) are widely believed to be accelerated by shock waves associated with the expansion of supernova ejecta into the interstellar medium.
It has long been speculated that supernova remnants represent a major source of cosmic rays in the Galaxy. Observations over the past decade have ceremoniously unveiled direct evidence of particle acceleration in SNRs to energies approaching the knee of the cosmic ray spectrum. Nonthermal X-ray emission from shell-type SNRs reveals multi-TeV electrons, and the dynamical properties of several SNRs point to efficient acceleration of ions. Observations of TeV gamma-ray emission have confirmed the presence of energetic particles in several remnants as well, but there remains considerable debate as to whether this emission originates with high energy electrons or ions. Equally uncertain are the exact conditions that lead to efficient particle acceleration. Based on the catalog of EGRET sources, we know that there is a large population of Galactic gamma-ray sources whose distribution is similar to that of SNRs. With the increased resolution and sensitivity of GLAST, the gamma-ray SNRs from this population will be identified. Their detailed emission structure, along with their spectra, will provide the link between their environments and their spectra in other wavebands to constrain emission models and to potentially identify direct evidence of ion acceleration in SNRs. Here I summarize recent observational and theoretical work in the area of cosmic ray acceleration by SNRs, and discuss the contributions GLAST will bring to our understanding of this problem.
It has long been speculated that supernova remnants represent a major source of cosmic rays in the Galaxy.
It is shown that the radio and gamma-ray emission observed from newly-found 'GeV-bright' supernova remnants (SNRs) can be explained by a model, in which a shocked cloud and shock-accelerated cosmic rays (CRs) frozen in it are simultaneously compressed by the supernova blastwave as a result of formation of a radiative cloud shock. Simple reacceleration of pre-existing CRs is generally sufficient to power the observed gamma-ray emission through the decays of [pi]°-mesons produced in hadronic interactions between high-energy protons (nuclei) and gas in the compressed-cloud layer. This model provides a natural account of the observed synchrotron radiation in SNRs W51C, W44 and IC 443 with flat radio spectral index, which can be ascribed to a combination of secondary and reaccelerated electrons and positrons.
It is shown that the radio and gamma-ray emission observed from newly-found 'GeV-bright' supernova remnants (SNRs) can be explained by a model, in which a shocked cloud and shock-accelerated cosmic rays (CRs) frozen in it are simultaneously ...
Harry Potter a quatorze ans et entre en quatrième année au collège de Poudlard. Une grande nouvelle attend Harry, Ron et Hermione à leur arrivée : la tenue d’un tournoi de magie exceptionnel entre les plus célèbres écoles de sorcellerie. Déjà les délégations étrangères font leur entrée. Harry se réjouit... Trop vite. Il va se trouver plongé au coeur des événements les plus dramatiques qu’il ait jamais eu à affronter. Dans ce quatrième tome bouleversant, drôle, fascinant, qui révèle la richesse des enjeux en cours, Harry Potter doitfaire face et relever d’immenses défis.
Harry Potter a quatorze ans et entre en quatrième année au collège de Poudlard.
Il confronto finale con Voldemort è imminente, una grande battaglia è alle porte e Harry, con coraggio, compirà ciò che dev’essere fatto. Mai i perché sono stati così tanti e mai come in questo libro si ha la soddisfazione delle risposte. Giunti all’ultima pagina si vorrà rileggere tutto daccapo, per chiudere il cerchio e ritardare il distacco dai meravigliosi personaggi che ci hanno accompagnato per così tanto tempo.
Il confronto finale con Voldemort è imminente, una grande battaglia è alle porte e Harry, con coraggio, compirà ciò che dev’essere fatto.
Revelations, battles, and betrayals await in Harry Potter's brilliant conclusion. Harry faces more danger than that found in all six books combined, from the direct threat of the Death Eaters and you-know-who, to the subtle perils of losing faith in himself.
Revelations, battles, and betrayals await in Harry Potter's brilliant conclusion.
Po śmierci Dumbledore'a Zakon Feniksa wzmaga swoją działalność, próbując przeciwstawić się coraz potężniejszym siłom śmierciożerców. Harry wraz z przyjaciółmi opuszcza Hogwart, by odnaleźć sposób na pokonanie Voldemorta. Wyprawa ta pełna niepewności i zwątpienia najeżona jest niebezpieczeństwami, a co gorsza nikt nie wie, czy zakończy się sukcesem i czy wszyscy dotrwają do jej końca. Dlaczego Dumbledore nie pozostawił Harry`emu czytelnych wskazówek? Czy przeszłość nieżyjącego dyrektora kryje jakieś niezwykłe tajemnice? Jaką rolę odegra Snape przy boku Voldemorta? Czy Harry`emu uda się dotrzeć do najważniejszych miejsc i faktów dotyczących jego rodziny?
In his fifth year at Hogwart's, Harry faces challenges at every turn, from the dark threat of He-Who-Must-Not-Be-Named and the unreliability of the government of the magical world, to the rise of Ron Weasley as the keeper of the Gryffindor Quidditch Team.
In his fifth year at Hogwart's, Harry faces challenges at every turn, from the dark threat of He-Who-Must-Not-Be-Named and the unreliability of the government of the magical world, to the rise of Ron Weasley as the keeper of the Gryffindor ...
