Cos­mic par­ti­cle ac­cel­er­a­tors

FrontLine - - OBITUARY -

MOST cos­mic rays are pro­tons or atomic nu­clei with en­er­gies around 108 (100 mil­lion) ev. Ul­tra High-en­ergy Cos­mic Rays (UHECRS) have en­er­gies above 1018 (bil­lion bil­lion) ev and have been spo­rad­i­cally de­tected on earth since the 1960s, though their sources have re­mained un­cer­tain.

Astro­physi­cists have the­o­rised that the mys­te­ri­ous UHECRS could come from su­per­mas­sive black hole­sand hence are called ac­tive ga­lac­tic nu­clei (AGNS). AGNS launch high-speed jets of plas­mathat shoot mil­lions of light years into space. A re­cent co­in­ci­dent de­tec­tion of a high-en­ergy neu­trino and a gamma ray from an AGN, both of which could be ex­plained by the ac­cel­er­a­tion of pro­tons to very high en­er­gies, was re­garded as strong ev­i­dence for AGNS as UHECR sources. But one had to rule out other vi­able Uhecr-source can­di­dates, such as rel­a­tivis­tic su­per­novae and gamma-ray bursts. A team of re­searchers from the SLAC Na­tional Ac­cel­er­a­tor Lab­o­ra­tory in Cal­i­for­nia has used sim­u­la­tions of par­ti­cle emis­sions from dis­tant ac­tive gal­ax­ies at an un­prece­dented scale to pro­pose a mech­a­nism. The re­sults lend fur­ther cre­dence to the idea of AGNS as UHECR sources. The re­sults of the sim­u­la­tions sug­gest that mag­netic field lines tan­gled like spaghetti in a bowl might be be­hind the most pow­er­ful par­ti­cle ac­cel­er­a­tors in the uni­verse.

“The mech­a­nism that cre­ates these ex­treme par­ti­cle en­er­gies isn’t known yet,” said SLAC staff sci­en­tist Fred­erico Fi­uza, the prin­ci­pal in­ves­ti­ga­tor of this new study that has been pub­lished in “Phys­i­cal Re­view Let­ters”. “But based on our sim­u­la­tions, we’re able to pro­pose a new mech­a­nism that can po­ten­tially ex­plain how these cos­mic par­ti­cle ac­cel­er­a­tors work.”

“We knew that the mag­netic fields can be­come un-

stable,” said lead au­thor Paulo Alves. “But what ex­actly hap­pens when they be­come dis­torted, and could this process ex­plain how par­ti­cles gain tremen­dous en­ergy in these jets? That’s what we wanted to find out.”

The re­searchers sim­u­lated the mo­tions of up to 550 bil­lion par­ti­cles—a minia­ture ver­sion of a cos­mic jet—on the Mira su­per­com­puter at the Ar­gonne Lead­er­ship Com­put­ing Fa­cil­ity at the Ar­gonne Na­tional Lab­o­ra­tory in Le­mont, Illi­nois. Then, they scaled up their re­sults to cos­mic di­men­sions and com­pared them with as­tro­phys­i­cal ob­ser­va­tions.

The sim­u­la­tions showed that when the he­li­cal mag­netic field is strongly dis­torted, the mag­netic field lines be­come highly tan­gled and a large elec­tric field is pro­duced in­side the jet. This ar­range­ment of elec­tric and mag­netic fields can, in­deed, ef­fi­ciently ac­cel­er­ate elec­trons and pro­tons to ex­treme en­er­gies. While high-en­ergy elec­trons ra­di­ate their en­ergy away in the form of X-rays and gamma rays, pro­tons can es­cape the jet into space and reach the earth’s at­mos­phere as cos­mic ra­di­a­tion.

GREG STE­WART/SLAC NA­TIONAL AC­CEL­ER­A­TOR LAB­O­RA­TORY

SLAC RE­SEARCHERS have found a new mech­a­nism that could ex­plain how plasma jets emerg­ing from the cen­tre of ac­tive gal­ax­ies ac­cel­er­ate par­ti­cles to ex­treme en­er­gies. Com­puter sim­u­la­tions (cir­cled area) showed that tan­gled mag­netic field lines cre­ate strong elec­tric fields in the di­rec­tion of the jets, lead­ing to dense elec­tric cur­rents of high-en­ergy par­ti­cles stream­ing away from the galaxy.

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