Cosmic mystery 'solved' after decades
Nov 16,2007 00:00 by Bend_Weekly_News_Sources

The most en­er­get­ic par­t­i­cles known, called ul­tra­high-en­er­gy cos­mic rays, pro­b­ab­ly come from super massive black holes in the hearts of bright near­by ga­lax­ies, a new study con­cludes. If cor­rect, that solves a dec­ades-long mys­tery: the source of these sub­a­tom­ic par­t­i­cles, which can slam in­to our at­mos­phere with the en­er­gy of a speed­ing base­ball.

Centaurus A, an active galaxy. (Courtesy European Southern Observatory)

“Ga­lax­ies which host vi­o­lent black holes,” also called act­ive ga­la­xies, now seem to be the cul­prit, said Mi­guel Mos­ta­fa, a Un­ivers­ity of Utah phys­i­cist col­la­bo­rat­ing in the work. This puts sci­en­tists “one step clos­er to know­ing what phys­i­cal pro­cess can ac­cel­er­ate par­t­i­cles” so pow­er­ful­ly, he added. “Right now, we don’t know.”

Mostafa is part of a 17-na­t­ion col­labora­t­ion that oper­ates the $54 mil­lion Pierre Au­ger Ob­serv­a­to­ry in Ar­gen­ti­na, which was used for the stu­dy. The find­ings are to ap­pear in the Nov. 9 is­sue of the re­search jour­nal Sci­ence

Black holes are ex­tremely com­pact ob­jects with gra­vity so strong that noth­ing—not even light—can es­cape them. Sci­en­tists be­lieve the cores of most ga­lax­ies, in­clud­ing ours, con­tain su­per­mas­sive black holes, which can con­tain the weight equiv­a­lent of bil­lions of our suns crammed in­to a ti­ny space. They gobble near­by ma­te­ri­al, which in the pro­cess heats up and spew out par­t­i­cles and light be­fore va­nish­ing. Par­tic­u­larly bright ga­la­xy cores are known as ac­tive ga­lac­tic nu­clei.

Cos­mic rays, dis­cov­ered in 1912 by the Aus­tri­an Vic­tor Hess, aren’t really rays at all. They are sub­a­tom­ic par­t­i­cles, in­clud­ing nu­clei of cer­tain atoms that en­ter the at­mos­phere from space at nearly light speed. Low­er-en­er­gy cos­mic rays come from or­d­in­ary stars. Medium-en­er­gy rays are thought to come from ex­plod­ing stars. But the source of the most pow­er­ful ones—around 100 mil­lion times more en­er­get­ic than any that lab­o­r­a­to­ries on Earth can pro­duce—has been un­ex­plained. 

The highest-en­er­gy cos­mic ray de­tected was meas­ured in 1991 by the Un­ivers­ity of Utah’s Fly’s Eye ob­serv­a­to­ry, ac­cord­ing to sci­en­tists in­volved in the new stu­dy. Its en­er­gy was logged at 300 bil­lion elec­tron volts. It thus would have felt like a fast-pitched base­ball had it hit some­one on the head, though it would­n’t have, as the at­mos­phere ab­sorbs most cos­mic rays. They are de­tect­ed by ground in­stru­ments based on show­ers of sec­ond­ary part­i­cles that they give off on im­pact.

In the new stu­dy, sci­en­tists at the Au­ger Ob­serv­a­to­ry—the world’s larg­est for cos­mic rays—found that of the 27 most en­er­get­ic ones de­tected, 20 came from the di­rec­tion of ac­tive ga­lac­tic nu­clei. There’s only a one per­cent chance that such a cor­rela­t­ion could have hap­pened ran­dom­ly, Mos­ta­fa said. Most like­ly, he added, if the rays had been com­ing ran­domly from all di­rec­tions, only five or six would have seemed to come from such ob­jects.

Super-en­er­get­ic cos­mic rays al­so have to come from ga­lax­ies rel­a­tively close by, with­in 326 mil­lion light-years, Mos­ta­fa said. A light-year is the dis­tance light trav­els in a year; but even that many light-years “is our lo­cal neigh­bor­hood in cos­mic terms,” he added.

Courtesy University of Utah World Science staff