"Long before it's in the papers"
February 02, 2010

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Mystery rays probably from bursting stars, scientists say

March 30, 2010
Courtesy U.S. Department of Energy
and World Science staff

In all di­rec­tions of the sky, cos­mic rays rock­et through space with in­cred­i­ble speed. These “rays”—which mostly con­sist of elec­tric­ally charged par­t­i­cles called pro­ton­s—are some of the most en­er­get­ic par­t­i­cles in the uni­verse. 

For nearly 100 years, they have al­so been some of the most en­ig­mat­ic, their ori­gins un­cer­tain. Now, re­search­ers have found ev­i­dence to back up a long­stand­ing the­o­ry that the rays come from re­mains of su­per­novas, or ex­plod­ed stars.

Super­nova rem­nant W44 as im­aged by the Fer­mi te­le­scope's Large Area Te­le­scope and en­hanced with a re­stor­a­tion tech­nique. Brigh­ter co­lors indicate areas from which great­er num­bers of gam­ma rays are ar­riv­ing. The green con­tours in­di­cate the rem­nant seen with in­fra­red light. (Im­age cour­tesy of NA­SA/DOE/LAT col­labor­a­tion.)
The new find­ings come from the space-based Fer­mi Gamma-ray Space Tel­e­scope, a col­la­bora­t­ion of NASA and in­sti­tu­tions in sev­er­al coun­tries. They are de­scribed in the Jan. 7 is­sue of the re­search jour­nal Sci­ence and al­so in a U.S. De­part­ment of En­er­gy on­line mag­a­zine, Sym­me­try.

How cos­mic rays at­tain such high speeds has been a mys­tery for a cen­tu­ry. The idea that they may come from su­per­novas was pro­posed dec­ades ago, but there was lit­tle di­rect ev­i­dence to back it up.

As a star dies and runs out of nu­clear fuel, atom­ic pro­cesses cause it to ex­plode. The stel­lar ma­te­ri­al then plows in­to the gas among the stars, com­press­ing the gas and form­ing shock waves, which are mov­ing ar­eas of ex­tremely high com­pres­sion in a gas or flu­id. Re­search­ers have sup­posed that these shock waves are the most likely where charged par­t­i­cles speed up to be­come cos­mic rays.

But “ob­serva­t­ions had yet to pin­point where the par­t­i­cle ac­celera­t­ion really oc­curs,” Ya­sunobu Uchiyama of the Kavli In­sti­tute for Par­t­i­cle As­t­ro­phys­ics and Cos­mol­o­gy at Stan­ford Univers­ity in Cal­i­for­nia, told Sym­me­try.

In the new re­search, the Large Ar­ea Tel­e­scope col­la­bora­t­ion, led by re­search­ers Takaaki Tanaka, Uchiyama, and Hi­roy­asu Tajima at the in­sti­tute, re­leased the first im­age of a su­per­no­va rem­nant in the giga-electronvolt en­er­gy range, about 200 mil­lion times the en­er­gy of vis­i­ble light. The im­ages re­veal where cos­mic rays are dis­trib­ut­ed in the rem­nant, sci­en­tists said.

“We fi­nally have suc­ceeded in get­ting in­forma­t­ion about spa­tial dis­tri­bu­tion from a su­per­no­va rem­nant in this en­er­gy band,” Tanaka told Sym­me­try.

To de­tect where the cos­mic rays were lurk­ing, the re­search­ers tracked gam­ma-ray light, a high-en­er­gy form of light, com­ing from a su­per­no­va rem­nant dubbed W44. Cos­mic rays tend to pro­duce gam­ma rays through suba­tom­ic pro­cesses as they in­ter­act with the dif­fuse gas among the stars. The re­search­ers de­duced that the gam­ma rays they de­tected were very likely cre­at­ed in this way based on the ob­served gam­ma-ray spec­trum, or the amount of light com­ing in at dif­fer­ent en­er­gies.

“This pa­per proves Fer­mi ca­pa­ble of de­ter­min­ing the or­i­gin of gam­ma rays,” said Tanaka, ac­cord­ing to Sym­me­try. As the col­la­bora­t­ion gath­ers more da­ta, he con­tin­ued, the cer­tainty will in­crease.

“We can­not de­clare for cer­tain that we’ve fi­nally seen the sig­na­ture of these pro­tons,” Uchiyama said, ac­cord­ing to the pub­lica­t­ion. “There is an­oth­er pos­si­bil­ity we need to rule out. But if we can prove this con­nec­tion, it will be a huge break­through. Re­search­ers have been chas­ing this for nearly 100 years, ev­er since cos­mic rays were first un­der­stood.”

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In all directions of the sky, cosmic rays rocket through space with incredible speed. These “rays”—which mostly consist of electrically charged particles called protons—are some of the most energetic particles in the universe. For nearly 100 years, they have also been some of the most enigmatic, their origins uncertain. Now, researchers have found evidence to back up a longstanding theory that the rays come from remains of supernovas, or exploded stars. The new findings come from the space-based Fermi Gamma-ray Space Telescope, a collaboration of NASA and institutions in several countries. They are described in the Jan. 7 issue of the research journal Scienceand also in a U.S. Department of Energy online magazine, Symmetry. How cosmic rays attain such high speeds has been a mystery for a century. The idea that they may come from supernovas was proposed decades ago, but there was little direct evidence to back it up. As a star dies and runs out of nuclear fuel, atomic processes cause it to explode. The stellar material then plows into the gas among the stars, compressing the gas and forming shock waves, which are moving areas of extremely high compression in a gas or fluid. Researchers have supposed that these shock waves are the most likely where charged particles speed up to become cosmic rays. But “observations had yet to pinpoint where the particle acceleration really occurs,” Yasunobu Uchiyama of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in California, told Symmetry. In the new research, the Large Area Telescope collaboration, led by researchers Takaaki Tanaka, Uchiyama, and Hiroyasu Tajima, released the first image of a supernova remnant in the giga-electronvolt energy range, about 200 million times the energy of visible light. The images reveal where cosmic rays are distributed in the remnant, scientists said. “We finally have succeeded in getting information about spatial distribution from a supernova remnant in this energy band,” Tanaka told Symmetry. To detect where the cosmic rays were lurking, the researchers tracked gamma-ray light, a high-energy form of light, coming from a supernova remnant dubbed W44 in gamma-ray light. Cosmic rays tend to produce gamma rays through subatomic processes as they interact with the diffuse gas among the stars. The researchers deduced that the gamma rays they detected were very likely created in this way based on the observed gamma-ray spectrum, or the amount of light coming in at different energies. “This paper proves Fermi capable of determining the origin of gamma rays,” said Tanaka, according to Symmetry. As the collaboration gathers more data, he continued, the certainty will increase. “We cannot declare for certain that we’ve finally seen the signature of these protons,” Uchiyama said, according to the publication. “There is another possibility we need to rule out. But if we can prove this connection, it will be a huge breakthrough. Researchers have been chasing this for nearly 100 years, ever since cosmic rays were first understood.”