Featured Research

from universities, journals, and other organizations

Cosmic accelerators discovered in our galaxy

Date:
August 18, 2010
Source:
University of California - Los Angeles
Summary:
Physicists have discovered evidence of "natural nuclear accelerators" at work in our Milky Way galaxy, based on an analysis of data from the world's largest cosmic ray detector.

Physicists using the Pierre Auger Observatory in Argentina, the world's largest cosmic ray observatory, made a surprising discovery: Many of the energetic cosmic rays found in the Milky Way are not actually protons but nuclei -- and the higher the energy, the greater the nuclei-to-proton ratio.
Credit: Image courtesy of Pierre Auger Observatory

Physicists from UCLA and Japan have discovered evidence of "natural nuclear accelerators" at work in our Milky Way galaxy, based on an analysis of data from the world's largest cosmic ray detector.

The research is published Aug. 20 in the journal Physical Review Letters.

Cosmic rays of the highest energies were believed by physicists to come from remote galaxies containing enormous black holes capable of consuming stars and accelerating protons at energies comparable to that of a bullet shot from a rifle. These protons -- referred to individually as "cosmic rays" -- travel through space and eventually enter our galaxy.

But earlier this year, physicists using the Pierre Auger Observatory in Argentina, the world's largest cosmic ray observatory, published a surprising discovery: Many of the energetic cosmic rays found in the Milky Way are not actually protons but nuclei -- and the higher the energy, the greater the nuclei-to-proton ratio.

"This finding was totally unexpected because the nuclei, more fragile than protons, tend to disintegrate into protons on their long journey through space," said Alexander Kusenko, UCLA professor of physics and astronomy and co-author of the Physical Review Letters research. "Moreover, it is very unlikely that a cosmic accelerator of any kind would accelerate nuclei better than protons at these high energies."

The resolution to the paradox of the nuclei's origin comes from an analysis by Kusenko; Antoine Calvez, a UCLA graduate student of physics who is part of Kusenko's research group; and Shigehiro Nagataki, an associate professor of physics at Japan's Kyoto University. They found that stellar explosions in our own galaxy can accelerate both protons and nuclei. But while the protons promptly leave the galaxy, the heavier and less mobile nuclei become trapped in the turbulent magnetic field and linger longer.

"As a result, the local density of nuclei is increased, and they bombard Earth in greater numbers, as seen by the Pierre Auger Observatory," said Kusenko, who is also a senior scientist at the University of Tokyo's Institute for Physics and Mathematics of the Universe (IPMU).

These ultra-high-energy nuclei have been trapped in the web of galactic magnetic fields for millions of years, and their arrival directions as they enter the Earth's atmosphere have been "completely randomized by numerous twists and turns in the tangled field," he said.

"When the data came out, they were so unexpected that many people started questioning the applicability of known laws of physics at high energy," Kusenko said. "The common lore has been that all ultra-high-energy cosmic rays must come from outside the galaxy. The lack of plausible sources and the arrival-direction anisotropy (the nuclei have different physical properties when measured in different directions) have been used as arguments in favor of extragalactic sources.

"However, since the cosmic rays in question turned out to be nuclei, the galactic field can randomize their arrival directions, taking care of the anisotropy puzzle. As for the plausible sources, the enormous stellar explosions responsible for gamma ray bursts can accelerate nuclei to high energies. When we put these two together, we knew we were on the right track. Then we calculated the spectra and the asymmetries, and both agreed with the data very well."

Kusenko hopes this research will enhance the understanding of "astrophysical archeology."

"We can study the collective effects of gamma ray bursts that have taken place in the past of our own galaxy over millions of years," he said.

Stellar explosions capable of accelerating particles to ultra-high energies have been seen in other galaxies, where they produce gamma-ray bursts. The new analysis provides evidence that such powerful explosions occur in our galaxy as well, at least a few times per million years, Kusenko said.

Kusenko and his colleagues predict that the protons escaping from other galaxies should still be seen at the highest energies and should point back to their sources, providing Pierre Auger Observatory with valuable data.

The Pierre Auger Observatory records cosmic ray showers through an array of 1,600 particle detectors placed about one mile apart in a grid spread across 1,200 square miles, complemented by specially designed telescopes. The observatory is named for the French physicist Pierre Victor Auger, who in the 1920s discovered air showers.

Kusenko's research was federally funded by the U.S. Department of Energy and NASA. Nagataki's research was funded by the Japan Society for the Promotion of Science.


Story Source:

The above story is based on materials provided by University of California - Los Angeles. The original article was written by Stuart Wolpert. Note: Materials may be edited for content and length.


Journal Reference:

  1. Antoine Calvez, Alexander Kusenko, Shigehiro Nagataki. The role of Galactic sources and magnetic fields in forming the observed energy-dependent composition of ultrahigh-energy cosmic rays. Physical Review Letters, 2010; (accepted for publication) [link]

Cite This Page:

University of California - Los Angeles. "Cosmic accelerators discovered in our galaxy." ScienceDaily. ScienceDaily, 18 August 2010. <www.sciencedaily.com/releases/2010/08/100817151451.htm>.
University of California - Los Angeles. (2010, August 18). Cosmic accelerators discovered in our galaxy. ScienceDaily. Retrieved September 17, 2014 from www.sciencedaily.com/releases/2010/08/100817151451.htm
University of California - Los Angeles. "Cosmic accelerators discovered in our galaxy." ScienceDaily. www.sciencedaily.com/releases/2010/08/100817151451.htm (accessed September 17, 2014).

Share This



More Space & Time News

Wednesday, September 17, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

East Coast Treated To Rare Meteor Sighting

East Coast Treated To Rare Meteor Sighting

Newsy (Sep. 16, 2014) — Numerous residents along the East Coast reported seeing a bright meteor flash through the sky Sunday night. Video provided by Newsy
Powered by NewsLook.com
Space Race Pits Bezos Vs Musk

Space Race Pits Bezos Vs Musk

Reuters - Business Video Online (Sep. 16, 2014) — Amazon CEO Jeff Bezos' startup will team up with Boeing and Lockheed to develop rocket engines as Elon Musk races to have his rockets certified. Fred Katayama reports. Video provided by Reuters
Powered by NewsLook.com
NASA’s Curiosity Rover Finally Reaches Long-Term Goal

NASA’s Curiosity Rover Finally Reaches Long-Term Goal

Newsy (Sep. 15, 2014) — After more than two years, NASA’s Mars Curiosity Rover reached Mount Sharp, its long-term destination. Video provided by Newsy
Powered by NewsLook.com
SpaceX's Elon Musk Really Wants To Colonize Mars

SpaceX's Elon Musk Really Wants To Colonize Mars

Newsy (Sep. 14, 2014) — Elon Musk has been talking about his goal of colonizing Mars for years now, but how much of it does he actually have figured out, and is it possible? Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

    Environment News

      Technology News



      Save/Print:
      Share:  

      Free Subscriptions


      Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

      Get Social & Mobile


      Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

      Have Feedback?


      Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
      Mobile iPhone Android Web
      Follow Facebook Twitter Google+
      Subscribe RSS Feeds Email Newsletters
      Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins