Universe Today

Image credit: Hubble
University of California scientists working at Los Alamos National Laboratory have proposed a new theory to explain the movement of vast energy fields in giant radio galaxies (GRGs). The theory could be the basis for a whole new understanding of the ways in which cosmic rays — and their signature radio waves — propagate and travel through intergalactic space.

In a paper published this month in The Astrophysical Journal Letters, the scientists explain how magnetic field reconnection may be responsible for the acceleration of relativistic electrons within large intergalactic volumes. That is, the movement of charged particles in space that are originally energized by massive black holes.

“If our understanding of this process is correct,” says Los Alamos astrophysicist Philipp Kronberg, “it could be a paradigm shift in current thinking about the nature of GRGs and cosmic rays.”

Researchers still do not fully understand why magnetic field reconnection occurs, but this much is known: a deeper understanding of the mechanism could have important applications here on Earth, such as the creation of a system of magnetic confinement for fusion energy reactors.

If the Los Alamos scientists’ theory is correct, the discovery also has wide-ranging astrophysical consequences. It implies that magnetic field reconnection or some other highly efficient field-to-particle energy conversion process could be a principal source of all extragalactic radio sources, and possibly also the mysterious “Ultra High Energy Cosmic Ray particles”.

Giant radio galaxies are vast celestial objects that emit a continuum of radio wavelengths detectable with radio telescopes like those at the Very Large Array in Socorro, N.M. Using comprehensive data on seven of the largest radio galaxies in the Universe gathered over the past two decades, the researchers were able to study cosmic ray energy fields that are expelled from the GRGs centers — which are almost certain to contain supermassive black holes — outward as much as a few millions of light years into intergalactic space (1 light year = 5,900,000,000,000 miles).

What the Los Alamos researchers concluded was that the high energy content of these giant radio galaxies, their large ordered magnetic field structures, the absence of strong large-scale shocks and very low internal gas densities point to a direct and efficient conversion of the magnetic field to particle energy in a process that astrophysicists call magnetic field reconnection. Magnetic field reconnection is a process where the lines of a magnetic field connect and vanish, converting the field’s energy into particle energy. Reconnection is considered a key process in the sun’s corona for the production of solar flares and in fusion experiment devices called tokamaks. It also occurs in the interaction between the solar wind and the Earth’s magnetic field and is considered a principal cause of magnetospheric storms.

The research determined that the measurement of the total energy content of at least one of these giant radio galaxies — which is believed to have at its center a black hole with a mass equal to 100 million times that of our sun — was 10 61 ergs. Ergs are a measure of energy where one erg is the amount of energy needed to lift one gram of weight a distance of one centimeter. This energy level of 10 61 ergs is several times more than the thermonuclear energy that could be released by all the stars in a galaxy, offering substantial proof to the researchers that the source of the measured energy could not be typical solar fusion or even supernovae.

In addition to the high energy content, the large, orderly structure of the magnetic field and the absence of strong large-scale shocks — like those that might be present from a supernova explosion — led the scientists to believe that the process of magnetic field reconnection is at work.

In addition to Kronberg, the theory is the result of work by Los Alamos scientists Stirling Colgate, Hui Li and Quentin Dufton. The research was funded by Los Alamos Laboratory-Directed Research and Development (LDRD) funding. LDRD funds basic and applied research and development focusing on creative concepts selected at the discretion of the Laboratory Director.

Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA’s Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos enhances global security by ensuring safety and confidence in the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction and improving the environmental and nuclear materials legacy of the cold war. Los Alamos’ capabilities assist the nation in addressing energy, environment, infrastructure and biological security problems.

Original Source: Los Alamos National Laboratory News Release

Image credit: University of Chicago
University scientists are preparing to run the most advanced supercomputer simulation of an exploding star ever attempted.

Tomasz Plewa, Senior Research Associate in the Center for Astrophysical Thermonuclear Flashes and Astronomy & Astrophysics, expects the simulation to reveal the mechanics of exploding stars, called supernovae, in unprecedented detail.

The simulation is made possible by the U.S. Department of Energy?s special allocation of an extraordinary 2.7 million hours of supercomputing time to the Flash Center, which typically uses less than 500,000 hours of supercomputer time annually.

?This is beyond imagination,? said Plewa, who submitted the Flash Center proposal on behalf of a research team at the University and Argonne National Laboratory.

The Flash Center project was one of three selected to receive supercomputer time allocations under a new competitive program announced last July by Secretary of Energy Spencer Abraham.

The other two winning proposals came from the Georgia Institute of Technology, which received 1.2 million processor hours, and the DOE?s Lawrence Berkeley National Laboratory, which received one million processor hours.

The supercomputer time will help the Flash Center more accurately simulate the explosion of a white dwarf star, one that has burned most or all of its nuclear fuel. These supernovae shine so brightly that astronomers use them to measure distance in the universe. Nevertheless, many details about what happens during a supernova remain unknown.

Simulating a supernova is computationally intensive because it involves vast scales of time and space. White dwarf stars gravitationally accumulate material from a companion star for millions of years, but ignite in less than a second. Simulations must also account for physical processes that occur on a scale that ranges from a few hundredths of an inch to the entire surface of the star, which is comparable in size to Earth.

Similar computational problems vex the DOE?s nuclear weapons Stockpile Stewardship and Management Program. In the wake of the Comprehensive Test Ban Treaty, which President Clinton signed in 1996, the reliability of the nation?s nuclear arsenal must now be tested via computer simulations rather than in the field.

?The questions ultimately are how is the nuclear arsenal aging with time, and is your code predicting that aging process correctly?? Plewa said.

Flash Center scientists verify the accuracy of their supernovae code by comparing the results of their simulations both to laboratory experiments and to telescopic observations. Spectral observations of supernovae, for example, provide a sort of bar code that reveals which chemical elements are produced in the explosions. Those observations currently conflict with simulations.

?You want to reconcile current simulations with observations regarding chemical composition and the production of elements,? Plewa said.

Scientists also wish to see more clearly the sequence of events that occurs immediately before a star goes supernova. It appears that a supernova begins in the core of a white dwarf star and expands toward the surface like an inflating balloon.

According to one theory, the flame front initially expands at a relatively ?slow? subsonic speed of 60 miles per second. Then, at some unknown point, the flame front detonates and accelerates to supersonic speeds. In the ultra-dense material of a white dwarf, supersonic speeds exceed 3,100 miles per second.

Another possibility: the initial subsonic wave fizzles when it reaches the outer part of the star, leading to a collapse of the white dwarf, the mixing of unburned nuclear fuel and then detonation.

?It will be very nice if in the simulations we could observe this transition to detonation,? Plewa said.

Flash Center scientists already are on the verge of recreating this moment in their simulations. The extra computer time from the DOE should push them across the threshold.

The center will increase the resolution of its simulations to one kilometer (six-tenths of a mile) for a whole-star simulation. Previously, the center could achieve a resolution of five kilometers (3.1 miles) for a whole-star simulation, or 2.5 kilometers (1.5 miles) for a simulation encompassing only one-eighth of a star.

The latter simulations fail to capture perturbations that may take place in other sections of the star, Plewa said. But they may soon become scientific relics.

?I hope by summer we?ll have all the simulations done and we?ll move on to analyze the data,? he said.

Original Source: University of Chicago News Release

Image credit: NASA/JPL
The only known gravitationally bound pair of pulsars — extremely dense, spinning stars that beam radio waves — may be pirouetting around each other in an intricate dance.

“Pulsars are intriguing and puzzling objects. They pack as much mass as the Sun crammed into an object with a cross-sectional area about as large as Boston,” said Fredrick Jenet of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Jenet and Scott Ransom of McGill University, Montreal, Quebec, Canada, have developed a theoretical model to explain the behavior of this one-of-a-kind set of pulsars.

“The physics of radio pulsar emission has eluded researchers for more than three decades,” Jenet said. “This system may be the ‘Rosetta stone’ of radio pulsars, and this model is one step toward its translation.”

The research appears in the April 29 issue of the journal Nature. Jenet and Ransom studied the recently-discovered double pulsar system, in which two spinning pulsars orbit each other.

The discovery of the two-star system, officially named PSR J0737- 3039B, was announced in 2003 by a multinational team of researchers from Italy, Australia, the United Kingdom and the United States. Those researchers proposed that the duo contained one spinning pulsar and a neutron star. Later in 2003, scientists working at the Parkes Observatory in New South Wales, Australia, determined that both stars are actually pulsars. This discovery marked the first known example of a “binary,” or double, pulsar system. The stars are referred to as A and B.

Pulsars emit high-intensity radio radiation into a narrow beam. As the pulsar rotates, this beam moves in and out of our line of sight. Hence, we see periodic bursts of radio radiation. In this sense, a pulsar works like a lighthouse, in which the light may be on all the time, but it appears to blink on and off. Scientists were surprised to find that the B pulsar is on only at certain locations in its orbit. “It’s as though something is turning B on and off,” Jenet said.

According to Jenet and Ransom, this “something” is closely related to the radio emission beam emanating from the A pulsar. They believe that B becomes bright when it is illuminated by emission from A. Jenet and Ransom used Einstein’s Theory of General Relativity to predict the future evolution of this pulsar system. The theory implies that gravitational effects will change the emission pattern of A, which will then alter the exact orbital locations where B becomes bright.

The double pulsar system is located about 2,000 light years, or 10 million billion miles, from Earth. Jenet and Ransom based their research on observations made at the Green Bank Telescope in West Virginia.

Original Source: NASA/JPL News Release