UNDERSTANDING A STAR'S SURFACE AND THE EXTREME FORCES OF NATURE

ILLUSTRATION FROM:  NASA:  A neutron star is the densest object astronomers can observe directly, crushing half a million times Earth's mass into a sphere about 12 miles across, or similar in size to Manhattan Island, as shown in this illustration.
Credit: NASA's Goddard Space Flight Center
Neutron Stars’ X-ray 


STORY FROM:  LOS ALAMOS NATIONAL LABORATORY 

Superbursts Mystify, Inspire Los Alamos Scientists

New neutrino cooling theory changes understanding of stars’ surface

LOS ALAMOS, N.M., Dec. 6, 2013—Massive X-ray superbursts near the surface of neutron stars are providing a unique window into the operation of fundamental forces of nature under extreme conditions.

“Scientists are intrigued by what exactly powers these massive explosions, and understanding this would yield important insights about the fundamental forces in nature, especially on the astronomical/cosmological scale,” said Peter Moller of Los Alamos National Laboratory’s Theoretical Division.



A neutron star is created during the death of a giant star more massive than the sun, compressed to a tiny size but with gravitational fields exceeded only by those of black holes. And in the intense, neutron-rich environment, nuclear reactions cause strong explosions that manifest themselves as X-ray bursts and the X-ray superbursts that are more rare and 1000 times more powerful.

Los Alamos researchers and former postdocs contributed to the paper “Strong neutrino cooling by cycles of electron capture and beta decay in neutron star crusts” that was published in Nature’s online edition of Dec. 1, 2013.
The importance of discovering an unknown energy source of titanic magnitude in the outermost layers of accreting neutron star surfaces is heightened by the unresolved issue of neutrino masses, the recent discovery of the Higgs boson and the fact that highly-neutron-rich nuclei with low-lying states enable “Weak Interactions,” prominent in stellar explosions. (The weak nuclear force is one of four fundamental sources, such as gravity, which interacts with the neutrinos; it is responsible for some types of radioactive decay.)

These hitherto celestially operative nuclei are expected to be within the experimental reach of the Facility for Rare Isotope Beams (FRIB), a proposed user facility at Michigan State University funded by the U.S. Department of Energy Office of Science.

“The terrestrial experimental study of Weak Interactions in highly deformed, neutron-rich nuclei that FRIB can potentially provide is lent support by this ground-breaking Nature letter, since Los Alamos has been one of the few homes to theoretical studies of deformed nuclei and their role in astrophysics, and remains so to this day,” said Moller, who coauthored the paper with a multidisciplinary team including former Los Alamos postdoctoral researchers Sanjib Gupta, now a faculty member at the Indian Institute of Technology (IIT), Ropar and Andrew Steiner, now a research assistant professor at INT, Seattle.

Previously a common assumption was that that the energy released in these radioactive decays would power the X-ray superburst explosions. This was based on simple models of nuclear beta-decay, sometimes postulating the same decay properties for all nuclei. It turns out, however, that it is of crucial importance to develop computer models that realistically describe the shape of each individual nuclide since they are not all spherical.

At Los Alamos scientists have carried out detailed calculations of the specific, individual beta-decay properties of thousands of nuclides, all with different decay properties, and created databases with these calculated properties.

The databases are then used at MSU as input into models that trace the decay pathways with the passage of time in accreting neutron stars and compute the total energy that is released in these reactions.

The new, unexpected result is that so much energy escapes by neutrino emission that the remaining energy released in the beta decays is not sufficient to ignite the X-ray superbursts that are observed. Thus the superbursts’ origin has now become a puzzle.

Solving the puzzle will require that we calculate in detail the consequences of shapes of neutron-rich nuclei, the authors said, and it requires that they simultaneously analyze the role played by neutrinos in neutron star X-ray bursts whose energetic magnitudes are exceeded only by explosions in the nova/supernova class.

The strong nuclear deformations that formed the basis for the neutrino cooling in neutron star crusts also play a role in a number of astrophysical settings, and have been taken into account in studies of supernovae explosions and subsequent collapses, funded by Los Alamos’ Laboratory Directed Research and Development (LDRD) programs.

Nuclear-structure databases valued worldwide

The large databases compiled by use of these and other nuclear-structure models are also used in several other Los Alamos programs. For example in modeling nuclear-reactor behavior, researchers have had to take into account beta-decay both because delayed neutrons are emitted, which governs the criticality of the reactor, and because it generates heat, just as in the neutron star.

Another current application is in nuclear non-proliferation programs. One method for detecting clandestine nuclear material in cargo shipments is to bombard cargoes with a small number of neutrons. If emission of delayed neutrons is detected after neutron bombardment, scientists have a sure signature of fissile nuclear material. The theoretical databases compiled at Los Alamos are not just used internally but are also part of nuclear-structure databases maintained by the International Atomic Energy Agency.

The authors, an international team

The authors on the paper are Hendrik Schatz from MSU; Sanjib Gupta from IIT Ropar; Peter Mller from LANL; Mary Beard and Michael Wiescher from the University of Notre Dame; Edward F. Brown, Alex T. Deibel, Laurens Keek, and Rita Lau from MSU; Leandro R. Gasques from the Universidade de Sao Paulo; William Raphael Hix from Oak Ridge National Laboratory and the University of Tennessee; and Andrew W. Steiner from the University of Washington.

FINDING THE ORIGINS OF COSMIC RAYS AT THE BOTTOM OF THE WORLD

FROM:  NATIONAL SCIENCE FOUNDATION
Ice Cube Neutrino Observatory Provides New Insights Into Origin of Cosmic Rays
April 18, 2012
Analysis of data from the IceCube Neutrino Observatory, a massive detector deployed in deep ice at the U.S. Amundsen-Scott South Pole Station in Antarctica at the geographic South Pole, recently provided new insight into one of the most enduring mysteries in physics, the production of cosmic rays.

Cosmic rays were discovered 100 years ago, but only now are scientists homing in on how the highest energy cosmic rays are produced.

Cosmic rays are electrically charged particles, such as protons, that strike Earth from all directions with energies up to one hundred million times higher than those created in man-made accelerators.

The intense conditions needed to generate such energetic particles have focused physicists' interest on two potential sources: the massive black holes at the centers of active galaxies and exploding fireballs observed by astronomers called gamma-ray bursts or GRBs.

"Although we have not discovered where cosmic rays come from, we have taken a major step towards ruling out one of the leading predictions," said Francis Halzen, a physicist at the University of Wisconsin-Madison and the IceCube principal investigator.

In a paper published in the April 19 issue of the journal Nature, the IceCube collaboration describes a search for neutrinos emitted from 300 gamma ray bursts observed between May 2008 and April 2010 in coincidence with the SWIFT and Fermi satellites.

Surprisingly, the scientists found no neutrinos--a result that contradicts 15 years of predictions and challenges the theory that gamma-ray bursts produce the highest energy cosmic rays.

"The result of this neutrino search is significant because for the first time we have an instrument with sufficient sensitivity to open a new window on cosmic ray production and the interior processes of GRBs," said Greg Sullivan, a physicist at the University of Maryland and IceCube spokesman.

"The unexpected absence of neutrinos from GRBs has forced a re-evaluation of the theory for production of cosmic rays and neutrinos in a GRB fireball and possibly the theory that high-energy cosmic rays are generated in fireballs," he said.

IceCube observes neutrinos by detecting the faint blue light produced in neutrino interactions in ice. Neutrinos are of a ghostly nature; they can easily travel through people, walls, or the planet Earth. To compensate for the antisocial nature of neutrinos and detect their rare interactions, IceCube is built on an enormous scale. One cubic kilometer of glacial ice, enough to fit the great pyramid of Giza 400 times, is instrumented with 5,160 optical sensors embedded up to 2.5 kilometers deep in the ice.

GRBs, the universe's most powerful explosions, are usually first observed by satellites using X-rays and/or gamma rays. GRBs are seen about once per day, and are so bright that they can be seen from half way across the visible Universe. The explosions usually last only a few seconds, and during this brief time they can outshine everything else in the universe.

The IceCube Neutrino Observatory was built under a National Science Foundation (NSF) Major Research Equipment and Facilities Construction grant, with assistance from partner funding agencies around the world.

NSF continues to support the project with a Maintenance and Operations grant co-funded by the Division of Antarctic Sciences and the Division of Physics. IceCube construction was finished in December 2010. A collaboration of 250 physicists and engineers from the United States, Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia and Barbados operate the observatory.

"Building the IceCube Neutrino Observatory at the geographic South Pole was a major effort made possible through many collaborating institutions and the U.S. Antarctic Program," said Scott Borg, division director for Antarctic Sciences in NSF's Office of Polar Programs. "The IceCube Collaboration has been busy analyzing data and the finding published in Nature is an early and significant, result. We are pleased with this achievement but we also anticipate many more important discoveries to follow."

NSF, an independent U.S. government agency, manages the U.S. Antarctic Program, through which it coordinates all U.S. scientific research on the southernmost continent and aboard ships in the Southern Ocean as well as related logistics support.

Improved theoretical understanding and continued data collection from the complete and fully calibrated IceCube detector will help scientists better uncover the mystery of cosmic ray production.

ANTARCTIC TELESCOPE SUPPORTS EXPLANATION OF DARK ENERGY FORCE

FROM NATIONAL SCIENCE FOUNDATION
Credit: Daniel Luong-Van, National Science Foundation

NSF-funded 10-meter South Pole Telescope in Antarctica provides new support for the most widely accepted explanation of dark energy, the source of the mysterious force that is responsible for the accelerating expansion of the universe.

April 2, 2012
Analysis of data from the National Science Foundation- (NSF) funded 10-meter South Pole Telescope (SPT) in Antarctica provides new support for the most widely accepted explanation of dark energy, the source of the mysterious force that is responsible for the accelerating expansion of the universe.

The results begin to hone in on the tiny mass of the neutrinos, the most abundant particles in the universe, which until recently were thought to be without mass.
The SPT data strongly support Albert Einstein's cosmological constant--the leading model for dark energy--even though researchers base the analysis on only a fraction of the SPT data collected and only 100 of the over 500 galaxy clusters detected so far.

"With the full SPT data set we will be able to place extremely tight constraints on dark energy and possibly determine the mass of the neutrinos," said Bradford Benson, an NSF-funded postdoctoral scientist at the University of Chicago's Kavli Institute for Cosmological Physics.

Benson presented the SPT collaboration's latest findings, Sunday, April 1, at the American Physical Society meeting in Atlanta.

These most recent SPT findings are only the latest scientifically significant results produced by NSF-funded researchers using the telescope in the five years since it became active, noted Vladimir Papitashvili, Antarctic Astrophysics and Geospace Sciences program director in NSF's Office of Polar Programs.

"The South Pole Telescope has proven to be a crown jewel of astrophysical research carried out by NSF in the Antarctic," he said. "It has produced about two dozen peer-reviewed science publications since the telescope received its 'first light' on Feb. 17, 2007. SPT is a very focused, well-managed and amazing project."

The 280-ton SPT stands 75 feet tall and is the largest astronomical telescope ever built in the clear and dry air of Antarctica. Sited at NSF's Amundsen-Scott South Pole station at the geographic South Pole, it stands at an elevation of 9,300 feet on the polar plateau. Because of its location at the Earth's axis, it can conduct long-term observations.

NSF manages the U.S. Antarctic Program through which it coordinates all U.S. scientific research on the southernmost continent and aboard ships in the Southern Ocean as well as providing the necessary related logistics support.

An international research collaboration led by the University of Chicago manages the South Pole Telescope. The collaboration includes research groups at Argonne National Laboratory; Cardiff University in Wales; Case Western Reserve University; Harvard University; Ludwig-Maximilians-Universität in Germany; the Smithsonian Astrophysical Observatory; McGill University in Canada; the University of California, Berkeley; the University of California, Davis; the University of Colorado Boulder; and the University of Michigan, as well as individual scientists at several other institutions.

SPT specifically was designed to tackle the dark-energy mystery. The 10-meter telescope operates at millimeter wavelengths to make high-resolution images of Cosmic Microwave Background (CMB) radiation, the light left over from the big bang.

Scientists use the CMB to search for distant, massive galaxy clusters that can be used to pinpoint the properties of dark energy and also help define the mass of the neutrino.
"The CMB is literally an image of the universe when it was only 400,000 years old, from a time before the first planets, stars and galaxies formed in the universe," Benson said. "The CMB has travelled across the entire observable universe, for almost 14 billion years, and during its journey is imprinted with information regarding both the content and evolution of the universe."

The new SPT results are based on a new method that combines measurements taken by the telescope and by NASA and European Space Agency X-ray satellites, and extends these measurements to larger distances than previously achieved.

The most widely accepted property of dark energy is that it leads to a pervasive force acting everywhere and at all times in the universe. This force could be the manifestation of Einstein's cosmological constant that assigns energy to space, even when it is free of matter and radiation.

Einstein considered the cosmological constant to be one of his greatest blunders after learning that the universe is not static, but expanding.

In the late 1990s, astronomers discovered the universe's expansion appears to be accelerating according to cosmic distance measurements based on the relatively uniform luminosity of exploding stars. The finding was a surprise because gravity should have been slowing the expansion, which followed the big bang.

Einstein introduced the cosmological constant into his theory of general relativity to accommodate a stationary universe, the dominant idea of his day. But his constant fits nicely into the context of an accelerating universe, now supported by countless astronomical observations.

Others hypothesize that gravity could operate differently on the largest scales of the universe. In either case, the astronomical measurements point to new physics that have yet to be understood.

As the CMB passes through galaxy clusters, the clusters effectively leave "shadows" that allow astronomers to identify the most massive clusters in the universe, nearly independent of their distance.

"Clusters of galaxies are the most massive, rare objects in the universe, and therefore they can be effective probes to study physics on the largest scales of the universe," said John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics, who heads the SPT collaboration.

"The unsurpassed sensitivity and resolution of the CMB maps produced with the South Pole Telescope provides the most detailed view of the young universe and allows us to find all the massive clusters in the distant universe," said Christian Reichardt, a postdoctoral researcher at the University of California, Berkeley and lead author of the new SPT cluster catalog paper.

The number of clusters that formed over the history of the universe is sensitive to the mass of the neutrinos and the influence of dark energy on the growth of cosmic structures.
"Neutrinos are amongst the most abundant particles in the universe," Benson said. "About one trillion neutrinos pass through us each second, though you would hardly notice them because they rarely interact with 'normal' matter."

The existence of neutrinos was proposed in 1930. They were first detected 25 years later, but their exact mass remains unknown. If they are too massive they would significantly affect the formation of galaxies and galaxy clusters, Benson said.

The SPT team has been able to improve estimates of neutrino masses, yielding a value that approaches predictions stemming from particle physics measurements.

"It is astounding how SPT measurements of the largest structures in the universe lead to new insights on the evasive neutrinos," said Lloyd Knox, professor of physics at the University of California at Davis and member of the SPT collaboration. Knox will also highlight the neutrino results in his presentation on Neutrinos in Cosmology at a special session of the APS on Tuesday, April 3.

NSF's Office of Polar Programs primarily funds the SPT. The NSF-funded Physics Frontier Center of the Kavli Institute for Cosmological Physics, the Kavli Foundation and the Gordon and Betty Moore Foundation provide partial support.