The
IceCube Collaboration, in which Berkeley Lab is a crucial contributor, has
taken the first steps toward clearing up a cosmic mystery – and made the mystery
more intriguing
The IceCube neutrino telescope
encompasses a cubic kilometer of clear Antarctic ice under the South Pole, a
volume seeded with an array of 5,160 sensitive digital optical modules (DOMs)
that precisely track the direction and energy of speeding muons, massive
cousins of the electron that are created when neutrinos collide with atoms in
the ice. The IceCube Collaboration recently announced the results of an
exhaustive search for high-energy neutrinos that would likely be produced if
the violent extragalactic explosions known as gamma-ray bursts (GRBs) are the
source of ultra-high-energy cosmic rays.
“According to a leading model, we would
have expected to see 8.4 events corresponding to GRB production of neutrinos in
the IceCube data used for this search,” says Spencer Klein of the U.S.
Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab),
who is a long-time member of the IceCube Collaboration. “We didn’t see any,
which indicates that GRBs are not the source of ultra-high-energy cosmic rays.”
“This result represents a coming-of-age
of neutrino astronomy,” says Nathan Whitehorn from the University of
Wisconsin-Madison, who led the recent GRB research with Peter Redl of the
University of Maryland. “IceCube, while still under construction, was able to
rule out 15 years of predictions and has begun to challenge one of only two
major possibilities for the origin of the highest-energy cosmic rays, namely
gamma-ray bursts and active galactic nuclei.”
Redl says, “While not finding a neutrino
signal originating from GRBs was disappointing, this is the first neutrino
astronomy result that is able to strongly constrain extra-galactic astrophysics
models, and therefore marks the beginning of an exciting new era of neutrino
astronomy.”
The IceCube Collaboration’s report on
the search appears in the April 19, 2012, issue of the journal Nature.
Blazing
fireballs and nature’s accelerators
Cosmic rays are energetic particles from
deep in outer space – predominately protons, the bare nuclei of hydrogen atoms,
plus some heavier atomic nuclei. Most probably acquire their energy when
naturally accelerated by exploding stars. A few rare cosmic rays pack an
astonishing wallop, however, with energies prodigiously greater than the
highest ever attained by human-made accelerators like CERN’s Large Hadron
Collider. Their sources are a mystery.
“Nature is capable of accelerating
elementary particles to macroscopic energies,” says Francis Halzen, IceCube’s
principal investigator and a professor of physics at the University of
Wisconsin-Madison. “There are basically only two ideas on how she does this: in
gravitationally driven particle flows near the supermassive black holes at the
centers of active galaxies, and in the collapse of stars to a black hole, seen
by astronomers as gamma ray bursts.”
Klein, the deputy director of Berkeley
Lab’s Nuclear Science Division (NSD, explains that in active galactic nuclei
(AGNs) “the black holes suck in matter and eject enormous particle jets,
perpendicular to the galactic disk, which could act as strong linear
accelerators.” Of gamma-ray bursts he says, “Some GRBs are thought to be
collapses of supermassive stars – hypernova – while others are thought to be
collisions of black holes with other black holes or neutron stars. Both types
produce brief but intense blasts of radiation.”
The massive fireballs move away from the
explosion at nearly the speed of light, releasing most of their energy as gamma
rays. The fireballs that give rise to this radiation might also accelerate
particles to very high energies through a jet mechanism similar to that in
AGNs, although compressed into a much smaller volume.
Accelerated protons in a GRB’s jets
should interact with the intense gamma-ray background and strong magnetic
fields to produce neutrinos with energies about five percent of the proton
energy, together with much higher-energy neutrinos near the end of the
acceleration process.
Neutrinos come in three different types
that change and mix as they travel to Earth; the total flux can be estimated
from the muon neutrinos that IceCube concentrates on. The muons these neutrinos
create can travel up to 10 kilometers through the Antarctic ice. Thus many
neutrino interactions occur outside the actual dimensions of the IceCube array
but are nevertheless visible to IceCube’s detectors, effectively enlarging the
telescope’s aperture.
“The way we search for GRB neutrinos is
that we build a huge detector and then we just watch and wait,” says Klein.
“When it comes to detecting neutrinos, size really does matter.”
IceCube watches with its over 5,000
DOMs, digital optical modules conceived, designed, and proven by Berkeley Lab
physicists and engineers, which detect the faint light from each passing muon.
Scientists can rely on their remarkable dependability to wait as long as
necessary. Almost no failures occurred after the DOMs were installed; 98
percent are working perfectly and another one percent are usable. Now frozen in
the ice, they will never be seen again.
IceCube records a million times more
muon tracks moving downward through the ice than upward, mainly debris from
direct cosmic-ray hits on the surface or secondary products of cosmic-ray
collisions with Earth’s atmosphere. Muons moving upward, however, signal
neutrinos that have passed all the way through Earth. When the telescope is
searching for bright neutrino sources in the northern sky, the planet makes a
marvelous filter.
Zeroing
in on gamma-ray bursts
A network of satellites circles the
globe and reports almost 700 GRBs each year, which readily stand out from the
cosmic background. They’re timed, their positions are triangulated, and the
data are distributed by an international group of researchers. Some blaze for
less than two seconds and others for a few minutes. Neutrinos they produce
should arrive at IceCube during the burst or close to it.
“IceCube’s precision timing and charge
resolution, plus its large size, allow it to precisely determine where a
neutrino comes from – often to within one degree,” says Lisa Gerhardt of
Berkeley Lab, whose research has focused on detecting ultra-high-energy
neutrino interactions. Indeed, a GRB neutrino should send a muon track through
the ice with an angular resolution of about one degree with respect to the
GRB’s position in the sky.
IceCube researchers sifted through data
on 307 GRBs from two periods in 2008 and 2009 when IceCube was still under
construction, looking for records of muon trails coincident in time and space
with GRBs. (Forty strings, with 60 DOMs each, had been installed by 2008, and
59 strings by 2009. The finished IceCube has 86 strings.) The fireball model
predicted that when the expected flux from all the samples had been summed, at
least 8.4 related muon events would be found within 10 degrees of a GRB during
the seconds or minutes when it was blazing brightly.
“Different calculations of the neutrino
flux from GRBs are based on slightly different assumptions about how the
neutrinos are produced and on uncertainties such as how fast the fireball is
moving toward us,” says Klein. “Among the published predictions, the lowest
estimate of neutrino production is about a quarter of what the fireball model
predicts. That’s barely consistent with our zero observations.”
Says Halzen, “After observing gamma-ray
bursts for two years, we have not detected the telltale neutrinos for cosmic
ray acceleration.”
If it’s likely that GRBs aren’t up to
the task of accelerating cosmic rays to ultra-high-energies, what are the
options? Klein points to a salient fact about natural accelerators: a small,
rapidly spinning object must accelerate particles very rapidly; this requires
an extremely energy-dense environment, and there are many ways the particles
could lose energy during the acceleration process.
“But remember the other popular model of
ultra-high-energy cosmic rays, active galactic nuclei,” says Klein. “GRBs are
small, but AGNs are big – great big accelerators that may be able to accelerate
particles to very high energies without significant loss.”
Are AGNs the real source of the
highest-energy cosmic rays? IceCube has looked for neutrinos from active
galactic nuclei, but as yet the data sets are not sensitive enough to set
significant limits. For now, IceCube has nothing to say on the subject – beyond
the fact that the fireball model of GRBs can’t meet the specs.
###
“An absence of neutrinos associated with
cosmic ray acceleration in gamma-ray bursts,” by R. Abbasi et al (the IceCube
Collaboration), appears in the April 19, 2012, issue of Nature and is available
online to subscribers at http://www.nature.com/nature/index.html. Collaboration
members currently or formerly with Berkeley Lab include Keith Beattie, Kirill
Filimonov, Lisa Gerhardt, Ariel Goldschmidt, Chang Hyon Ha, Spencer Klein,
Howard Matis, Sandra Miarecki, David Nygren, Gerald Przybylski, Thorsten
Stezelberger, and Robert Stokstad; Filimonov, Gerhardt, Ha, Klein, and Miarecki
are also with the University of California at Berkeley.
The IceCube Collaboration includes over
260 researchers from 42 institutions in 11 countries and is supported by
agencies and foundations in Belgium, Germany, Japan, and Sweden, with primary
funding from the National Science Foundation and major support from the U.S.
Department of Energy’s Office of Science. Visit the IceCube website at
http://icecube.wisc.edu/, read the press release concerning this work at http://icecube.wisc.edu/news/view/52, and
access a selection of images at http://icecube.wisc.edu/~norris/nature_press/.
At Berkeley Lab, DOE’s Office of Science
supports participation in IceCube primarily through the National Energy
Research Scientific Computing Center (NERSC). Visit http://www.nersc.gov/.
DOE’s Office of Science is the single
largest supporter of basic research in the physical sciences in the United States,
and is working to address some of the most pressing challenges of our time. For
more information, please visit science.energy.gov.
Lawrence Berkeley National Laboratory
addresses the world’s most urgent scientific challenges by advancing
sustainable energy, protecting human health, creating new materials, and
revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s
scientific expertise has been recognized with 13 Nobel prizes. The University
of California manages Berkeley Lab for the U.S. Department of Energy’s Office
of Science. For more, visit www.lbl.gov.
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