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  • richardmitnick 2:23 pm on December 8, 2016 Permalink | Reply
    Tags: Alpha Magnetic Spectrometer experiment, , Cosmic Rays, , , , Sam Ting   

    From Symmetry: “A syllabus in cosmic rays” 

    Symmetry Mag

    Symmetry

    12/08/16
    Kathryn Jepsen

    What have scientists learned in five years of studying cosmic rays with the Alpha Magnetic Spectrometer experiment?

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    NASA

    On May 19, 2011, astronauts used a remote-controlled robotic arm to attach a nearly 17,000-pound payload to the side of the International Space Station. That payload was the Alpha Magnetic Spectrometer, or AMS-02, an international experiment sponsored by the US Department of Energy and NASA.

    AMS was designed to detect cosmic rays, highly energetic particles and nuclei that bombard the Earth from space. Since its installation, AMS has collected data from more than 90 billion cosmic ray events, experiment lead Sam Ting reported today in a colloquium at the experiment’s headquarters, CERN European research center.

    Ting, a Nobel Laureate and Thomas Dudley Cabot Professor of Physics at the Massachusetts Institute of Technology, shared a mix of new and recent results during his talk. Together they spelled out the persistent message of the AMS experiment: We have a lot left to learn from cosmic rays.

    For one, cosmic rays could tell us about the imbalance between matter and antimatter in the universe.

    Because matter and antimatter particles are created in pairs, scientists think the Big Bang should have produced half of each. But those evenly matched partners would have annihilated one another, and we would not exist.

    The generally accepted theory is that this imbalance came about thanks to processes in the very young universe that favor matter over antimatter. But an alternative idea is that a large amount of antimatter is still out there; it just hasn’t had a chance to collide with our matter-filled universe.

    One clue that this is the case would be finding an antimatter nucleus in the wild.

    With the negligible amount of antimatter that exists in our universe, “it’s almost impossible to make anything bigger than a proton,” says AMS Deputy Principal Investigator Mike Capell of MIT. “Getting the antimatter together to collide into an antihelium or anticarbon nucleus is not very probable.”

    AMS scientists do not claim to have detected antihelium, but they did announce that they have not ruled out “a few” candidate events.

    “Given the success of the standard cosmological model and the absence of gamma rays from hypothetical matter-antimatter interfaces, I think it’s very implausible that there’d be whole galaxies made of antimatter,” says theoretical astrophysicist Roger Blandford of the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and SLAC National Accelerator Laboratory. “But it’s the sort of investigation that could still give us a surprising discovery.”

    Cosmic rays could also tell us something about dark matter, which has never been detected directly.

    Cosmic rays can consist of a variety of particles, such as electrons or their antimatter counterparts, positrons. In previous measurements, AMS detected a surprising number of positrons on the higher end of its energy range. It is possible that collisions between dark matter particles created this excess of antimatter particles.

    An updated analysis—this one using almost double the number of electrons and positrons—continues to show this excess. But dark matter isn’t the only possible cause, Blandford says.

    “One interpretation is that one is seeing the annihilation of dark matter particles,” he says. “But there might be equally reasonable explanations associated with traditional astrophysics that could make the same sort of signal.”

    Pulsars are a particularly difficult alternative source to rule out. But AMS scientists anticipate that they will collect enough data to better discriminate between models by 2024, Ting said in his presentation.

    Cosmic rays could tell us about their history.

    As particles in cosmic rays approach light speed, time effectively slows down for them, as Albert Einstein predicted in his theory of relativity. We can see evidence of time dilation in the extended lifetimes of particles traveling near light speed.

    In a forthcoming AMS result, scientists look at just how much the lifetimes of isotopes of beryllium stretch as they travel in cosmic rays. Based on that measurement, they estimate the cosmic rays we see in our galaxy are about 12 million years old.

    Cosmic rays could tell us about what they go through on their trip to Earth.

    Both observation and theory have a ways to go in this area, Blandford says. “They are both works in progress and, despite great advances, we still do not understand how cosmic rays propagate from their sources—mainly supernova remnants—to Earth. ”

    When cosmic rays get into collisions, they can produce secondary cosmic rays, which are made up of different ingredients. In a recently published result studying the ratio of boron (found only in secondary cosmic rays) to carbon (found in primary cosmic rays) at different energies, AMS scientists found possible evidence of turbulence in the cosmic rays’ path to our planet—but nothing that would explain the positron excess.

    Finally, cosmic rays could tell us that we don’t know what we think we know.

    In an unpublished analysis, AMS scientists found that their measurements of the spectra and ratios of different nuclei—protons, lithium and helium—did not fit well with predictions. This could mean that scientists’ assumptions about cosmic rays need to be reexamined.

    AMS scientists want to help with that. They plan to collect data from hundreds of billions of primary cosmic rays in the coming years as their experiment continues its orbit about 240 miles above the Earth.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 12:54 pm on November 30, 2016 Permalink | Reply
    Tags: Cosmic Rays, , , MicroBooNE, , ,   

    From FNAL: “Handy and trendy: MicroBooNE’s new look” 

    FNAL II photo

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    November 30, 2016
    Ricarda Laasch

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    MicroBooNE’s shiny new exterior helps scientists identify cosmic rays masquerading as neutrinos. From left: Elena Gramellini, Thomas Mettler. Martin Auger, Mark Shoun, John Voirin. Photo: Reidar Hahn

    The signals of cosmic rays

    Cosmic rays are a constant rain of particles that are created in our sun or faraway stars and travel through space to our planet.

    They’re subjects of many important physics studies, but for MicroBooNE’s research, they simply get in the way. That’s because MicroBooNE scientists are looking for something else — abundant, subtle particles called neutrinos.

    FNAL/MicrobooNE
    FNAL/MicrobooNE

    Unlocking the secrets neutrinos hold could help us understand the evolution of our universe, but they’re exceedingly difficult to measure. Fleeting neutrinos are rarely captured, even as they sail through detectors built for that purpose.

    Add to that the fact that their interactions are potentially drowned in a sea of cosmic rays rushing through the same detector, and you get a sense of the formidable challenge that neutrinos represent.

    The MicroBooNE experiment starts with Fermilab’s powerful accelerators, which create neutrino beams that are then propelled through the MicroBooNE detector.

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    July 8, 2015 Fermilab’s Main Injector accelerator, one of the most powerful particle accelerators in the world, has just achieved a world record for high-energy beams for neutrino experiments. Photo: Fermilab

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    Fermilab’s accelerator complex comprises seven particle accelerators and storage rings. It produces the world’s most powerful, high-energy neutrino beam and provides proton beams for a variety of experiments and R&D programs.

    Fermilab is currently upgrading its accelerator complex to deliver high-intensity neutrino beams and to provide beams for a broad range of new and existing experiments, including the Long-Baseline Neutrino Experiment, Muon g-2 and Mu2e.

    “The neutrino beam here at the lab gives us the right conditions to study neutrinos,” said Elena Gramellini, a Yale University graduate student on the MicroBooNE experiment. “Our challenge is to pick out neutrinos from many cosmic rays passing through the detector.”

    Since cosmic rays are made of some of the same particles produced when a neutrino interacts with matter, they leave signals in the MicroBooNE detector that are often similar to the sought-after neutrino signals. Scientists need to be able to sort the cosmic rays in the MicroBooNE data from the neutrino signals.

    Tagging and sorting

    Even several feet of concrete enclosure would not completely block cosmic rays from hitting a detector such as MicroBooNE, not to mention that such a structure would be inconvenient and expensive. Instead, MicroBooNE uses the aforementioned panels, called a cosmic ray tagger, or CRT. While the panels don’t block cosmic rays, they do detect them.

    Each CRT panel has particle-detecting components – strips of scintillator – that lie beneath its shiny aluminum enclosure. Cosmic ray particles can easily pass through aluminum and the scintillator — a clear, plastic-like material — on their way toward the MicroBooNE detector.

    The cosmic ray particles deposit energy in the plastic scintillator, which then emits light. An optical fiber buried inside the scintillator captures the emitted light and transmits it to devices that generate the digital information that tells scientists where and when the cosmic ray struck.

    “With our current layout of scintillator strips in each panel, we are able to tell precisely where the cosmic ray enters the MicroBooNE detector after it left the panel,” said Igor Kreslo, professor at the University of Bern who designed the CRT panels for MicroBooNE. “Our design effort really paid off and now ensures thorough cosmic ray tracking.“

    So why the shiny aluminum shell? It blocks unwanted light from the detector’s immediate surroundings so that only light created by cosmic rays inside a CRT panel reaches the optical fiber and is detected.

    Putting up panels

    The 49 rectangular CRT panels are the contribution of the University of Bern in Switzerland, one of the 28 institutions collaborating on MicroBooNE worldwide. They produced the panels last winter and shipped them to Fermilab during the spring.

    “This was a large project for us, and it took everyone in Bern to finish everything in time,” said Martin Auger, scientist at the University of Bern who planned the arrangement of the CRT panels. “A key moment was the test of the CRT panels after the long journey to Fermilab. All the panels arrived in good shape!”

    The installation team overcame a number of challenges —including the tight space in which MicroBooNE stands — to successfully place the panels around the detector.

    “The installation crew is a crack team of veteran Fermilab employees,” said John Voirin, who leads experiment installations at the laboratory. “In the end we have a very elegant, safe operating product that is a valuable asset to the experiment.”

    Later this year the group will complete the installation by placing the final layer on top of the MicroBooNE detector. Even without it, the CRT already greatly enhances the capabilities of the experiment.

    “We started taking data just in time for the first neutrinos delivered to the experiment,” Gramellini said.

    See the full article here .

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 11:12 am on November 30, 2016 Permalink | Reply
    Tags: , , Cosmic Rays,   

    From Physics: “Focus: More Hints of Exotic Cosmic-Ray Origin” 

    Physics LogoAbout Physics

    Physics Logo 2

    Physics

    November 28, 2016
    Michael Schirber

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    The Alpha Magnetic Spectrometer (AMS) aboard the International Space Station

    New Space Station data support a straightforward model of cosmic-ray propagation through the Galaxy but also add to previous signs of undiscovered cosmic-ray sources such as dark matter.

    Observing the constant rain of cosmic rays hitting Earth can provide information on the “magnetic weather” in other parts of the Galaxy. A new high-precision measurement of two cosmic-ray elements, boron and carbon, supports a specific model of the magnetic turbulence that deflects cosmic rays on their journey through the Galaxy. The data, which come from the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station, appear to rule out alternative models for cosmic-ray propagation. The failure of these models—which were devised to explain recent observations of cosmic-ray antimatter—implies a possible exotic origin for some cosmic rays.

    The majority of cosmic rays are particles or nuclei produced in supernovae or other astrophysical sources. However, as these so-called primary cosmic rays travel through the Galaxy to Earth, they collide with gas atoms in the interstellar medium. The collisions produce a secondary class of cosmic rays with masses and energies that differ from primary cosmic rays. To investigate the relationship of the two classes, astrophysicists often look at the ratio of the number of detections of two nuclei, such as boron and carbon. For the most part, carbon cosmic rays have a primary origin, whereas boron is almost exclusively created in secondary processes. A relatively high boron-to-carbon (B/C) ratio in a certain energy range implies that the relevant cosmic rays are traversing a lot of gas before reaching us. “The B/C ratio tells you how cosmic rays propagate through space,” says AMS principal investigator Samuel Ting of MIT.

    Previous measurements of the B/C ratio have had large errors of 15% or more, especially at high energy, mainly because of the brief data collection time available for balloon-based detectors. But the AMS has been operating on the Space Station for five years, and over this time it has collected more than 80 billion cosmic rays. The AMS detectors measure the charges of these cosmic rays, allowing the elements to be identified. The collaboration has detected over ten million carbon and boron nuclei, with energies per nucleon ranging from a few hundred MeV up to a few TeV.

    The B/C ratio decreases with energy because higher-energy cosmic rays tend to take a more direct path to us (and therefore experience fewer collisions producing boron). By contrast, lower-energy cosmic rays are diverted more strongly by magnetic fields, so they bounce around like pinballs among magnetic turbulence regions in the Galaxy. Several theories have been proposed to describe the size and spacing of these turbulent regions, and these theories lead to predictions for the energy dependence of the B/C ratio. However, previous B/C observations have not been precise enough to favor one theory over another. The AMS data show very clearly that the B/C ratio is proportional to the energy raised to the -1/3 power. This result matches a prediction based on a theory of magnetohydrodynamics developed in 1941 by the Russian mathematician Andrey Kolmogorov [1].

    These results conflict with models that predict that the B/C ratio should exhibit some more complex energy dependence, such as kinks in the B/C spectrum at specific energies [2]. Theorists proposed these models to explain anomalous observations—by AMS and other experiments—that showed an increase in the number of positrons (anti-electrons) reaching Earth relative to electrons at high energy (see 3 April 2013 Viewpoint). The idea was that these “excess” positrons are—like boron—produced in collisions between cosmic rays and interstellar gas. But such a scenario would require that cosmic rays encounter additional scattering sites, not just magnetically turbulent regions. By ruling out these models, the AMS results support the alternative explanation—a new primary cosmic ray source that emits positrons. Candidates include pulsars and dark matter, but a lot of mystery still surrounds the unexplained positron data.

    Igor Moskalenko from Stanford University is very surprised at the close match between the data and the Kolmogorov model. He expected that the ratio would deviate from a single power law in a way that might provide clues to the origin of the excess positrons. “This is a dramatic result that should lead to much better understanding of interstellar magnetohydrodynamic turbulence and propagation of cosmic rays,” he says. “On the other hand, it is very much unexpected in that it makes recent discoveries in astrophysics of cosmic rays even more puzzling.”

    This research is published in Physical Review Letters.

    References

    A. N. Kolmogorov, The Local Structure of Turbulence in Incompressible Viscous Fluid for Very Large Reynolds Numbers, Dokl. Akad. Nauk SSSR 30, 301 (1941).
    A. E. Vladimirov, G. Jóhannesson, I. V. Moskalenko, and T. A. Porter, Testing the Origin of High-Energy Cosmic Rays, Astrophys. J. 752, 68 (2012).

    See the full article here .

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    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

     
  • richardmitnick 11:09 am on November 8, 2016 Permalink | Reply
    Tags: , Cosmic Rays, ,   

    From Pierre Auger Observatory: “Evidence for a mixed mass composition at the ‘ankle’ in the cosmic ray spectrum” 

    pierre-auger-observatory-bloc

    Pierre Auger Observatory

    Undated
    No writer credit found

    Pierre Auger Observatory Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes
    Pierre Auger Observatory Pierre Auger Observatory in the western Mendoza Province, Argentina, near the Andes

    The highest energy cosmic rays remain elusive and mysterious, and their study requires extraordinary efforts. At the Pierre Auger Observatory in Argentina, the giant air showers of particles created by these cosmic rays are detected when they slam into the ground by a large array of water tanks equipped with electronic detectors. But on dark nights they are simultaneously detected by telescopes sensitive to the faint sky glow left by the air showers. The new report by the Pierre Auger Collaboration correlates in detail the signals in the water tanks with those from the telescopes. The correlation is uniquely sensitive to the presence in the primary beam of nuclei with different masses, and is used in particular to help resolve how many types of atomic nuclei contribute to the cosmic ray flux.

    Evidence for a mixed mass composition at the ‘ankle’ in the cosmic ray spectrum

    Cosmic rays are energetic particles (atomic nuclei) impinging upon the Earth from the vast reaches of the cosmos. They can have tremendous energies and thus must originate in remarkable but still mysterious astrophysical sources. If we understand the nature of these particles, we may be able to find the extragalactic sources of the highest energy cosmic rays.

    The distribution of particle numbers with energy, or ‘spectrum’, shows a striking and very rapid reduction in numbers with increasing energy. At an energy of E ≈ 5 × 1018 eV a flattening in the spectrum (slightly slower rate of reduction with energy) is observed. This feature is called the ‘ankle’ in the cosmic ray spectrum.

    The transition from cosmic rays from a Galactic origin to an extragalactic component may cause such a flattening. Alternatively, the ‘dip’ model in which highly energetic extragalactic protons interact with photons from the cosmic microwave background also creates a flattening of the spectrum. The different models can be distinguished by the predicted cosmic ray composition in the ankle region.

    In this paper the important characteristics of the mass composition of the cosmic rays — the spread of their masses — is measured using a method relatively insensitive to either experimental uncertainties or uncertainties in the particle interaction models. The method takes advantage of the hybrid design of the Pierre Auger Observatory, which measures both the cascade of particles when it reaches the ground and the development of the shower through the atmosphere using specialized fluorescence telescopes on dark clear nights.

    The method uses the independent information on the depth of shower maximum (Xmax) from the fluorescence telescopes and the signal at 1000 m from the shower axis, S(1000), from the surface detector. The original idea of the method is that a direct and robust estimation of the spread of masses in the primary beam can be obtained via a measurement of the correlation between Xmax and S(1000). For any given type of cosmic ray nucleus this correlation is close to or larger than zero as shown in the first figure (left) for proton and iron air showers generated with the particle interaction model EPOS-LHC (the correlation coefficient is denoted as rG). For mixed compositions a negative correlation emerges due to a very general characteristic of air showers: showers from heavier nuclei have smaller Xmax and larger S(1000) (due to a larger number of muons). Thus in a mixed composition shallower showers have on average larger signals. The correlation becomes more negative the larger the spread of masses.

    For events successfully reconstructed with both Auger fluorescence and surface detectors in the energy range around the ankle E = 1018.5 – 1019.0 eV a significant negative correlation was found rG = -0.125 ± 0.024 (stat), as illustrated in the first figure (right). This value is at least 5 standard deviations away from predictions for pure compositions or any composition made up of protons and helium only.

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    Correlation between X*max (Xmax scaled to 1019 eV) and S*38 (S(1000) scaled to 1019 eV,
    38° of zenith angle). Left: simulations for protons and iron nuclei with EPOS-LHC. Right: Auger data.

    To estimate the spread of the primary mass numbers σ(ln A) the value of the correlation found in the data is compared to values from simulations for compositions with varying fractions of protons, helium, oxygen and iron. As illustrated in the second figure, for different particle interaction models (EPOS-LHC or QGSJetII-04) the data can be described with compositions having a spread in atomic mass numbers within the range 1.0 ≲ σ(ln A)≲ 1.7. The results are practically unaffected by systematic uncertainties on Xmax and S(1000), or by modifications of the key parameters of the particle interaction models.

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    Correlation coefficient as a function of spread of cosmic ray mass number, in simulated (with two separate particle interaction models, EPOS-LHC and QGSJetII-04) mixtures with different fractions of protons, helium, oxygen and iron nuclei (points) compared to the correlation value found in Auger data (shaded area). The ranges of cosmic ray mass spread compatible with the data are marked by vertical lines.

    The conclusion that the mass composition around the cosmic ray ankle energy is not pure but mixed has important consequences for theoretical source models. Proposals of almost pure compositions, such as the dip scenario, are disfavored as a sole explanation of the ultra-high energy cosmic rays. These findings, together with other observations made at the Pierre Auger Observatory, indicate that nuclei with mass number A > 4 are accelerated to ultra-high energies E ≥ 1018.5 eV and are able to escape the source environment. The search for the final resolution of the ankle puzzle is continuing and new astrophysical models are already emerging, e.g., including modifications of the nuclear composition in the environment of the acceleration sites.

    Related paper:
    Evidence for a mixed mass composition at the ‘ankle’ in the cosmic-ray spectrum
    A. Aab et al. (Pierre Auger Collaboration), Phys.Lett. B762 (2016) 288-295
    https://doi.org/10.1016/j.physletb.2016.09.039
    https://arxiv.org/abs/1609.08567

    See the full article here .

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    Pierre Augur Observatory

    The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling nearly at the speed of light and each with energies beyond 1018 eV. In Earth’s atmosphere such particles interact with air nuclei and produce various other particles. These effect particles (called an “air shower”) can be detected and measured. But since these high energy particles have an estimated arrival rate of just 1 per km2 per century, the Auger Observatory has created a detection area of 3,000 km2 (1,200 sq mi) — the size of Rhode Island, or Luxembourg — in order to record a large number of these events. It is located in the western Mendoza Province, Argentina, near the Andes.

    Construction began in 2000,[1] the observatory has been taking production-grade data since 2005 and was officially completed in 2008.

    The observatory was named after the French physicist Pierre Victor Auger. The project was proposed by Jim Cronin and Alan Watson in 1992. Today, more than 500 physicists from nearly 100 institutions around the world[2] are collaborating to maintain and upgrade the site in Argentina and collect and analyse the measured data. The 15 participating countries shared the $50 million construction budget, each providing a small portion of the total cost.

     
  • richardmitnick 9:07 am on October 8, 2016 Permalink | Reply
    Tags: , , Cosmic Rays, , High energy neutrinos, , ,   

    From IceCube: “Neutrinos and gamma rays, a partnership to explore the extreme universe” 

    icecube
    IceCube South Pole Neutrino Observatory

    07 Oct 2016
    Sílvia Bravo

    Solving the mystery of the origin of cosmic rays will not happen with a “one-experiment show.” High-energy neutrinos might be produced by galactic supernova remnants or by active galactic nuclei as well as other potential sources that are being sought. And, if our models are right, gamma rays at lower energies could also help identify neutrino sources and, thus, cosmic-ray sources. It’s sort of a “catch one, get them all” opportunity.

    IceCube’s collaborative efforts with gamma-ray, X-ray, and optical telescopes started long ago. Now, the IceCube, MAGIC and VERITAS collaborations present updates to their follow-up programs that will allow the gamma-ray community to collect data from specific sources during periods when IceCube detects a higher number of neutrinos.

    MAGIC Cherenkov gamma ray telescope  on the Canary island of La Palma, Spain
    MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

    CfA/VERITAS, AZ, USA
    “CfA/VERITAS, AZ, USA

    Details of the very high energy gamma-ray follow-up program have been submitted to the Journal of Instrumentation.

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    Image: Juan Antonio Aguilar and Jamie Yang. IceCube/WIPAC

    From efforts begun by its predecessor AMANDA, IceCube initiated a gamma-ray follow-up program with MAGIC for sources of electromagnetic radiation emissions with large time variations. If we can identify periods of increased neutrino emission, then we can look for gamma-ray emission later on from the same direction.

    For short transient sources, such as gamma-ray bursts and core-collapse supernovas, X-ray and optical wavelength telescopes might also detect the associated electromagnetic radiation. In this case, follow-up observations are much more time sensitive, with electromagnetic radiation expected only a few hours after neutrino emission from a GRB or a few weeks after a core-collapse supernova.

    Updates to this transient follow-up system will use a multistep high-energy neutrino selection to send alerts to gamma-ray telescopes, such as MAGIC and VERITAS, if clusters of neutrinos are observed from a predefined list of potential sources. The combined observation of an increased neutrino and gamma-ray flux could point us to the first source of astrophysical neutrinos. Also, the information provided by both cosmic messengers will improve our understanding of the physical processes that power those sources.

    The initial selection used simple cuts on a number of variables to discriminate between neutrinos and the atmospheric muon background. IceCube, MAGIC, and VERITAS are currently testing a new event selection that uses learning machines and other sophisticated discrimination algorithms to take into account the geometry and time evolution of the hit pattern in IceCube events. Preliminary studies show that this advanced event selection has a sensitivity comparable to offline point-source samples, with a 30-40% sensitivity increase in the Northern Hemisphere with respect to the old selection. The new technique does not rely only on catalogues of sources and allows observing neutrino flares in the Southern Hemisphere. Thus, those alerts will also be forwarded to the H.E.S.S. collaboration, expanding the gamma-ray follow-up program to the entire sky.

    HESS Cherenko Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg
    HESS Cherenko Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg

    During the last few years, IceCube has sent several alerts to VERITAS and MAGIC that have not yet resulted in any significant correlation between neutrino and gamma-ray emission. For some of those, however, the source was not in the reach of the gamma-ray telescopes, either because it was out of the field of view or due to poor weather conditions. Follow-up studies have allowed setting new limits on high-energy gamma-ray emission.

    With the increased sensitivity in the Northern Hemisphere and new alerts to telescopes in the Southern Hemisphere, the discovery potential of these joint searches for neutrino and gamma-ray sources is greatly enhanced. Stay tuned for new results!

    See the full article here .

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    ICECUBE neutrino detector

    IceCube neutrino detector interior

    IceCube is a particle detector at the South Pole that records the interactions of a nearly massless sub-atomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. In addition, exploring the background of neutrinos produced in the atmosphere, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice.

     
  • richardmitnick 10:22 am on October 6, 2016 Permalink | Reply
    Tags: Cherenkov radiation, , Cosmic Rays, , Scientists Catch The Highest Energy Particles By Making Them Go Faster Than Light   

    From Ethan Siegel: “Scientists Catch The Highest Energy Particles By Making Them Go Faster Than Light” 

    Ethan Siegel

    Oct 6, 2016

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    Cosmic rays shower particles by striking protons and atoms in the atmosphere, but they also emit light due to Cherenkov radiation. Image credit: Simon Swordy (U. Chicago), NASA.

    If you pump more and more energy into a massive particle, it moves faster and faster, asymptotically approaching the speed of light. But if there’s too much energy in your particle, then your standard way of building a detector — to force the particle to collide with another and detect the properties of what comes out — simply won’t work. The faster particles go, the faster and more indeterminate the detector tracks are, meaning that your attempts to reconstruct the original particle’s energy, mass, charge and other properties fare worse and worse. The “brute force” solution of building larger and more sensitive detectors becomes prohibitively expensive very quickly; that simply won’t do. But there’s a trick that physicists can use: slow down the speed of light, and force that particle to spontaneously slow down.

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    Particle accelerators on Earth, like the LHC at CERN, can accelerate particles very close to — but not quite up to — the speed of light. Image credit: LHC / CERN.

    It’s true that Einstein had it right all the way back in 1905: there is a maximum speed to anything in the Universe, and that speed is the speed of light in a vacuum (c), 299,792,458 m/s. Cosmic ray particles can go faster than anything on Earth, even at the LHC. Here’s a fun list of how fast various particles can go at a variety of accelerators, and from space:

    980 GeV: fastest Fermilab proton, 0.99999954c, 299,792,320 m/s.
    6.5 TeV: fastest LHC proton, 0.9999999896c, 299,792,455 m/s.
    104.5 GeV: fastest LEP electron (fastest accelerator particle ever), 0.999999999988c, 299,792,457.9964 m/s.
    5 x 10^19 GeV: highest energy cosmic rays ever (assumed to be protons), 0.99999999999999999999973c, 299,792,457.999999999999918 m/s.

    When it comes to the absolute fastest particles of all, Earth-based accelerators don’t stand a chance.

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    The high energy radiation and particles from the active galaxy NGC 1275 are only one example of astrophysical high-energy phenomena that far exceed anything on Earth. Image credit: NASA, ESA, Hubble Heritage (STScI/AURA).

    As good as our control of electric and magnetic fields are, to bend charged particles into a ring and accelerate them with a “kick” each time they go by, we can’t compete with the natural phenomena of the Universe. Black holes, neutron stars, merging stellar systems, supernovae and other astrophysical catastrophes can accelerate particles to far greater speeds than anything we could ever do on Earth. The highest energy cosmic rays travel so close to the speed of light in a vacuum that if you were to race a proton of this energy and a photon to the nearest star-and-back, the photon would arrive first… with the proton just 22 microns behind, arriving 700 femtoseconds later.

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    A portion of the digitized sky survey with the nearest star to our Sun, Proxima Centauri, shown in red in the center. Image credit: David Malin, UK Schmidt Telescope, DSS, AAO.

    1.2m UK Schmidt Telescope at Siding Spring Observatory
    AAO UK Schmidt Telescope Interior
    AAO 1.2m UK Schmidt Telescope at Siding Spring Observatory, near Coonabarabran, New South Wales, Australia

    But photons only move at that perfect speed-of-light (c) if they’re in a vacuum, or the complete emptiness of space. Put one in a medium — like water, glass, or acrylic — and they’ll move at the speed of light in that medium, which is less than 299,792,458 m/s by quite a bit. Even air, which is pretty close to a vacuum, slows down light by 0.03% from its maximum possible speed. This isn’t that much, but it does mean something remarkable: these high-energy particles that come into the atmosphere are now moving faster than light in that medium, which means they emit a special type of radiation known as Cherenkov radiation.

    5
    The Advanced Test Reactor core, Idaho National Laboratory. Image credit: Argonne National Laboratory.

    When you move faster than light in a medium, you emit photons radially outward in all directions, but they make a “cone” of light because the particle emitting them is moving so fast. Nuclear reactors, which emit fast particles, are surrounded by water to shield people from the particles the reactor emits. But, because those particles move faster than the speed of light in water, that water has a characteristic blue glow due to this radiation! The atmosphere doesn’t quite glow blue, but when a cosmic ray in a certain energy range passes through the atmosphere, the Cherenkov radiation is emitted at a different specific frequency, and is detectable on the ground by an array of telescopes of the right size.

    6
    CfA/VERITAS, AZ, USA

    Presently, observatories such as H.E.S.S., MAGIC and VERITAS are set up to be atmospheric imaging Cherenkov telescopes, and have provided locations and energies for the sources of Very High Energy Cosmic Rays like never before.

    HESS Cherenko Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg
    HESS Cherenko Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg

    MAGIC Cherenkov gamma ray telescope  on the Canary island of La Palma, Spain
    “MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

    But, as scientists, we want to do better. This year, for the first time, construction has begun on the most ambitious attempt to view the sources of these types of particles: the Cherenkov Telescope Array. All told, the array will consist of 118 dishes: 19 in the northern hemisphere and 99 in the southern hemisphere, with the northern array focusing on “lower” energies and sources outside of the galaxy, and the southern array focusing on the full spectrum of energies and sources inside the galaxy. All told, 32 countries are presently involved in this nearly $300 million project, with ESO’s Paranal–Armazones site in the Atacama Desert of Chile hosting the greatest number of dishes.

    7
    An artist’s concept for the conceptual design of the Cherenkov Telescope Array. Image credit: G. Pérez, IAC.

    If you want to catch particles as they were before they ever reached Earth, you need to go to space to see them. But that’s expensive; the Fermi gamma-ray telescope (which detects individual high energy photons, not cosmic rays directly) cost approximately $690 million total. For less than half that cost, you can catch the particles that result from cosmic rays hitting the atmosphere in more than 100 locations across the globe, all because we understand the physics of particles that move faster-than-light through the atmosphere. More than that, the science prospects include understanding the origin of relativistic cosmic particles, the acceleration mechanisms around neutron stars and black holes and might even improve astrophysical searches for dark matter. You might not ever break Einstein’s laws, but figuring out the tricks to take advantage of their intricacies might be an even better solution!

    See the full article here .

    Please help promote STEM in your local schools.

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 9:47 am on October 6, 2016 Permalink | Reply
    Tags: , , Cosmic Rays, , , , Reconstruction of the muon production depth in the atmosphere with the surface detectors of the Pierre Auger Observatory   

    From Pierre Auger Observatory: “Reconstruction of the muon production depth in the atmosphere with the surface detectors of the Pierre Auger Observatory” 

    pierre-auger-observatory-bloc

    10.6.16
    No writer credit found

    Pierre Auger Observatory

    Cosmic rays are energetic particles, mostly atomic nuclei, raining down upon the Earth from the depths of the cosmos. Understanding their detailed nature and origins remains a primary goal in modern-day astroparticle physics.

    When cosmic rays enter the Earth’s atmosphere they produce a shower of billions of particles. These particles travel nearly at the speed of light and a large part of them will reach the ground and can be detected by the water-Cherenkov stations of the Pierre Auger Observatory. The most energetic cosmic rays are able to produce particle showers which have a footprint at the ground of a few km2. Among the produced particles there are muons, elementary particles similar to electrons but with a much greater mass. The paper shows a novel technique to estimate where in the atmosphere the muons that we measure at Earth are produced. As the muon signal is a measure for the nature of the primary cosmic ray, this technique may help solve one of the most persisting questions surrounding ultra-high energy cosmic rays: What are they?

    Identifying the muons using the surface detector of the Pierre Auger Observatory is not an easy task. However, each type of particle (muon, electron, photon) produces a signal with a characteristic amplitude and time structure. Muonic signals are spiky and have a narrow time distribution (tens to hundreds of nanoseconds) while signals produced by electrons and photons are small, smoother-looking and characterised by a wide time distribution (microseconds). This is especially true for stations far from the impact point of the shower at the ground.

    By applying a “low-pass” filter to the signal repeatedly, it is possible to gradually separate the low-frequency smooth electromagnetic signal from the high-frequency component which is primarily due to muons. The technique is effective over a large range of arrival directions (i.e. for zenith angles between 45° and 65°), and for energies greater than 1.5 x 1019 eV. Once the muon signal is estimated station by station, together with its time structure, the atmospheric depth at which muons had been produced is obtained by applying a model of their arrival time at the ground.

    The geometry used to reconstruct the muon production point is depicted in the figure on the left. The model is based on the fact that muons are produced close to the shower axis and that they travel to the ground following straight lines. For each muon sampled at the ground, its atmospheric production depth is estimated: the set of these forms the Muon Production Depth (MPD) distribution, as shown in the figure on the right.

    2
    Left: Schematic view of the geometry used to obtained the muon production point. Muons are produced at the position z along the shower axis and, after traveling a distance l, they reach the ground and may hit a station of the surface detector. Right: The reconstructed MPD distribution for a imulated shower induced by a proton with θ=48° and E = 6.3 x 1019 eV.

    The maximum of the distribution, which is called Xμmax , is the point at which the maximum number of muons is produced, which is a function of the mass of the cosmic ray. Heavy primaries induce showers which reach maximum production higher in the atmosphere compared to light primaries.This method can thus be exploited to study the mass composition of the most energetic cosmic rays detected by the Pierre Auger Observatory.

    In addition, Xμmax depends sensitively on the properties of the hadronic particle interactions taking place in the atmosphere. Its measure is a nearly optimal tool to test hadronic interaction models at energies well above those attainable with accelerators such as the Large Hadron Collider (LHC) at CERN.

    This novel technique is described in detail in the paper below. Auger results on the mass composition of the highest energy cosmic rays will follow in future publications.

    Related paper: Measurement of the Muon Production Depths at the Pierre Auger Observatory,
    Laura Collica for the Pierre Auger Collaboration: Eur. Phys. J. Plus (2016) 131: 301,

    See the full article here .

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    Pierre Augur Observatory

    The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling nearly at the speed of light and each with energies beyond 1018 eV. In Earth’s atmosphere such particles interact with air nuclei and produce various other particles. These effect particles (called an “air shower”) can be detected and measured. But since these high energy particles have an estimated arrival rate of just 1 per km2 per century, the Auger Observatory has created a detection area of 3,000 km2 (1,200 sq mi) — the size of Rhode Island, or Luxembourg — in order to record a large number of these events. It is located in the western Mendoza Province, Argentina, near the Andes.

    Construction began in 2000,[1] the observatory has been taking production-grade data since 2005 and was officially completed in 2008.

    The observatory was named after the French physicist Pierre Victor Auger. The project was proposed by Jim Cronin and Alan Watson in 1992. Today, more than 500 physicists from nearly 100 institutions around the world[2] are collaborating to maintain and upgrade the site in Argentina and collect and analyse the measured data. The 15 participating countries shared the $50 million construction budget, each providing a small portion of the total cost.

     
  • richardmitnick 12:37 pm on July 29, 2016 Permalink | Reply
    Tags: , , Cosmic Rays, , What Are The Most Energetic Particles In The Universe?   

    From Ethan Siegel: “What Are The Most Energetic Particles In The Universe?” 

    From Ethan Siegel

    Jul 29, 2016

    1
    The production of a cosmic ray shower, produced by an incredibly energetic particle from far outside our Solar System. Image credit: Pierre Auger Observatory, via http://apcauger.in2p3.fr/Public/Presentation/.

    You might think of the largest and most powerful particle accelerators in the world — places like SLAC, Fermilab and the Large Hadron Collider — as the source of the highest energies we’ll ever see. But everything we’ve ever done here on Earth has absolutely nothing on the natural Universe itself! In fact, if you were interested in the most energetic particles on Earth, looking at the Large Hadron Collider — at the 13 TeV collisions occurring inside — you wouldn’t even be close to the highest energies. Sure, those are the highest human-made energies for particles, but we’re constantly bombarded all the time by particles far, far greater in energy from the depths of space itself: cosmic rays.

    2
    An illustration of a very high energy process in the Universe: a gamma-ray burst. Image credit: NASA / D. Berry.

    You didn’t need to be in space, or even to have any type of flight, to know that these particles existed. Even before the first human beings ever left the surface of the Earth, it was widely known that up there, above the protection of the Earth’s atmosphere, outer space was filled with high-energy radiation. How did we know?

    The first clues came from looking at one of the simplest electricity experiments you can do on Earth, involving an electroscope. If you’ve never heard of an electroscope, it’s a simple device: take two thin pieces of conducting, metal foil, place them in an airless vacuum and connect them to a conductor on the outside that you can control the electric charge of.

    3
    The electric charge on an electroscope, depending on what you charge it with, and how the leaves inside respond. Image credit: Figure 16-8 from Boomeria’s Honors Physics page, via http://boomeria.org/physicstextbook/ch16.html.

    If you place an electric charge on one of these devices — where two conducting metal leaves are connected to another conductor — both leaves will gain the same electric charge, and repel one another as a result. You’d expect, over time, for the charge to dissipate into the surrounding air, which it does. So you might have the bright idea to isolate it as completely as possible, perhaps creating a vacuum around the electroscope once you charge it up.

    But even if you do, the electroscope still slowly discharges! In fact, even if you placed lead shielding around the vacuum, it would still discharge, and experiments in the early 20th century gave us a clue as to why: if you went to higher and higher altitudes, the discharge happened more quickly. A few scientists put forth the hypothesis that the discharge was happening because high-energy radiation — radiation with both extremely large penetrating power and an extraterrestrial origin — was responsible for this.

    4
    Victor Hess in his balloon-borne, cosmic ray experiment. Image credit: American Physical Society.

    Well, you know the deal when it comes to science: if you want to confirm or refute your new idea, you test it! So in 1912, Victor Hess conducted balloon-borne experiments to search for these high-energy cosmic particles, discovering them immediately in great abundance and henceforth becoming the father of cosmic rays.

    The early detectors were remarkable in their simplicity: you set up some sort of emulsion (or later, a cloud chamber) that’s sensitive to charged particles passing through it and place a magnetic field around it. When a charged particle comes in, you can learn two extremely important things:

    The particle’s charge-to-mass ratio and
    its velocity,

    simply dependent on how the particle’s track curves, something that’s a dead giveaway so long as you know the strength of the magnetic field you applied.

    In the 1930s, a number of experiments — both in early terrestrial particle accelerators and via more sophisticated cosmic ray detectors — turned up some interesting information. For starters, the vast majority of cosmic ray particles (around 90%) were protons, which came in a wide range of energies, from a few mega-electron-Volts (MeV) all the way up to as high as they could be measured by any known equipment! The vast majority of the rest of them were alpha-particles, or helium nuclei with two protons and two neutrons, with comparable energies to the protons.

    5
    An illustration of cosmic rays striking Earth’s atmosphere. Image credit: Simon Swordy (U. Chicago), NASA.

    When these cosmic rays hit the top of the Earth’s atmosphere, they interacted with it, producing cascading reactions where the products of each new interaction led to subsequent interactions with new atmospheric particles. The end result was the creation of what’s called a shower of high-energy particles, including two new ones: the positron — hypothesized in 1930 by Dirac, the antimatter counterpart of the electron with the same mass but a positive charge — and the muon, an unstable particle with the same charge as the electron but some 206 times heavier! The positron was discovered by Carl Anderson in 1932 and the muon by him and his student Seth Neddermeyer in 1936, but the first muon event was discovered by Paul Kunze a few years earlier, which history seems to have forgotten!

    One of the most amazing things is that even here on the surface of the Earth, if you hold out your hand so that it’s parallel to the ground, about one muon passes through it every second.

    5
    Image credit: Konrad Bernlöhr of the Max Planck Institute for Nuclear Physics.

    Every muon that passes through your hand originates from a cosmic ray shower, and every single one that does so is a vindication of the theory of special relativity! You see, these muons are created at a typical altitude of about 100 km, but a muon’s mean lifetime is only about 2.2 microseconds! Even moving at the speed of light (299,792.458 km/sec), a muon would only travel about 660 meters before it decays. Yet because of time dilation — or the fact that particles moving close to the speed of light experience time passing at a slower rate from the point-of-view of a stationary outside observer — these fast-moving muons can travel all the way to the surface of the Earth before they decay, and that’s where muons on Earth originate!

    Fast-forward to the present day, and it turns out that we’ve accurately measured both the abundance and energy spectrum of these cosmic particles!

    7
    The spectrum of cosmic rays. Image credit: Hillas 2006, preprint arXiv:astro-ph/0607109 v2, via University of Hamburg.

    Particles with about 100 GeV worth of energy and under are by far the most common, with about one 100 GeV particle (that’s 10^11 eV) hitting every square-meter cross-section of our local region of space every second. Although higher-energy particles are still there, they’re far less frequent as we look to higher and higher energies.

    For example, by time you reach 10,000,000 GeV (or 10^16 eV), you’re only getting one-per-square-meter each year, and for the highest energy ones, the ones at 5 × 10^10 GeV (or 5 × 10^19 eV), you’d need to build a square detector that measured about 10 kilometers on a side just to detect one particle of that energy per year!

    8
    How to detect a cosmic ray shower: build a giant array on the ground to — to quote Pokémon — catch ‘em all. Image credit: ASPERA / G.Toma / A.Saftoiu.

    Seems like a crazy idea, doesn’t it? It’s asking for a huge investment of resources to detect these incredibly rare particles. And yet there’s an extraordinarily compelling reason that we’d want to do so: there should be a cutoff in the energies of cosmic rays, and a speed limit for protons in the Universe! You see, there might not be a limit to the energies we can give to protons in the Universe: you can accelerate charged particles using magnetic fields, and the largest, most active black holes in the Universe could give rise to protons with energies even greater than the ones we’ve observed!

    But they have to travel through the Universe to reach us, and the Universe — even in the emptiness of deep space — isn’t completely empty. Instead, it’s filled with large amounts of cold, low-energy radiation: the cosmic microwave background!

    9
    An illustration of the radiation background at various redshifts in the Universe. Image credits: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP.

    The only places where the highest energy particles are created are around the most massive, active black holes in the Universe, all of which are far beyond our own galaxy. And if particles with energies in excess of 5 × 10^10 GeV are created, they can only travel a few million light years — max — before one of these photons, left over from the Big Bang, interacts with it and causes it to produce a pion, radiating away the excess energy and falling down to this theoretical cosmic energy limit, known as the GZK cutoff. There’s even more braking radiation — or Bremsstrahlung radiation — that arises from interactions with any particles in the interstellar/intergalactic medium. Even lower-energy particles are subject to it, and radiate energy away in droves as electron/positron pairs (and other particles) are produced. (More details here.)

    So we did the only reasonable thing for physicists to do: we built a detector that ridiculously large and looked, and saw if this cutoff existed!

    10
    The largest cosmic ray detector in the world. Image credit: Pierre Auger Observatory in Malargüe, Argentina / Case Western Reserve U.

    The Pierre Auger Observatory has done exactly this, verifying that cosmic rays exist up to but not over this incredibly high-energy threshold, a literal factor of about 10,000,000 larger than the energies reached at the LHC! This means the fastest protons we’ve ever seen evidence for in the Universe are moving almost at the speed-of-light, which is exactly 299,792,458 m/s, but just a tiny bit slower. How much slower?

    The fastest protons — the ones just at the GZK cutoff — move at 299,792,457.999999999999918 meters-per-second, or if you raced a photon and one of these protons to the Andromeda galaxy and back, the photon would arrive a measly six seconds sooner than the proton would… after a journey of more than five million years! But these ultra-high-energy cosmic rays don’t come from Andromeda (we believe); they come from active galaxies with supermassive black holes like NGC 1275, which tend to be hundreds of millions or even billions of light years away.

    11
    Galaxy NGC 1275, as imaged by Hubble. Image credit: NASA, ESA, Hubble Heritage (STScI/AURA).

    We even know — thanks to NASA’s Interstellar Boundary Explorer (IBEX) — that there are about 10 times as many cosmic rays out there in deep space as we detect here on-and-around Earth, as the Sun’s heliosheath protects us from the vast majority of them!

    NASA IBEX
    NASA/IBEX

    (Although the Sun does the worst job of protecting us from the most energetic particles.) In theory, there are collisions occurring everywhere in space between these cosmic rays, and so in a very real sense of the word, the Universe itself is our ultimate Large Hadron Collider: up to ten million times more energetic than what we can perform here on Earth. And when we’ve finally reached the limits of what a collider experiment can perform on Earth, it will be back to the same techniques we used in the earliest days of cosmic ray experiments.

    11
    Exterior view of the ISS with the AMS-02 visible in the foreground. Image credit: NASA.

    It will be back to space, to wait and see what the Universe delivers to us, and to detect the aftermath of the most energetic cosmic collisions of all.

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 8:27 pm on April 29, 2016 Permalink | Reply
    Tags: , , , Cosmic Rays   

    From astrobites: “A PeVatron at the Galactic Center” 

    Astrobites bloc

    Astrobites

    Apr 29, 2016
    Kelly Malone

    Science paper: Acceleration of petaelectronvolt protons in the Galactic Centre
    Authors: The HESS Collaboration
    Status: Published in Nature

    In the past, we’ve talked on this website a bit about the mysteries of galactic cosmic rays, or charged particles from outer space that are mainly made up of protons. These particles can reach PeV energies and beyond, but the shocks of supernova remnants (the origin of most galactic cosmic rays) cannot accelerate particles to these high energies. The HESS Collaboration analyzed 10 years of gamma-ray observations and have seen evidence of a PeVatron (PeV accelerator) in the center of our galaxy. If confirmed, this would be the first PeVatron in our galaxy.

    As mentioned above, the HESS Collaboration used observations of gamma rays from their array of telescopes to do this analysis.

    HESS Cherenko Array
    HESS Cherenko Array

    Gamma rays are often used to probe the nature of cosmic ray accelerators; this is because they are associated with these sites, but unlike the charged cosmic rays, they are electrically neutral and therefore don’t bend in magnetic fields on their way to Earth (i.e. they point back to the source).

    1
    Figure 1: HESS’s very high energy gamma ray map of the Galactic Center region. The color scale shows the number of gamma rays per pixel, while the white contour lines illustrate the distribution of molecular gas. Their correlation points to a hadronic origin of gamma ray emission. The right panel is simply a zoomed view of the inner portion. (Source: Figure 1 from the paper)

    2
    Figure 2: The red shaded area shows the 1 sigma confidence band of the measured gamma-ray spectrum of the diffuse emission in the region of interest. The red lines show different models, assuming that the gamma rays are coming from neutral pion decay after the pions have been produced in proton-proton interactions. Note the lack of cutoff at high energies, indicating that the parent protons have energies in the PeV range. The blue data points refer to another gamma-ray source in the region, HESS J1745-290. The link between these two objects is currently unknown.

    The area they studied is known as the Central Molecular Zone, which surrounds the Galactic Center. They found that the distribution of gamma rays mirrored the distribution of the gas-rich areas, which points to a hadronic (coming from proton interactions) origin of the gamma rays. From the gamma-ray luminosity and amount of gases in the area, it can be shown that there must be at least one cosmic ray accelerator in the region. Additionally, the energy spectrum of the diffuse gamma-ray emission from the region around Sagittarius A* (the location of the black hole at at the Galactic Center) does not have an observed cutoff or a break in the TeV energy range. This means that the parent proton population that created these gamma rays should have energies of ~1 PeV (the PeVatron). Just to refresh everyone’s memory, a TeV is 10^12 electronvolts, while a PeV is 10^15 electronvolts. A few TeV is about the limit of what can be produced in particle laboratories on Earth (the LHC reaches 14 TeV). A PeV is roughly 1000 times that!

    What is the source of these protons? The typical explanation for Galactic cosmic rays, supernova remnants, is unlikely here: in order to match the data and inject enough cosmic rays into the Central Molecular Zone, the authors estimate that we would need more than 10 supernova events over 1000 years. This is a very high rate that is improbable.

    Instead, they hypothesize that Sgr A* is the source of these protons.

    Sag A* NASA's Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
    Sag A* Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    They could either be accelerate in the accretion flow immediately outside the black hole, or further away where the outflow terminates. They do note that the required acceleration rate is a few orders of magnitude above the current luminosity, but that the black hole may have been much more active in the past, leading to higher production rates of the protons and other nuclei. If this is true, it could solve one of the most puzzling mysteries in cosmic ray physics: the origin of the higher energy galactic cosmic rays.

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 4:12 pm on April 21, 2016 Permalink | Reply
    Tags: , , Cosmic Rays,   

    From Goddard: “Microscopic “Timers” Reveal Likely Source of Galactic Space Radiation” 

    NASA Goddard Banner
    NASA Goddard Space Flight Center

    April 21, 2016
    Karen C. Fox
    NASA Goddard Space Flight Center, Greenbelt, Maryland
    301-286-6284
    karen.c.fox@nasa.gov

    Most of the cosmic rays that we detect at Earth originated relatively recently in nearby clusters of massive stars, according to new results from NASA’s Advanced Composition Explorer (ACE) spacecraft.

    NASA ACE
    NASA ACE

    ACE allowed the research team to determine the source of these cosmic rays by making the first observations of a very rare type of cosmic ray that acts like a tiny timer, limiting the distance the source can be from Earth.

    Nebula in the constellation Carina, contains the central cluster of huge, hot stars, called NGC 3603. NASA ESA Hubble
    Nebula in the constellation Carina, contains the central cluster of huge, hot stars, called NGC 3603. NASA/ESA Hubble.

    “Before the ACE observations, we didn’t know if this radiation was created a long time ago and far, far away, or relatively recently and nearby,” said Eric Christian of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Christian is co-author of a paper on this research published April 21 in Science.

    Cosmic rays are high-speed atomic nuclei with a wide range of energy — the most powerful race at almost the speed of light. Earth’s atmosphere and magnetic field shield us from less-energetic cosmic rays, which are the most common. However, cosmic rays will present a hazard to unprotected astronauts traveling beyond Earth’s magnetic field because they can act like microscopic bullets, damaging structures and breaking apart molecules in living cells. NASA is currently researching ways to reduce or mitigate the effects of cosmic radiation to protect astronauts traveling to Mars.

    Cosmic rays are produced by a variety of violent events in space. Most cosmic rays originating within our solar system have relatively low energy and come from explosive events on the Sun, like flares and coronal mass ejections. The highest-energy cosmic rays are extremely rare and are thought to be powered by massive black holes gorging on matter at the center of other galaxies. The cosmic rays that are the subject of this study come from outside our solar system but within our Galaxy and are called galactic cosmic rays. They are thought to be generated by shock waves from exploding stars called supernovae.

    Supernova remnant Crab nebula. NASA/ESA Hubble
    Supernova remnant Crab nebula. NASA/ESA Hubble

    The galactic cosmic rays detected by ACE that allowed the team to estimate the age of the cosmic rays, and the distance to their source, contain a radioactive form of iron called Iron-60 (60Fe). It is created inside massive stars when they explode and then blasted into space by the shock waves from the supernova. Some 60Fe in the debris from the destroyed star is accelerated to cosmic-ray speed when another nearby massive star in the cluster explodes and its shock wave collides with the remnants of the earlier stellar explosion.

    60Fe galactic cosmic rays zip through space at half the speed of light or more, about 90,000 miles per second. This seems very fast, but the 60Fe cosmic rays won’t travel far on a galactic scale for two reasons. First, they can’t travel in straight lines because they are electrically charged and respond to magnetic forces. Therefore they are forced to take convoluted paths along the tangled magnetic fields in our Galaxy. Second, 60Fe is radioactive and over a period of about 2.6 million years, half of it will self-destruct, decaying into other elements (Cobalt-60 and then Nickel-60). If the 60Fe cosmic rays were created hundreds of millions of years or more ago, or very far away, eventually there would be too little left for the ACE spacecraft to detect.

    “Our detection of radioactive cosmic-ray iron nuclei is a smoking gun indicating that there has likely been more than one supernova in the last few million years in our neighborhood of the Galaxy,” said Robert Binns of Washington University, St. Louis, Missouri, lead author of the paper.

    “In 17 years of observing, ACE detected about 300,000 galactic cosmic rays of ordinary iron, but just 15 of the radioactive Iron-60,” said Christian. “The fact that we see any Iron-60 at all means these cosmic ray nuclei must have been created fairly recently (within the last few million years) and that the source must be relatively nearby, within about 3,000 light years, or approximately the width of the local spiral arm in our Galaxy.” A light year is the distance light travels in a year, almost six trillion miles. A few thousand light years is relatively nearby because the vast swarm of hundreds of billions of stars that make up our Galaxy is about 100,000 light years wide.

    There are more than 20 clusters of massive stars within a few thousand light years, including Upper Scorpius (83 stars), Upper Centaurus Lupus (134 stars), and Lower Centaurus Crux (97 stars). These are very likely major contributors to the 60Fe that ACE detected, owing to their size and proximity, according to the research team.

    ACE was launched on August 25, 1997 to a point 900,000 miles away between Earth and the Sun where it has acted as a sentinel, detecting space radiation from solar storms, the Galaxy, and beyond. This research was funded by NASA’s ACE program.

    Additional co-authors on this paper were: Martin Israel and Kelly Lave at Washington University, St. Louis, Missouri; Alan Cummings, Rick Leske, Richard Mewaldt and Ed Stone at Caltech in Pasadena, California; Georgia de Nolfo and Tycho von Rosenvinge at Goddard; and Mark Wiedenbeck at NASA’s Jet Propulsion Laboratory in Pasadena, California.

    See the full article here.

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    Stem Education Coalition

    NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

    Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

    NASA Goddard Campus
    NASA/Goddard Campus
    NASA

     
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