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  • richardmitnick 2:38 pm on May 1, 2019 Permalink | Reply
    Tags: , Charged particles travel faster than light through the quantum vacuum of space that surrounds pulsars., Cherenkov radiation, Dame Susan Jocelyn Bell Burnell (1943 – ) still working, , ,   

    From SPACE.com: “Faster-Than-Light Particles Emit Superbright Gamma Rays that Circle Pulsars” 

    space-dot-com logo

    From SPACE.com

    5.1.19
    Yasemin Saplakoglu

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    The Vela pulsar that lives 1,000 light years from our planet. (Image: © NASA/CXC/Univ of Toronto/M.Durant et al)

    Charged particles travel faster than light through the quantum vacuum of space that surrounds pulsars. As these electrons and protons fly by pulsars, they create the ultrabright gamma-ray flashes emitted by the rapidly twirling neutron stars, new research reveals.

    These gamma-rays, called Cherenkov emissions, are also found in powerful particle accelerators on Earth, such as the Large Hadron Collider near Geneva, Switzerland. The rays are also the source of the bluish-white glow in the waters of a nuclear reactor.

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    Daya Bay, nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China

    But until now, no one thought that pulsar emissions consisted of Cherenkov radiation.

    That’s in part because of Albert Einstein’s famous theory of relativity, which holds that nothing can travel faster than light in a vacuum. Because of those propositions, scientists previously thought that Cherenkov emissions couldn’t happen in the quantum vacuum of space surrounding pulsars. That area is mostly devoid of matter but home to ghostly quantum particles that flicker in and out of existence.

    So, does this new research mean Einstein’s landmark theory was just violated? Not at all, said study co-author Dino Jaroszynski, a professor of physics at the University of Strathclyde in Scotland.

    Pulsars create crushingly strong electromagnetic fields in the quantum vacuum surrounding the stars. These fields warp, or polarize, the vacuum, essentially creating speed bumps that slow down light particles, Jaroszynski told Live Science. Meanwhile, charged particles such as protons and electrons zoom through these fields, racing past light.

    As charged particles fly through this field, they displace electrons along their path and emit radiation, which gathers into an electromagnetic wave. This wave, like an optical version of a sonic boom, is what we see as the gamma-ray flash, according to a statement.

    The team still doesn’t know exactly how bright these gamma-ray flashes are, Jaroszynski said.

    “What we do know is that, under the right conditions, vacuum Cherenkov radiation outshines synchrotron radiation,” he added, referring to another type of radiation that is emitted from pulsars by charged particles moving along a curved path.

    But the new findings could have implications beyond pulsars, the researchers said.

    “This is a very exciting new prediction because it could provide answers to basic questions such as what is the origin of the gamma-ray glow at the centre of galaxies?” Jaroszynski said in the statement. “It provides a new way of testing some of the most fundamental theories of science by pushing them to their limits.”

    The researchers reported their findings April 25 in the journal Physical Review Letters.

    See the full article here .

    Women in STEM – Dame Susan Jocelyn Bell Burnell

    Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

    Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.

    Dame Susan Jocelyn Bell Burnell 2009

    Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

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  • richardmitnick 8:57 am on March 31, 2019 Permalink | Reply
    Tags: "Physicists predict a way to squeeze light from the vacuum of empty space", Cherenkov radiation, , ,   

    From Science Magazine: “Physicists predict a way to squeeze light from the vacuum of empty space” 

    AAAS
    From Science Magazine

    Mar. 29, 2019
    Adrian Cho

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    Charged particles zipping through water in a nuclear reactor produce Cherenkov radiation. Credit: Argonne National Laboratory/Wikimedia commons (CC BY-SA 2.0)

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    Cherenkov Radiation. Credit Nuclear-Power.net

    Talk about getting something for nothing. Physicists predict that just by shooting charged particles through an electromagnetic field, it should be possible to generate light from the empty vacuum. In principle, the effect could provide a new way to test the fundamental theory of electricity and magnetism, known as quantum electrodynamics, the most precise theory in all of science. In practice, spotting the effect would require lasers and particle accelerators far more powerful than any that exist now.

    “I’m quite confident about [the prediction] simply because it combines effects that we understand pretty well,” says Ben King, a laser-particle physicist at the University of Plymouth in the United Kingdom, who was not involved in the new analysis. Still, he says, an experimental demonstration “is something for the future.”

    Physicists have long known that energetic charged particles can radiate light when they zip through a transparent medium such as water or a gas. In the medium, light travels slower than it does in empty space, allowing a particle such as an electron or proton to potentially fly faster than light. When that happens, the particle generates an electromagnetic shockwave, just as a supersonic jet creates a shockwave in air. But whereas the jet’s shockwave creates a sonic boom, the electromagnetic shockwave creates light called Cherenkov radiation. That effect causes the water in the cores of nuclear reactors to glow blue, and it’s been used to make particle detectors.

    However, it should be possible to ditch the material and produce Cherenkov light straight from the vacuum, predict Dino Jaroszynski, a physicist at the University of Strathclyde in Glasgow, U.K., and colleagues. The trick is to shoot the particles through an extremely intense electromagnetic field instead.

    According to quantum theory, the vacuum roils with particle-antiparticle pairs flitting in and out of existence too quickly to observe directly. The application of a strong electromagnetic field can polarize those pairs, however, pushing positive and negative particles in opposite directions. Passing photons then interact with the not-quite-there pairs so that the polarized vacuum acts a bit like a transparent medium in which light travels slightly slower than in an ordinary vacuum, Jaroszynski and colleagues calculate.

    Putting two and two together, an energetic charged particle passing through a sufficiently strong electromagnetic field should produce Cherenkov radiation, the team reports in a paper in press at Physical Review Letters. Others had suggested vacuum Cherenkov radiation should exist in certain situations, but the new work takes a more fundamental and all-encompassing approach, says Adam Noble, a physicist at Strathclyde.

    Spotting vacuum Cherenkov radiation would be tough. First, the polarized vacuum slows light by a tiny amount. The electromagnetic fields in the strongest pulses of laser light reduce light’s speed by about a millionth of a percent, Noble estimates. In comparison, water reduces light’s speed by 25%. Second, charged particles in an electromagnetic field spiral and emit another kind of light called synchroton radiation that, in most circumstances, should swamp the Cherenkov radiation.

    Still, in principle, it should be possible to produce vacuum Cherenkov radiation by firing high-energy electrons or protons through overlapping pulses from the world’s highest intensity lasers, which can pack a petawatt, or 1015 watts, of power. However, Jaroszynski and colleagues calculate that in such fields, even particles from the world’s highest energy accelerators would produce much more synchrotron radiation than Cherenkov radiation.

    Space could be another place to look for the effect. Extremely high-energy protons passing through the intense magnetic field of a spinning neutron star—also known as a pulsar—should produce more Cherenkov radiation than synchrotron radiation, the researchers predict. However, pulsars don’t produce many high-energy protons, says Alice Harding, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the particles that do enter a pulsar’s magnetic field should quickly lose energy and spiral instead of zipping across it. “I’m not terribly excited about the prospect for pulsars,” she says.

    Nevertheless, King says, experimenters might see the effect someday. Physicists in Europe are building a trio of 10 petawatt lasers in Romania, Hungary, and the Czech Republic, and their counterparts in China are developing a 100 petawatt laser.

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    A laser in Shanghai, China, has set power records yet fits on tabletops. Credit: KAN ZHAN

    Scientists are also trying to create compact laser-driven accelerators that might produce highly energetic particle beams far more cheaply. If those things come together, physicists might be able to spot vacuum Cherenkov radiation, King says.

    Others are devising different ways to use high-power lasers to probe the polarized vacuum. The ultimate aim of such work is to test quantum electrodynamics in new ways, King says. Experimenters have confirmed the theory’s predictions are accurate to within a few parts in a billion. But the theory has never been tested in the realm of extremely strong fields, King says, and such tests are now becoming possible. “The future of this field is quite exciting.”

    See the full article here .


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  • richardmitnick 4:04 pm on July 12, 2018 Permalink | Reply
    Tags: , , , Cherenkov radiation, , ,   

    From Ethan Siegel: “How A Failed Nuclear Experiment Accidentally Gave Birth To Neutrino Astronomy” 

    From Ethan Siegel
    Jul 10, 2018

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    A neutrino event, identifiable by the rings of Cerenkov radiation that show up along the photomultiplier tubes lining the detector walls, showcase the successful methodology of neutrino astronomy. This image shows multiple events. Super Kamiokande collaboration

    Sometimes, the best-designed experiments fail. The effect you’re looking for might not even occur, meaning that a null result should always be a possible outcome you’re prepared for. When that happens, the experiment is often dismissed as a failure, even though you never would have known the results without performing it.

    Yet, every once in a while, the apparatus that you build might be sensitive to something else entirely. When you do science in a new way, at a new sensitivity, or under new, unique conditions, that’s often where the most surprising, serendipitous discoveries are made. In 1987, a failed experiment for detecting proton decay detected neutrinos, for the first time, from beyond not only our Solar System, but from outside of the Milky Way. This is how neutrino astronomy was born.

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    The conversion of a neutron to a proton, an electron, and an anti-electron neutrino is how Pauli hypothesized resolving the energy non-conservation problem in beta decay. Joel Holdsworth

    The neutrino is one of the great success stories in all the history of theoretical physics. Back in the early 20th century, three types of radioactive decay were known:

    Alpha decay, where a larger atom emits a helium nucleus, jumping two elements down the periodic table.
    Beta decay, where an atomic nucleus emits a high-energy electron, moving one element up the periodic table.
    Gamma decay, where an atomic nucleus emits an energetic photon, remaining in the same location on the periodic table.

    In any reaction, under the laws of physics, whatever the total energy and momentum of the initial reactants are, the energy and momentum of the final products need to match. For alpha and gamma decays, they always did. But for beta decays? Never. Energy was always lost.

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    The V-shaped track in the center of the image is likely a muon decaying to an electron and two neutrinos. The high-energy track with a kink in it is evidence of a mid-air particle decay. This decay, if the (undetected) neutrino is not included, would violate energy conservation.The Scottish Science & Technology Roadshow

    In 1930, Wolfgang Pauli proposed a new particle that could solve the problem: the neutrino. This small, neutral particle could carry both energy and momentum, but would be extremely difficult to detect. It wouldn’t absorb or emit light, and would only interact with atomic nuclei extremely rarely.

    Upon its proposal, rather than confident and elated, Pauli felt ashamed. “I have done a terrible thing, I have postulated a particle that cannot be detected,” he declared. But despite his reservations, the theory was vindicated by experiment.

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    Reactor nuclear experimental RA-6 (Republica Argentina 6), en marcha, showing the characteristic Cherenkov radiation from the faster-than-light-in-water particles emitted. The neutrinos (or more accurately, antineutrinos) first hypothesized by Pauli in 1930 were detected from a similar nuclear reactor in 1956. Centro Atomico Bariloche, via Pieck Darío

    In 1956, neutrinos (or more specifically, antineutrinos) were first directly detected as part of the products of a nuclear reactor. When neutrinos interact with an atomic nucleus, two things can result:

    they either scatter and cause a recoil, like a billiard ball knocking into other billiard balls,
    or they cause the emission of new particles, which have their own energies and momenta.

    Either way, you can build specialized particle detectors around where you expect the neutrinos to interact, and look for them. This was how the first neutrinos were detected: by building particle detectors sensitive to neutrino signatures at the edges of nuclear reactors. If you reconstructed the entire energy of the products, including neutrinos, energy is conserved after all.

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    Schematic illustration of nuclear beta decay in a massive atomic nucleus. Only if the (missing) neutrino energy and momentum is included can these quantities be conserved. Wikimedia Commons user Inductiveload

    In theory, neutrinos should be produced wherever nuclear reactions take place: in the Sun, in stars and supernovae, and whenever an incoming high-energy cosmic ray strikes a particle from Earth’s atmosphere. By the 1960s, physicists were building neutrino detectors to look for both solar (from the Sun) and atmospheric (from cosmic ray) neutrinos.

    A large amount of material, with mass designed to interact with the neutrinos inside of it, would be surrounded by this neutrino detection technology. In order to shield the neutrino detectors from other particles, they were placed far underground: in mines. Only neutrinos should make it into the mines; the other particles should be absorbed by the Earth. By the end of the 1960s, solar and atmospheric neutrinos had both successfully been found.

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    The Homestake Gold Mine sits wedged in the mountains in Lead, South Dakota. It began operation over 123 years ago, producing 40 million ounces of gold from the 8,000 foot deep underground mine and mill. In 1968, the first Solar neutrinos were detected at an experiment here, devised by John Bahcall and Ray Davis. (Jean-Marc Giboux/Liaison)

    The particle detection technology that was developed for both neutrino experiments and high-energy accelerators was found to be applicable to another phenomenon: the search for proton decay. While the Standard Model of particle physics predicts that the proton is absolutely stable, in many extensions — such as Grand Unification Theories — the proton can decay into lighter particles.

    In theory, whenever a proton does decay, it will emit lower-mass particles at very high speeds. If you can detect the energies and momenta of those fast-moving particles, you can reconstruct what the total energy is, and see if it came from a proton.

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    High-energy particles can collide with others, producing showers of new particles that can be seen in a detector. By reconstructing the energy, momentum, and other properties of each one, we can determine what initially collided and what was produced in this event. Fermilab

    If protons decay, their lifetime must be extremely long. The Universe itself is 1010 years old, but the proton’s lifetime must be much longer. How much longer? The key is to look not at one proton, but at an enormous number. If a proton’s lifetime is 1030 years, you can either take a single proton and wait that long (a bad idea), or take 1030 protons and wait 1 year to see if any decay.

    A liter of water contains a little over 1025 molecules in it, where each molecule contains two hydrogen atoms: a proton orbited by an electron. If the proton is unstable, a large enough tank of water, with a large set of detectors around it, should allow you to either measure or constrain its stability/instability.

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    A schematic layout of the KamiokaNDE apparatus from the 1980s. For scale, the tank is approximately 15 meters (50 feet) tall.©Jnn / Wikimedia Commons

    In Japan, in 1982, they began constructing a large underground detector in the Kamioka mines. The detector was named KamiokaNDE: Kamioka Nucleon Decay Experiment.

    Super-Kamiokande Detector, located under Mount Ikeno near the city of Hida, Gifu Prefecture, Japan

    It was large enough to hold over 3,000 tons of water, with around a thousand detectors optimized to detect the radiation that fast-moving particles would emit.

    By 1987, the detector had been running for years, without a single instance of proton decay. With around 1033 protons in that tank, this null result completely eliminated the most popular model among Grand Unified Theories. The proton, as far as we could tell, doesn’t decay. KamiokaNDE’s main objective was a failure.

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    A supernova explosion enriches the surrounding interstellar medium with heavy elements. The outer rings are caused by previous ejecta, long before the final explosion. This explosion also emitted a huge variety of neutrinos, some of which made it all the way to Earth. ESO / L. Calçada

    But then something unexpected happened. 165,000 years earlier, in a satellite galaxy of the Milky Way, a massive star reached the end of its life and exploded in a supernova. On February 23, 1987, that light reached Earth for the first time.

    This is an artist’s impression of the SN 1987A remnant. The image is based on real data and reveals the cold, inner regions of the remnant, in red, where tremendous amounts of dust were detected and imaged by ALMA. This inner region is contrasted with the outer shell, lacy white and blue circles, where the blast wave from the supernova is colliding with the envelope of gas ejected from the star prior to its powerful detonation. Image credit: ALMA / ESO / NAOJ / NRAO / Alexandra Angelich, NRAO / AUI / NSF.

    But a few hours before that light arrived, something remarkable happened at KamiokaNDE: a total of 12 neutrinos arrived within a span of about 13 seconds. Two bursts — the first containing 9 neutrinos and the second containing 3 — demonstrated that the nuclear processes that create neutrinos occur in great abundance in supernovae.

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    Three different detectors observed the neutrinos from SN 1987A, with KamiokaNDE the most robust and successful. The transformation from a nucleon decay experiment to a neutrino detector experiment would pave the way for the developing science of neutrino astronomy.Institute for Nuclear Theory / University of Washington

    For the first time, we had detected neutrinos from beyond our Solar System. The science of neutrino astronomy had just begun. Over the next few days, the light from that supernova, now known as SN 1987A, was observed in a huge variety of wavelengths by a number of ground-based and space-based observatories. Based on the tiny difference in the time-of-flight of the neutrinos and the arrival time of the light, we learned that neutrinos:

    traveled that 165,000 light years at a speed indistinguishable from the speed of light,
    that their mass could be no more than 1/30,000th the mass of an electron,
    and that neutrinos aren’t slowed down as they travel from the core of the collapsing star to its photospher, the way that light is.

    Even today, more than 30 years later, we can examine this supernova remnant and see how it’s evolved.

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    The outward-moving shockwave of material from the 1987 explosion continues to collide with previous ejecta from the formerly massive star, heating and illuminating the material when collisions occur. A wide variety of observatories continue to image the supernova remnant today.NASA, ESA, and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)

    The scientific importance of this result cannot be overstated. It marked the birth of neutrino astronomy, just as the first direct detection of gravitational waves from merging black holes marked the birth of gravitational wave astronomy. It was the birth of multi-messenger astronomy, marking the first time that the same object had been observed in both electromagnetic radiation (light) and via another method (neutrinos).

    It showed us the potential of using large, underground tanks to detect cosmic events. And it causes us to hope that, someday, we might make the ultimate observation: an event where light, neutrinos, and gravitational waves all come together to teach us all about the workings of the objects in our Universe.

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    The ultimate event for multi-messenger astronomy would be a merger of either two white dwarfs or two neutrons stars that was close enough. If such an event occurred in near-enough proximity to Earth, neutrinos, light, and gravitational waves could all be detected.NASA, ESA, and A. Feild (STScI)

    The ultimate event for multi-messenger astronomy would be a merger of either two white dwarfs or two neutrons stars that was close enough. If such an event occurred in near-enough proximity to Earth, neutrinos, light, and gravitational waves could all be detected.NASA, ESA, and A. Feild (STScI)

    See the full article here .


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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 5:03 pm on April 14, 2017 Permalink | Reply
    Tags: and HESS, , , , , Cherenkov radiation, , , Extensive air shower, Ground-based gamma-ray astrophysics, MAGIC, TeV gamma rays, TeV gamma-ray astronomy, VERITAS,   

    From astrobites: “The birth of a new field” 

    Astrobites bloc

    Astrobites

    Apr 14, 2017
    Kelly Malone

    Title: Observation of TeV gamma rays from the Crab Nebula using the Atmospheric Cerenkov Imaging technique
    Authors: Weekes et. al
    First Author’s Institution: Harvard-Smithsonian Center of Astrophysics

    Status: Published in The Astrophysical Journal (1989), [open access]

    Today’s paper is historical in nature rather than a current summary – it describes the 1989 paper that essentially birthed the field of ground-based gamma-ray astrophysics by making the first > 5 sigma detection of a TeV gamma-ray source!

    A brief history of TeV gamma-ray astronomy

    Gamma rays lie on the highest-frequency end of the electromagnetic spectrum and have been observed spanning a few orders of magnitude in energy, starting from a few hundred keV, going through the MeV range, the GeV range, and beyond. The most energetic gamma rays observed to date have been in the TeV range, which is roughly the same energy the proton collisions at the Large Hadron Collider take place at. TeVCat currently lists 198 known TeV gamma-ray sources.

    2
    http://tevcat.uchicago.edu/

    They are associated with some of the most energetic and violent things in our universe, including supernova explosions and active galactic nuclei.

    TeV gamma-ray sources are of particular interest because of their ability to probe phenomena associated with some of the big unsolved problems in astroparticle physics – they are associated with the acceleration sites of charged cosmic rays, but are somewhat easier to study since gamma rays are electrically neutral and don’t curve in magnetic fields on their way to us. This means that they point directly back to their sources. The origins and acceleration sites of charged cosmic rays are still open questions – we know a large portion of the galactic cosmic rays originate in supernova explosions, but don’t know a whole lot else. They can also be used for other science; for example, many current gamma-ray observatories are involved in finding electromagnetic counterparts to gravitational waves.

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    The Whipple 10 m telescope used to make the observations described in this Astrobite. (Source: Michael Richmond, used under a Creative Commons license)

    CfA Whipple Observatory, near Amado, Arizona on the slopes of Mount Hopkins

    When a gamma ray hits the Earth’s atmosphere, it interacts with the air molecules and creates what is known as an extensive air shower. This means that it is not possible to directly observe the gamma ray from the Earth and its indirect products must be studied instead. The extensive air shower consists of many electrons and positrons, some of which are traveling faster than the phase velocity of light in air. This leads to the emission of a type of radiation known as Cherenkov radiation. Detecting this radiation is one of the ways that we can indirectly detect gamma rays on the Earth, and many currently running experiments (such as VERITAS, MAGIC, and HESS) have used this technique to great success.

    CfA/VERITAS, AZ, USA

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

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

    A few decades ago, the prospects for detecting gamma rays from the Earth was not so rosy. Techniques to separate the showers caused by gamma rays from those caused by the very large background caused by hadrons were still in their infancy. Experiments were publishing only weak detections (~3 sigma) and contradictory results. Statistical tests that we use today to check the validity of results were not widely used yet. An overview of the field from 1988 stresses that it is likely that some of the “sources” in their list will likely be removed as techniques are refined. (For more information about this time period, see section 2.2 of this history of gamma-ray astronomy, written in 2012.)

    A pioneering observation

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    The spectrum of the Crab Nebula. The “W” is the measurement from this paper, the others are results from earlier experiments, which were less significant detections. The lines are the predicted spectrum for two different values of the magnetic field.

    In 1989, everything changed for the field of ground-based gamma-ray astrophysics. That was the year that scientists published a 9 sigma detection of TeV gamma rays from the Crab Nebula, the first unambiguous detection of gamma rays (from any source) at TeV energies.

    Supernova remnant Crab nebula. NASA/ESA Hubble

    The data was collected at the Whipple Observatory in Arizona, which had a 10 m reflector outfitted with a 37 pixel camera to detect the Cherenkov radiation described in the preceding section. The 37 phototubes were arranged in a hexagonal pattern and were capable of tracking sources across the sky.

    It was the improved gamma/background discrimination that led to the unambiguous detection. After each observation, the data was calibrated, the observed showers parameterized, and then candidate gamma rays were selected. Monte Carlo simulations were used to predict how the camera would respond to gamma-ray initiated showers and hadron-initiated background showers. When the analysis was finished, the Crab Nebula was seen with a significance of 9 sigma above an energy threshold of 0.7 TeV. No variability was observed over the months or years the data was taken over, and it was established that the emission was likely coming from the hard Compton synchrotron spectrum in the Nebula.

    The authors close the paper by noting that observing a steady source such as the Crab Nebula is important for the field of TeV gamma ray astronomy, since such a source can be used as a standard candle in calibrating new detectors. In fact, this is still true today. Nearly every gamma-ray experiment starts off their life by publishing a paper with their observations of the Crab Nebula, as it is still the most significant source in the gamma ray sky!

    See the full article here .

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    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.
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  • richardmitnick 10:22 am on October 6, 2016 Permalink | Reply
    Tags: Cherenkov radiation, , , , 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.

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    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.

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    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.

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    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 .

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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

     
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