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  • richardmitnick 8:55 pm on October 5, 2017 Permalink | Reply
    Tags: , , , , , Crab Pulsar- a fast-rotating neutron star, CTA-Cherenkov Telescope Array, , Gamma Rays,   

    From H.E.S.S.: “The Crab Nebula is Extended” 

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

    H.E.S.S.

    October 2017

    The Crab Nebula is one of the best-studied astrophysical objects of all time and shines across the whole accessible electromagnetic spectrum, from radio wavelengths up to very high energy (VHE, E > 100 GeV*) gamma-rays ([1], [2]).

    Supernova remnant Crab nebula. NASA/ESA Hubble

    X-ray picture of Crab pulsar, taken by Chandra

    It is powered by the Crab Pulsar, a fast-rotating neutron star (period P = 33 ms) possessing an ultra-strong magnetic field. It was created during a supernova explosion seen in the constellation Taurus in 1054 A.D. A part of the rotational energy of the Crab Pulsar is converted into electron-positron pairs, forming a pulsar wind. The electrons and positrons are shocked and accelerated to ultra-relativistic energies at the wind termination shock. These high-energy particles propagate outwards and lose energy via the emission of synchrotron and Inverse Compton (IC) radiation resulting from interactions with magnetic and photon fields, respectively. The synchrotron radiation ranges from the radio regime up to around 1 GeV, and its emissivity depends on the density of high-energy particles and the strength of the magnetic field. Thus, the spatial extent of the synchrotron emission is determined by a convolution of the local electron distributions with the magnetic field, where the latter is expected to vary significantly through the nebula. The second important emission mechanism is IC scattering: Energetic electrons and positrons can transfer a fraction of their energy onto photons thereby transforming these to high-energy and VHE gamma-rays. In the case of the Crab Nebula, the dominant target photon field is the synchrotron emission of the nebula itself, generated by the same particle population. This photon field is expected to be more homogeneous than the magnetic field. Thus the IC emission can be taken as a much more reliable tracer of the distribution of relativistic electrons, revealing more about the underlying physics of one of the Milky Way’s most prominent and interesting particle accelerators.

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    Fig 1: Top: Distribution of gamma-ray candidates from the Crab Nebula (Data ON) together with background events (Data OFF) as a function of the squared distance to the source position. The simulated PSF and the PSF convolved with the best-fit Gaussian are shown for comparison as well. Bottom: Significance of the bin-wise deviation (MC – Data) of the measured events when compared to the PSF (black) and the convolved one (orange).

    Until now, the morphology of the Crab Nebula has only been resolved with radio, optical, and X-ray telescopes, up to photon energies of around 80 keV [1,3]; at higher energies, no extension could be measured mainly due to the worse angular resolution of the corresponding instruments. For telescopes like H.E.S.S., the expected size of the Crab Nebula is several times smaller than the point spread function (PSF**). In such a case, the intrinsic extension of the source only leads to a slight broadening of the signal as compared to the PSF, which itself however strongly depends on the actual observation and instrument conditions. For the first time, we employ simulations that take into account all these conditions and thus lead to a considerably improved PSF description [4]. This allows to probe source extensions well below one arcminute***, which corresponds to a new level of resolving source sizes in VHE gamma-ray astronomy.

    Here we used 25.7 hours of high-quality observations of the Crab Nebula, taken with all four of the small telescopes of H.E.S.S. The analysis settings were chosen to achieve a good PSF; for this source and observation conditions, 68% of the gamma-rays from a point source are reconstructed within 0.05° (3 arcminutes) of the source direction.

    The distribution of events from the Crab Nebula as a function of the (squared) distance to the source is shown in Figure 1 (blue crosses). For comparison, the PSF is shown as well (black) and is obviously highly inconsistent with the data, where the probability for consistency of the two distributions amounts to merely around 10-14. The broadening of the data distribution with respect to the PSF can only be explained when assuming an intrinsic source extension. The PSF was iteratively convolved with a Gaussian source model of different width. The extension is obtained by comparing the compatibility of the data and the convolved PSF each time, and the best-fit extension is found to be σCrab = 52.2” ± 2.9” ± 7.8”sys. This Gaussian width σCrab corresponds to 39% source containment. The size of the Crab Nebula in gamma-rays has been measured for the first time and can now be put in context to its morphology seen at other wavelengths.

    2
    Fig 2: Extension of the Crab Nebula as seen with H.E.S.S. (solid white circles, corresponding to Gaussian width), overplotted on the UV (top) and X-ray (bottom) image. The bright dot in the middle corresponds to emission from the Crab Pulsar, whereas the inner ring around the pulsar that is visible in the X-ray image is supposed to be related to the wind termination shock ([5], [6]). For illustration purposes, the VHE extension circle is centered on the pulsar position.

    Our resulting Gaussian width is overplotted on images of the Crab Nebula at UV wavelengths (λ = 291 nm) and X-ray energies (0.1 – 10 keV) on the top and bottom of Figure 2, respectively. While the gamma-ray extension is obviously small compared to the optical/UV size, it is the other way around when comparing the gamma-ray to X-ray extension. This can be understood when considering the energetics of the electron population responsible for the respective emission.

    The X-ray emission of the Crab Nebula is confined to a smaller region than the UV emission, because the latter is mainly from electrons with E ~ 1 TeV, whereas the former, the X-ray emission, is mainly from electrons with larger energies (~ 10 TeV). Since electrons lose energy more efficiently at higher energies, a shrinking of the pulsar wind nebula with increasing energy is indeed expected. Until now, the window between UV and X-ray energies and the corresponding morphology has never been constrained observationally. Now with our new H.E.S.S. measurement, we are closing that gap by measuring the extension of the IC emission of the Crab Nebula in the range where electrons with energies of several TeV dominate. This naturally explains why the size we obtain lies in between the UV and X-ray extension.

    Resolving the extension of the Crab Nebula with the H.E.S.S. array is a milestone in the study of the gamma-ray sky with Cherenkov telescopes, and demonstrates the power of simulations to characterize the instrument performance, with potential applications for the upcoming CTA era.

    (*) 1 GeV = 109 eV and one eV (abbreviation of electron-volt) is a unit of energy which, by definition, represents the amount of energy gained by an electron when accelerated by an electric potential difference of 1 volt.

    (**) The PSF corresponds to the distribution of reconstructed event directions from a point source. In other words, it describes how a gamma-ray point source appears widened up the instrument.

    (***) One arcminute corresponds to 1/60th of 1°. An arcminute (or 1′) can be further subdivided into 60 arcseconds (60”). 1° thus equals 3600”, making it the angular equivalent of an hour.

    References [sorry, no links]:

    [1] Hester, J. J. The Crab Nebula: An Astrophysical Chimera. ARA&A 46, 127-155 (Sept. 2008)
    [2] Bühler, R. & Blandford, R.: The surprising Crab pulsar and its nebula: a review. Reports on Progress in Physics 77, 066901 (June 2014)
    [3] Madsen et al.: Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuStar. ApJ 801, 66 (March 2015)
    [4] Holler et al.: Run-Wise Simulations for Imaging Atmospheric Cherenkov Telescope Arrays. Proceedings of the 35th ICRC, contribution 755 (July 2017)
    [5] Weisskopf et al.: Discovery of Spatial and Spectral Structure in the X-ray Emission from the Crab Nebula. ApJ Letter 536, 81-84 (June 2000)
    [6] Gaensler, B. M. & Slane, P. O.: The Evolution and Structure of Pulsar Wind Nebulae. ARA&A 44, 17-47 (Sept. 2006)

    See the full article here .

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    The High Energy Stereoscopic System

    H.E.S.S. is a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays in the energy range from 10s of GeV to 10s of TeV. The name H.E.S.S. stands for High Energy Stereoscopic System, and is also intended to pay homage to Victor Hess , who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. The instrument allows scientists to explore gamma-ray sources with intensities at a level of a few thousandths of the flux of the Crab nebula (the brightest steady source of gamma rays in the sky). H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality. The first of the four telescopes of Phase I of the H.E.S.S. project went into operation in Summer 2002; all four were operational in December 2003, and were officially inaugurated on September 28, 2004. A much larger fifth telescope – H.E.S.S. II – is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity.

    crab
    Crab nebula

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  • richardmitnick 5:40 am on October 4, 2017 Permalink | Reply
    Tags: , , , , , , , ESA e-ASTROGAM, , Gamma Rays,   

    From astrobites: “Future Gamma-ray Telescopes and the Search for Dark Matter” 

    Astrobites bloc

    Astrobites

    Oct 3, 2017
    Nora Shipp

    Title: Resolving Dark Matter Subhalos With Future Sub-GeV Gamma-Ray Telescopes
    Authors: Ti-Lin Chou, Dimitrios Tanoglidis, and Dan Hooper
    First Author’s Institution: Dept. of Physics, University of Chicago, USA

    Status: Submitted to the Journal of Cosmology and Astroparticle Physics (open access)

    We are surrounded by undetected dark matter.

    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    In fact, our entire Galaxy is enveloped in a large halo of it, but because dark matter does not emit or reflect light, the halo is completely invisible.

    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    Milky Way NASA/JPL-Caltech /ESO R. Hurt

    Inside this halo, orbiting our galaxy, are hundreds of smaller, equally invisible dark matter halos (Figure 1).

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    Figure 1. Galaxies like the Milky Way are surrounded by small dark matter halos (blue blobs). Some of these halos contain no stars, but could still produce gamma-rays from dark matter annihilation! Source: ESO

    The larger ones contain their own dwarf galaxies, but the smallest halos are so tiny that they contain no stars at all. However, if the leading theory of WIMP (Weakly Interacting Massive Particle) dark matter is correct, there is one way that we could actually see these dark matter halos without the help of any stars. If dark matter particles are their own antiparticle, they would annihilate when they come into contact with each other, producing various particles, including highly energetic photons known as gamma-rays.

    Gamma-rays have millions of times more energy than the optical photons that human eyes can see, yet these energetic particles are quite difficult to detect. The current leader in gamma-ray detection is the Fermi Gamma-ray Space Telescope, a satellite that has been orbiting the Earth, searching the sky for gamma-rays, for almost 10 years.

    NASA/Fermi Telescope

    NASA/Fermi LAT

    Since Fermi was first launched, scientists have searched the gamma-ray sky for evidence of dark matter annihilation. What makes this search really tricky is that dark matter is not the only thing that produces gamma-rays. The sky is actually full of gamma-rays coming from all directions, produced by clouds of gas, pulsars, and active galactic nuclei, among many other sources (Figure 2 [not shown in article, replaced here).

    3
    Fermi’s Latest Gamma-ray Census Highlights Cosmic Mysteries

    However, those tiny dark matter halos that don’t contain stars or gas or any kind of non-dark matter should only be producing gamma-rays from dark matter annihilation. The catch is that we have no idea where these dark matter halos are. Scientists, therefore, have searched all across the sky for gamma-rays that might be coming from dark halos, and they just might have found a couple.

    Two sources of gamma-rays fit all the requirements – they are in the right part of the sky, do not emit any other kind of light (as you’d expect from a halo containing only dark matter), and appear to extend wider across the sky than the single point of a far away star. However, it’s impossible to tell whether these sources are really extended like a dark matter halo or whether they are just two star-like points so close to each other that they blur together, appearing to Fermi as a single blob. Today’s paper considers whether a proposed successor to Fermi called e-ASTROGAM (Figure 3) will be able to resolve the mystery of these gamma-ray blobs.

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    Figure 3. A model of e-Astrogam, one potential successor to the Fermi Gamma-ray Space Telescope. Source: ESA

    Are they in fact dark matter halos (in which case this would be the first confirmed detection of dark matter annihilation!) or are they simply two points blurred into one?

    e-ASTROGAM would be quite similar to Fermi, but with several important changes. The biggest difference is that it would be able to detect gamma-rays at a slightly lower energy than Fermi, giving us a brand new view of the gamma-ray sky. In the context of today’s paper, however, the most significant difference is the angular resolution. Angular resolution determines how close together two objects can get before they blur together into a single blob. The angular resolution of e-ASTROGAM will be about 4-6 times better than Fermi’s in the energy range of these mysterious gamma-ray sources. According to the authors of today’s paper, this should definitely be enough to tell whether they are single extended objects or two independent points that are just too close together for Fermi to see (Figure 4).

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    Figure 4. Simulated images of two point sources as seen by Fermi and e-ASTROGAM. On the left, Fermi is unable to distinguish between the two objects, seeing only a single blob of gamma-rays. On the right, e-ASTROGAM, with its superior angular resolution, can tell that the single blob is actually two individual objects. Source: Figure 3 of the paper.

    In order to see just how well e-ASTROGAM will be able to see these objects, the authors modeled fake observations of dark matter annihilation from an extended halo and from two point sources. They determined for different halo sizes and dark matter particles how well e-ASTROGAM will be able to tell whether an object is one extended source or two points. Figure 5 illustrates the difference e-ASTROGAM will make in confirming the nature of these gamma-ray sources. The green and red lines represent how easily Fermi and e-ASTROGAM can distinguish pairs of sources (x-axis) as a function of source brightness (y-axis). e-ASTROGAM reaches much farther along the x-axis, indicating that it can much more easily resolve two point sources. The precise numbers change for different dark matter halos and particles, but in all cases e-ASTROGAM shows a significant improvement over Fermi.

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    Figure 5. This plot illustrates how e-ASTROGAM will be able to help distinguish between extended dark matter halos and two nearby points. The y-axis shows how bright the object in question is, and the x-axis is related to how easily the telescope can distinguish between two nearby points and a single extended object. Even with really bright objects Fermi (green) has a hard time distinguishing between the two scenarios, while e-ASTROGAM (red) can more easily tell the difference. Source: Figure 5 of the paper.

    A future gamma-ray telescope like e-ASTROGAM will be an essential tool in determining whether Fermi has in fact detected dark matter annihilation from dark halos. In addition to determining whether the two potential halos detected by Fermi are actually just pairs of close-together point sources, e-ASTROGAM may be able to detect gamma-rays from even more dark matter halos that are too faint for Fermi to observe on its own. e-ASTROGAM with its superior angular resolution and lower energy range would provide a brand new view of the gamma-ray universe, giving us unexpected insight into known and unknown sources of gamma-rays, and perhaps finally revealing the nature of dark matter.

    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 12:25 pm on September 2, 2017 Permalink | Reply
    Tags: , Gamma Rays, , HESS Cherenko Array   

    From H.E.S.S. : “Probing Local Sources with High Energy Cosmic Ray Electrons” 

    HESS Cherenko Array

    September 2017

    Cosmic rays are high energy particles that pervade the Galaxy. Electrons represent only a small fraction of cosmic rays, which consist primarily of protons and nuclei. However, they are able to provide us with unique information complementary to what can be learnt from protons and nuclei. Due to the important difference in mass (an electron being about 1800 times lighter than a proton or any nuclei), electrons lose energy much more rapidly while propagating from their sources to Earth. Energy losses occur when the electrons interact with magnetic fields or scatter on ambient light in the Galaxy of different wavelengths: photons from the Cosmic Microwave Background or infrared photons or also photons emitted by stars for instance. Because of the strong radiative energy losses, very-high-energy cosmic-ray electrons can only travel short distances. Therefore, they provide us with information of the Earth’s local surroundings in the Galaxy. For example, electrons with an energy of 1 TeV (*) that reach the Earth are dominated by sources closer than ~1,000 light-years away. In comparison, the distance between the Sun and the centre of the Galaxy is about 24,000 light-year. For electrons with energies beyond 1 TeV, their sources must be even closer still….on our Galactic doorstep!

    Up to ∼1 TeV, cosmic-ray electrons can be measured using space based instruments such as AMS [1] or Fermi-LAT [2].


    NASA/AMS02 device

    NASA/Fermi LAT

    More dedicated space based instruments such as CALET or DAMPE are planning to measure the electron spectrum up to ∼10 TeV and recently CALET presented at this year’s International Cosmic Ray Conference first results up to ∼1 TeV, fully compatible with previous measurements.

    1
    CALET on the ISS

    2
    DAMPE DArk Matter Particle Explorer Chinese Academy of Sciences

    Above 1 TeV, the flux is very low and the use of ground-based Cherenkov telescopes, which feature very large effective areas, have proven to provide a robust probe of this flux up to high energies. Through measurements by H.E.S.S. [3], [4], MAGIC [5] and VERITAS [6], the frontier in the detected energy range of the cosmic-ray electron spectrum has been pushed up to ∼5 TeV.

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

    CfA/VERITAS, a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV – TeV energy range. Located at FLWO in AZ, USA

    These experiments are designed for gamma-ray observations: they detect gamma-rays through the cascade of secondary particles resulting from the interaction between a gamma-ray and a nucleus in the atmosphere. Their ability to measure electrons comes from the fact that both electrons and gamma-rays, upon their arrival at the Earth’s atmosphere, deposit their energy by the generation of essentially identical types of cascades.

    The main challenge of a cosmic-ray electron measurement is the distinction between electron and background events. This background can either be gamma-rays (which produce the same type of particle cascades) or protons and heavier nuclei (which massively outnumber the electrons). Since gamma-rays move in straight lines from their astrophysical sources, regions in the sky known for containing gamma-ray sources are excluded from the analysis. Cosmic-ray protons (and other nuclei) are the vast majority of cosmic rays, and a fraction of them can mimic atmospheric cascades induced by cosmic-ray electrons. Both protons and electrons seem to come from all directions of the sky with no preferred direction — at least to high degree of accuracy. This is due to their electric charge: whatever the sources of these charged particles, the magnetic fields in the Galaxy will affect their trajectories, leading them to a random walk through the Galaxy and eventually arriving at Earth isotropically. Therefore, protons cannot be excluded from the data in a similar fashion as for the gamma-rays. Thus, the distinction between electrons and protons is done using a specific algorithm based on the — sometimes very tiny — difference in shape of the cascades generated by electrons and protons [7].

    More than 9 years after the first electron spectrum measurement with H.E.S.S., subsequent observations have increased fourfold the amount of available data. In addition, analysis techniques have improved significantly, leading to a much better suppression of the background of cosmic-ray nuclei. These improvements allow for the first time a measurement of cosmic-ray electrons up to energies of ∼ 20 TeV (see Figure 1).

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    Fig 1: Cosmic-ray electrons energy spectrum measured with H.E.S.S. in 2017 (red dots) compared to previous measurements from various experiments.

    This new measurement from 0.25 TeV to ∼20 TeV reveals an electron spectrum that can be described by two regimes in the high energy region. The spectrum appears quite regular with a constant slope up to an energy of about 1 TeV. Above this energy the spectrum becomes steeper. This break in the spectral slope is the sign of some different physics phenomenon at play, most probably the transition between a regime where a large number of sources contribute to the spectrum, to a regime where only a few, the closest ones from Earth, are able to contribute. The very high energies reached in this measurement allow to test models of nearby sources of cosmic-ray electrons in which one source is very prominent. These models are very popular since those nearby sources of electrons (mainly pulsars) are often invoked as a possible explanation for the excess of positrons (**) measured by some experiments such as Pamela [8] and AMS [9].

    4
    Pamela, built by the Wizard collaboration, which includes Russia, Italy, Germany and Sweden.

    The steeply falling spectrum measured with H.E.S.S. from ∼1 TeV to ∼20 TeV allows to reject models with predictions of pronounced features in the spectrum as shown in Figure 2. The black line symbolises the individual contribution of two possible sources (the Vela and the Cygus Loop supernova remnants) for a given model presented in [10] that is obviously not reproducing the data. Therefore, this new measurement of cosmic-ray electrons reveals not only for the first time the shape of the cosmic-ray electron spectrum beyond ∼5 TeV, but also provides important information on cosmic-ray accelerators in Earth’s local neighbourhood, demanding that very local sources exist.

    5
    Fig 2: Comparison of the new measurement by H.E.S.S. (red dots) with some model predictions for two supernova remnants, Vela and Cygnus Loop (black lines). This specific model is clearly excluded by this measurement since the predicted feature for the Vela supernova remnant is not seen at all.

    Fig 2: Comparison of the new measurement by H.E.S.S. (red dots) with some model predictions for two supernova remnants, Vela and Cygnus Loop (black lines). This specific model is clearly excluded by this measurement since the predicted feature for the Vela supernova remnant is not seen at all.

    (*) 1 TeV = 1012 eV and one eV (abbreviation of electron-volt) is a unit of energy which, by definition, represents the amount of energy gained by an electron when accelerated by an electric potential difference of 1 volt.

    (**) The positron is the antiparticle of the electron.

    References: [sorry, no links]

    [1] AMS Collaboration, Phys. Rev. Lett. 113, 221102 (2014)
    [2] Fermi-LAT Collaboration, Physical Review D 95 (2017)
    [3] F. Aharonian et al., Phys. Rev. Lett. 101, 261104 (2008)
    [4] F. Aharonian et al., Astron. Astrophys. 508, 561 (2009)
    [5] D. Tridon et al., Proceedings of the 32nd ICRC (2011)
    [6] D. Staszak et al., proceedings of the 34th ICRC (2015)
    [7] M. de Naurois and L. Rolland, Astroparticle Physics, 32, 231 (2009)
    [8] PAMELA Collaboration, Nature 458, 607–609 (2009)
    [9] AMS Collaboration, Phys. Rev. Lett. 110, 141102 (2013)
    [10] T. Kobayashi et al., Astrophys. J. 601, 340 (2004)

    See the full article here .
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    The High Energy Stereoscopic System

    H.E.S.S. is a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays in the energy range from 10s of GeV to 10s of TeV. The name H.E.S.S. stands for High Energy Stereoscopic System, and is also intended to pay homage to Victor Hess , who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. The instrument allows scientists to explore gamma-ray sources with intensities at a level of a few thousandths of the flux of the Crab nebula (the brightest steady source of gamma rays in the sky). H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality. The first of the four telescopes of Phase I of the H.E.S.S. project went into operation in Summer 2002; all four were operational in December 2003, and were officially inaugurated on September 28, 2004. A much larger fifth telescope – H.E.S.S. II – is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity.

    crab
    Crab nebula

     
  • richardmitnick 4:41 pm on June 29, 2017 Permalink | Reply
    Tags: From the lab to the ports, Gamma Rays, Inspiration from light-emitting diodes lead to performance boost, , , Scintillating discovery at Sandia Labs, scintillator made of organic glass - much better   

    From Sandia: “Scintillating discovery at Sandia Labs” 


    Sandia Lab

    June 29, 2017

    Bright thinking leads to breakthrough in nuclear threat detection science.

    1
    Sandia National Laboratories researcher Patrick Feng, left, holds a trans-stilbene scintillator and Joey Carlson holds a scintillator made of organic glass. The trans-stilbene is an order of magnitude more expensive and takes longer to produce. (Photo by Randy Wong).

    Taking inspiration from an unusual source, a Sandia National Laboratories team has dramatically improved the science of scintillators — objects that detect nuclear threats. According to the team, using organic glass scintillators could soon make it even harder to smuggle nuclear materials through America’s ports and borders.

    The Sandia Labs team developed a scintillator made of an organic glass which is more effective than the best-known nuclear threat detection material while being much easier and cheaper to produce. Organic glass is a carbon-based material that can be melted and does not become cloudy or crystallize upon cooling. Successful results of the Defense Nuclear Nonproliferation project team’s tests on organic glass scintillators are described in a paper published this week in The Journal of the American Chemical Society.

    Sandia Labs material scientist and principal investigator Patrick Feng started developing alternative classes of organic scintillators in 2010. Feng explained he and his team set out to “strengthen national security by improving the cost-to-performance ratio of radiation detectors at the front lines of all material moving into the country.” To improve that ratio, the team needed to bridge the gap between the best, brightest, most sensitive scintillator material and the lower costs of less sensitive materials.

    Inspiration from light-emitting diodes lead to performance boost

    The team designed, synthesized and assessed new scintillator molecules for this project with the goal of understanding the relationship between the molecular structures and the resulting radiation detection properties. They made progress finding scintillators able to indicate the difference between nuclear materials that could be potential threats and normal, non-threatening sources of radiation, like those used for medical treatments or the radiation naturally present in our atmosphere.

    The team first reported [Science Direct] on the benefits of using organic glass as a scintillator material in June 2016. Organic chemist Joey Carlson said further breakthroughs really became possible when he realized scintillators behave a lot like light-emitting diodes.

    With LEDs, a known source and amount of electrical energy is applied to a device to produce a desired amount of light. In contrast, scintillators produce light in response to the presence of an unknown radiation source material. Depending on the amount of light produced and the speed with which the light appears, the source can be identified.

    Despite these differences in the ways that they operate, both LEDs and scintillators harness electrical energy to produce light. Fluorene is a light-emitting molecule used in some types of LEDs. The team found it was possible to achieve the most desirable qualities — stability, transparency and brightness — by incorporating fluorene into their scintillator compounds.

    2
    Sandia National Laboratories researcher Joey Carlson demonstrates the ease of casting an organic glass scintillator, which takes only a few minutes as compared to growing a trans-stilbene crystal, which can take several months. (Photo by Randy Wong).

    The gold standard scintillator material for the past 40 years has been the crystalline form of a molecule called trans-stilbene, despite intense research to develop a replacement. Trans-stilbene is highly effective at differentiating between two types of radiation: gamma rays, which are ubiquitous in the environment, and neutrons, which emanate almost exclusively from controlled threat materials such as plutonium or uranium. Trans-stilbene is very sensitive to these materials, producing a bright light in response to their presence. But it takes a lot of energy and several months to produce a trans-stilbene crystal only a few inches long. The crystals are incredibly expensive, around $1,000 per cubic inch, and they’re fragile, so they aren’t commonly used in the field.

    Instead, the most commonly used scintillators at borders and ports of entry are plastics. They’re comparatively inexpensive at less than a dollar per cubic inch, and they can be molded into very large shapes, which is essential for scintillator sensitivity. As Feng explained, “The bigger your detector, the more sensitive it’s going to be, because there’s a higher chance that radiation will hit it.”

    Despite these positives, plastics aren’t able to efficiently differentiate between types of radiation — a separate helium tube is required for that. The type of helium used in these tubes is rare, non-renewable and significantly adds to the cost and complexity of a plastic scintillator system. And plastics aren’t particularly bright, at only two-thirds the intensity of trans-stilbene, which means they do not do well detecting weak sources of radiation.

    For these reasons, Sandia Labs’ team began experimenting with organic glasses, which are able to discriminate between types of radiation. In fact, Feng’s team found the glass scintillators surpass even the trans-stilbene in radiation detection tests — they are brighter and better at discriminating between types of radiation.

    Another challenge: The initial glass compounds the team made weren’t stable. If the glasses got too hot for too long, they would crystallize, which affected their performance. Feng’s team found that blending compounds containing fluorene to the organic glass molecules made them indefinitely stable. The stable glasses could then also be melted and cast into large blocks, which is an easier and less expensive process than making plastics or trans-stilbene.

    From the lab to the ports

    The work thus far shows indefinite stability in a laboratory, meaning the material does not degrade over time. Now, the next step toward commercialization is casting a very large prototype organic glass scintillator for field testing. Feng and his team want to show that organic glass scintillators can withstand the humidity and other environmental conditions found at ports.

    The National Nuclear Security Administration has funded the project for an additional two years. This gives the team time to see if they can use organic glass scintillators to meet additional national security needs.

    Going forward, Feng and his team also plan to experiment with the organic glass until it can distinguish between sources of gamma rays that are non-threatening and those that can be used to make dirty bombs.

    See the full article here .

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    Sandia Campus
    Sandia National Laboratory

    Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.
    i1
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  • richardmitnick 2:37 pm on June 13, 2017 Permalink | Reply
    Tags: , , , , , Gamma Rays, ,   

    From FNAL: Searches for dark matter evidence with galactic gamma-rays 

    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.

    June 13, 2017
    Troy Rummler

    1
    Researchers believe that gamma rays — a very energetic form of light — could be produced when hypothetical dark matter particles decay or collide and destroy each other. Fermilab scientist Dan Hooper co-discovered more gamma-rays than he could explain at the center of our own galaxy in 2009 and sparked international interest. Whether dark matter particles or something else is responsible for these gamma rays remains an open and hotly debated question.

    See the full article here .

    Please help promote STEM in your local schools.

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    FNAL Icon
    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 1:24 pm on December 21, 2016 Permalink | Reply
    Tags: A Counterpart for Fast Radio Bursts, , , , , Gamma Rays, Neutron Star Merge   

    From astrobites: “A Counterpart for Fast Radio Bursts” 

    Astrobites bloc

    Astrobites

    Dec 20, 2016
    Joanna Bridge

    Title: Discovery of a Transient Gamma-Ray Counterpart to FRB 131104
    Authors: J. J. DeLaunay, D. B. Fox, K. Murase, P. Mézáros, A. Keivani, C. Messick, M. A. Mostafa, F. Oikonomou, G. Tešić, and C. F. Turley
    First Author’s Institution: Department of Physics, Pennsylvania State University
    Status: Published in ApJ Letters

    It’s a mysterious case worthy of Sherlock Holmes – seemingly random bursts of radio emission generated from somewhere outside the Milky Way, with no obvious source. These emissions, known as Fast Radio Bursts (FRB), have plagued astronomers over the last several years.

    FRB Fast Radio Bursts from NAOJ Subaru, Mauna Key, Hawaii, USA
    FRB Fast Radio Bursts from NAOJ Subaru, Mauna Key, Hawaii, USA

    They were initially discovered in data archives of the Parkes radio telescope in Australia, popping up as relatively strong radio bursts that lasted only 5 milliseconds.

    CSIRO/Parkes Observatory, located 20 kilometres north of the town of Parkes, New South Wales, Australia
    CSIRO/Parkes Observatory, located 20 kilometres north of the town of Parkes, New South Wales, Australia

    The usual radio bursts we detect are usually repeating, emanating from rapidly spinning neutron stars known as pulsars. This radio signal was all alone.

    Even stranger, the signal was dispersed, which means the higher frequency portion of the burst was detected before the lower frequencies. Dispersion is a result of the lower frequency signal being slowed preferentially compared to the high frequencies by the electron clouds between us and the source. Measuring the delay between low and high frequency gives us the distance the signal has traveled. This dispersion indicated that the burst must have originated from very far away – by at least 1 Gigaparsec. That’s a distance of over 3 billion light years!

    Since the archival discovery of FRBs, these sneaky radio bursts have been caught in the act by Parkes telescope, as well as Arecibo in Puerto Rico and the Green Bank Telescope in West Virginia.

    NAIC/Arecibo Observatory, Puerto Rico, USA
    NAIC/Arecibo Observatory, Puerto Rico, USA

    gbo-logo
    GBO radio telescope, Green Bank, West Virginia, USA
    GBO radio telescope, Green Bank, West Virginia, USA

    But until now, there have been no detections in any other wavelengths than radio. In today’s paper, we see for the first time a possible counterpart for an FRB detected in very high energy gamma rays using the Swift telescope.

    NASA/SWIFT Telescope
    NASA/SWIFT Telescope

    Swift has onboard the Burst Alert Telescope (BAT) that is usually triggered when a gamma ray burst goes off somewhere in the universe. If a typical gamma ray burst was responsible for FRBs, then the BAT would have been triggered, providing almost instantaneous gamma ray measurements in the same general direction of the the FRB. The authors of today’s paper, however, wondered if perhaps there is a gamma ray counterpart to the FRBs, but is not luminous enough to trigger the BAT on Swift. They therefore went digging in the archival BAT data, looking for times when Swift just fortuitously happened to be looking in the same direction as an FRB when the radio signal was detected.

    1
    Figure 1 The Swift BAT detection of FRB 131104. (a) shows the portion of the field of view of the BAT where the gamma ray counterpart was detected, denoted by the black circle near the top of the image. The x- and y-axes denote the position on the sky in right ascension (RA) and declination (dec), and the color bar shows how well-detected the emission is, in units of signal above the noise. (b) gives the number of photons detected per second as a function of time for gamma rays in the 5-15 keV energy range.

    Sure enough, there were four times when an FRB was detected within the same field of view as the BAT on Swift. Of these four possibilities, a gamma ray counterpart was detected for the FRB 131104 (so named for two digits for the year, month, and date of observation.) This detection is shown in Figure 1. This FRB is located 3.2 Gigaparsecs away, which corresponds to a redshift of z ~ 0.55. The reason that the BAT was not triggered for this gamma ray burst was because it was on the very edge of the detector, illuminating only 2.9% of the telescope’s detector. The authors were very careful to rule out other causes for the gamma rays.

    Of course, there are still unanswered questions about this detection. One mystery is that while the FRB lasted only 5 milliseconds, the emission detected in gamma rays lasted several minutes and released significantly more energy — a billion times more — than was detected in the radio burst. One theory posits that the sources of FRBs are brightly flaring magnetars, which are neutron stars with extremely high magnetic fields.

    5
    Artist’s impression of the magnetar in star cluster Westerlund 1. Credit: ESO/L. Calçada

    However, if magnetars are the culprit, then the gamma ray emission should not be nearly that long nor that energetic. On the other hand, FRBs with gamma ray counterparts could be caused by binary neutron stars spiraling into each other.

    8
    Neutron Star Merge. NASA Goddard Media Studios

    However, models indicate that we should see only about 25 of these events a year, whereas the inferred rate of FRBs is thought to be on the order of thousands per day.

    Suffice it to say, I think this is a mystery that would stump even the great detective Holmes himself (minus the small detail that he wasn’t an astrophysicist.) In some ways, this new gamma ray counterpart discovery is extremely enlightening, giving clues as to where to look next for the source of FRBs. On the other hand, however, this detection has resulted in even more questions about FRB origins. As is often the case in science, more data are needed! In the future, we should be able to fine tune the threshold for triggering the BAT when the next fast radio burst goes off, allowing us to catch it the FRB and its gamma rays in the act.

    See the full article here .

    Please help promote STEM in your local schools.

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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 7:07 am on November 5, 2016 Permalink | Reply
    Tags: Gamma Rays, MAGIC at LAPALMA, QSO B0218+357 is a blazar   

    From MAGIC: “Detour via gravitational lens makes distant galaxy visible” 

    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

    MAGIC

    November 4, 2016
    No writer credit found.

    1
    The MAGIC telescopes on the canary island of La Palma are shown. Credit: Robert Wagner

    Never before have astrophysicists measured light of such high energy from a celestial object so far away. Around 7 billion years ago, a huge explosion occurred at the black hole in the center of a galaxy. This was followed by a burst of high-intensity gamma rays. A number of telescopes, MAGIC included, have succeeded in capturing this light. An added bonus: it was thus possible to reconfirm Einstein’s General Theory of Relativity, as the light rays encountered a less distant galaxy en route to Earth – and were deflected by this so-called gravitational lens.

    The object QSO B0218+357 is a blazar, a specific type of black hole. Researchers now assume that there is a supermassive black hole at the center of every galaxy. Black holes, into which matter is currently plunging are called active black holes. They emit extremely bright jets. If these bursts point towards Earth, the term blazar is used.

    Full moon prevents the first MAGIC observation

    The event now described in “Astronomy & Astrophysics” took place 7 billion years ago, when the universe was not even half its present age. “The blazar was discovered initially on 14 July 2014 by the Large Area Telescope (LAT) of the Fermi satellite,” explains Razmik Mirzoyan, scientist at the Max Planck Institute for Physics and spokesperson for the MAGIC collaboration. “The gamma ray telescopes on Earth immediately fixed their sights on the blazer in order to learn more about this object.”

    One of these telescopes was MAGIC, on the Canary Island of La Palma, specialized in high-energy gamma rays. It can capture photons – light particles – whose energy is 100 billion times higher than the photons emitted by our Sun and a thousand times higher than those measured by Fermi-LAT. The MAGIC scientists were initially out of luck, however: A full moon meant the telescope was not able to operate during the time in question.

    2
    Photons are emitted from a galaxy QSO B0218+357 in the direction of the Earth. Due to the gravitational effect of the intervening galaxy B0218+357G photons form two paths that reach Earth with a delay of about 11 days. Photons were observed by both the Fermi-LAT instrument and the MAGIC telescopes. Credit: Daniel Lopez/IAC; NASA/ESA; NASA E/PO – Sonoma State University, Aurore Simonnet

    Gravitational lens deflects ultra-high-energy photons

    Eleven days later, MAGIC got a second chance, as the gamma rays emitted by QSO B0218+357 did not take the direct route to Earth: One billion years after setting off on their journey, they reached the galaxy B0218+357G. This is where Einstein’s General Theory of Relativity came into play.

    This states that a large mass in the universe, a galaxy, for example, deflects light of an object behind it. In addition, the light is focused as if by a gigantic optical lens – to a distant observer, the object appears to be much brighter, but also distorted. The light beams also need different lengths of time to pass through the lens, depending on the angle of observation.

    This gravitational lens was the reason that MAGIC was able, after all, to measure QSO B0218+357 – and thus the most distant object in the high-energy gamma ray spectrum. “We knew from observations undertaken by the Fermi space telescope and radio telescopes in 2012 that the photons that took the longer route would arrive 11 days later,” says Julian Sitarek (University of ?ódz, Poland), who led this study. “This was the first time we were able to observe that high-energy photons were deflected by a gravitational lens.”

    Doubling the size of the gamma-ray universe

    The fact that gamma rays of such high energy from a distant celestial body reach Earth’s atmosphere is anything but obvious. “Many gamma rays are lost when they interact with photons which originate from galaxies or stars and have a lower energy,” says Mirzoyan. “With the MAGIC observation, the part of the universe that we can observe via gamma rays has doubled.”

    The fact that the light arrived on Earth at the time calculated could rattle a few theories on the structure of the vacuum – further investigations, however, are required to confirm this. “The observation currently points to new possibilities for high-energy gamma ray observatories – and provides a pointer for the next generation of telescopes in the CTA project,” says Mirzoyan, summing up the situation.

    See the full article here .

    magic-at-lapalma

    MAGIC (Major Atmospheric Gamma Imaging Cherenkov) is a system of two 17 m diameter, F/1.03 Imaging Atmospheric Cherenkov Telescopes (IACT). They are dedicated to the observation of gamma rays from galactic and extragalactic sources in the very high energy range (VHE, 30 GeV to 100 TeV).

    The MAGIC telescopes are currently run by an international collaboration of about 165 astrophysicists from 24 institutions and consortia from 11 countries.

    MAGIC in a Nutshell

    The main goal of the MAGIC project was to build an instrument that could perform measurements in an energy range below 100 GeV, down to about 30 GeV, up to the high-energetic “terra incognita” of the electromagnetic emission spectrum, traditionally considered as the classical domain of satellite-born instruments. MAGIC researchers were anticipating finding new classes of gamma-ray sources such as, for example, pulsars and Gamma Ray Bursts (GRB). Because of the strong absorption of TeV gamma rays by the extragalactic background light, MAGIC was aiming to measure sources at few tens of GeV, where the universe becomes progressively more transparent. At lower energies, one can search for powerful sources residing at large redshifts. The telescopes measure Cherenkov light images of extended air showers from a target source direction. The software analysis allows, with very high efficiency, to select neutral gamma-ray induced electromagnetic showers from the several orders of magnitudes more intense isotropic background due to the charged particle (mostly hadron) induced showers. The MAGIC telescopes are located at a height of 2200 m a.s.l. on the Roque de los Muchachos European Northern Observatory on the Canary Island of La Palma (28°N, 18°W).

     
  • richardmitnick 8:59 pm on July 5, 2016 Permalink | Reply
    Tags: , Gamma Rays, ,   

    From Symmetry: “Incredible hulking facts about gamma rays” 

    Symmetry

    07/05/16
    Matthew R. Francis

    2

    From lightning to the death of electrons, the highest-energy form of light is everywhere.

    Gamma rays are the most energetic type of light, packing a punch strong enough to pierce through metal or concrete barriers. More energetic than X-rays, they are born in the chaos of exploding stars, the annihilation of electrons and the decay of radioactive atoms. And today, medical scientists have a fine enough control of them to use them for surgery. Here are seven amazing facts about these powerful photons.

    Doctors conduct brain surgery using “gamma ray knives.”

    2

    Gamma rays can be helpful as well as harmful (and are very unlikely to turn you into the Hulk). To destroy brain cancers and other problems, medical scientists sometimes use a “gamma ray knife.” This consists of many beams of gamma rays focused on the cells that need to be destroyed. Because each beam is relatively small, it does little damage to healthy brain tissue. But where they are focused, the amount of radiation is intense enough to kill the cancer cells. Since brains are delicate, the gamma ray knife is a relatively safe way to do certain kinds of surgery that would be a challenge with ordinary scalpels.

    [My wife had gamma-knife brain surgery. Whn I asked her neursurgeon how they got gamma rays, he replied from cobalt.]

    3

    The name “gamma rays” came from Ernest Rutherford.

    French chemist Paul Villard first identified gamma rays in 1900 from the element radium, which had been isolated by Marie and Pierre Curie just two years before. When scientists first studied how atomic nuclei changed form, they identified three types of radiation based on how far they penetrated into a barrier made of lead. Ernest Rutherford named the radiation for the first three letters of the Greek alphabet. Alpha rays bounce right off, beta rays went a little farther, and gamma rays went the farthest. Today we know alpha rays are the same thing as helium nuclei (two protons and two neutrons), beta rays are either electrons or positrons (their antimatter versions), and gamma rays are a kind of light.

    4

    Nuclear reactions are a major source of gamma rays.

    When an unstable uranium nucleus splits in the process of nuclear fission, it releases a lot of gamma rays in the process. Fission is used in both nuclear reactors and nuclear warheads. To monitor nuclear tests in the 1960s, the United States launched gamma radiation detectors on satellites. They found far more explosions than they expected to see. Astronomers eventually realized these explosions were coming from deep space—not the Soviet Union—and named them gamma-ray bursts, or GRBs. Today we know GRBs come in two types: the explosions of extremely massive stars, which pump out gamma rays as they die, and collisions between highly dense relics of stars called neutron stars and something else, probably another neutron star or a black hole.

    6

    Gamma rays played a key role in the discovery of the Higgs boson.

    Most of the particles in the Standard Model of particle physics are unstable; they decay into other particles almost as soon as they come into existence. The Higgs boson, for example, can decay into many different types of particles, including gamma rays. Even though theory predicts that a Higgs boson will decay into gamma rays just 0.2 percent of the time, this type of decay is relatively easy to identify and it was one of the types that scientists observed when they first discovered the Higgs boson.

    NASA Fermi Gamma-ray Space Telescope  Gamma-ray Burst Monitor (GBM)
    NASA Fermi Gamma-ray Space Telescope Gamma-ray Burst Monitor

    To study gamma rays, astronomers build telescopes in space.

    Gamma rays heading toward the Earth from space interact with enough atoms in the atmosphere that almost none of them reach the surface of the planet. That’s good for our health, but not so great for those who want to study GRBs and other sources of gamma rays. To see gamma rays before they reach the atmosphere, astronomers have to build telescopes in space. This is challenging for a number of reasons. For example, you can’t use a normal lens or mirror to focus gamma rays, because the rays punch right through them. Instead an observatory like the Fermi Gamma-ray Space Telescope detects the signal from gamma rays when they hit a detector and convert into pairs of electrons and positrons.

    Some gamma rays come from thunderstorms.

    In the 1990s, observatories in space detected bursts of gamma rays coming from Earth that eventually were traced to thunderclouds. When static electricity builds up inside clouds, the immediate result is lightning. That static electricity also acts like a giant particle accelerator, creating pairs of electrons and positrons, which then annihilate into gamma rays. These bursts happen high enough in the air that only airplanes are exposed—and they’re one reason for flights to steer well away from storms.

    Gamma rays (indirectly) give life to Earth.

    Hydrogen nuclei are always fusing together in the core of the sun. When this happens, one byproduct is gamma rays. The energy of the gamma rays keeps the sun’s core hot. Some of those gamma rays also escape into the sun’s outer layers, where they collide with electrons and protons and lose energy. As they lose energy, they change into ultraviolet, infrared and visible light. The infrared light keeps Earth warm, and the visible light sustains Earth’s plants.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:36 am on June 4, 2016 Permalink | Reply
    Tags: , , Gamma Rays   

    From Astro Watch: “At the Cradle of Oxygen: Brand-new Detector to Reveal the Interiors of Stars” 

    Astro Watch bloc

    Astro Watch

    June 4, 2016
    No writer credit found

    1
    No image caption. No image credit

    The most intense source of gamma radiation constructed to date will soon become operational at the ELI Nuclear Physics research facility. It will be possible to study reactions that reveal the details of many processes occurring within stars, in particular those leading to the formation of oxygen. An important part of the equipment will rely on a particle detector built by physicists at the University of Warsaw, Poland. A prototype has recently concluded the first round of testing.

    Oxygen is essential for life: we are immersed in it yet none of it actually originates from our own planet. All oxygen was ultimately formed through thermonuclear reactions deep inside stars. Laboratory studies of the astrophysical processes leading to the formation of oxygen are extremely important. A big step forward in these studies will be possible when work commences in 2018 at the Extreme Light Infrastructure – Nuclear Physics (ELI-NP) facility near Bucharest, using a state-of-the-art source of intense gamma radiation. High energy protons will be intercepted using a specially-designed particle detector acting as a target. A demonstrator version of the detector, constructed at the Faculty of Physics, University of Warsaw (FUW), has recently completed the first round of tests in Romania.

    In terms of mass, the most abundant elements in the Universe are hydrogen (74%) and helium (24%). The percentage by mass of other, heavier elements is significantly lower: oxygen comprises just 0.85% and carbon 0.39% (in contrast, oxygen comprises 65% of the human body and carbon 18% by mass). In nature, conditions supporting the formation of oxygen are present only within evolutionarily-advanced stars which have converted almost all their hydrogen into helium. Helium becomes then their main fuel. At this stage, three helium nuclei start combining into a carbon nucleus. By adding another helium nucleus, this in turn forms an oxygen nucleus and emits one or more gamma photons.

    “Oxygen can be described as the ‘ash’ from the thermonuclear ‘combustion’ of carbon. But what mechanism explains why carbon and oxygen are always formed in stars at more or less the same proportion of 6 to 10?” asks Dr. Chiara Mazzocchi (FUW). She goes on to explain: “Stars evolve in stages. During the first stage, they convert hydrogen into helium, then helium into carbon, oxygen and nitrogen, with heavier elements formed in subsequent stages. Oxygen is formed from carbon during the helium-burning phase. The thing is that, in theory, oxygen could be produced at a faster rate. If the star were to run out of helium and shift to the next stage of its evolution, the proportions between carbon and oxygen would be different.”

    The experiments planned for ELI-NP will not actually recreate thermonuclear reactions converting carbon into oxygen and photons gamma. In fact, researchers are hoping to observe the reverse reaction: collisions between high-energy photons with oxygen nuclei to produce carbon and helium nuclei. Registering the products of this decay should make it possible to study the characteristics of the reaction and fine-tune existing theoretical models of thermonuclear synthesis.

    “We are preparing an eTPC detector for the experiments at ELI-NP. It is an electronic-readout time-projection chamber, which is an updated version of an earlier detector built at the Faculty’s Institute of Experimental Physics. The latter was successfully used by our researchers for the world’s first observations of a rare nuclear process: two-proton decay,” says Dr. Mikolaj Cwiok (FUW).

    The main element of the eTPC detector is a chamber filled with gas comprising many oxygen nuclei (e.g. carbon dioxide). The gas acts as a target. The gamma radiation beam passes through the gas, with some of the photons colliding with oxygen nuclei to produce carbon and helium nuclei. The nuclei formed through the reaction, which are charged particles, ionize the gas. In order to increase their range, the gas is kept at a reduced pressure, around 1/10 of the atmospheric one. The released electrons are directed using an electric field towards the Gas Electron Multiplier (GEM) amplification structures followed by readout electrodes. The paths of the particles are registered electronically using strip electrodes. Processing the data using specialized FPGA processors makes it possible to reconstruct the 3D paths of the particles.

    The active region of the detector will be 35x20x20 cm3, and at nominal intensity of the photon beam it should register up to 70 collisions of gamma photons with oxygen nuclei per day. Tests at ELI-NP used a demonstrator:a smaller but fully functional version of the final detector, named mini-eTPC. The device was tested with a beam of alpha particles (helium nuclei).

    “We are extremely pleased with the results of the tests conducted thus far. The demonstrator worked as we expected and successfully registered the tracks of charged particles. We are certain to use it in future research as a fully operational measuring device. In 2018, ELI-NP will be equipped with a larger detector which we are currently building at our laboratories,” adds Dr. Mazzocchi.

    The project is carried out in collaboration with researchers from ELI-NP/IFIN-HH (Magurele, Romania) and the University of Connecticut in the US. The Warsaw team, led by Prof. Wojciech Dominik, brings together physicists and engineers from the Division of Particles and Fundamental Interactions and the Nuclear Physics Division and students from the University of Warsaw: Jan Stefan Bihalowicz, Jerzy Manczak, Katarzyna Mikszuta and Piotr Podlaski.

    Extreme Light Infrastructure (ELI) is a research project valued at 850 million euro, conducted as part of the European Strategy Forum on Research Infrastructures roadmap. The ELI scientific consortium will encompass three centers in the Czech Republic, Romania and Hungary, focusing on research into the interactions between light and matter under the conditions of the most powerful photon beams and at a wide range of wavelengths and timescales measured in attoseconds (a billionth of a billionth of a second). The Romanian ELI – Nuclear Physics center, in Magurele near Bucharest, conducts research into two sources of radiation: high-intensity radiation lasers (of the order of a 1023 watts per square centimeter), and high-intensity sources of monochromatic gamma radiation. The gamma beam will be formed by scattering laser light off the electrons accelerated by a linear accelerator to speeds nearing the speed of light.

    Credit: fuw.edu.pl
    Dr. Chiara Mazzocchi
    Institute of Experimental Physics, Faculty of Physics, University of Warsaw
    tel. +48 22 5532666
    email: chiara.mazzocchi@fuw.edu.pl

    See the full article here .

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  • richardmitnick 11:40 am on May 9, 2016 Permalink | Reply
    Tags: , , Gamma Rays,   

    From New Scientist: “Water telescope’s first sky map shows flickering black holes” 

    NewScientist

    New Scientist

    18 April 2016 [just appeared on social media]

    Lisa Grossman

    HAWC High Altitude Cherenkov Experiment, HAWC Collaboration
    HAWC High Altitude Cherenkov Gamma-Ray Collaboration, HAWC Observatory, Sierra Negra volcano near Puebla, Mexico

    Twinkle, twinkle, little black hole. The High Altitude Water Cherenkov observatory has released its first map of the sky, including the first measurements of how often black holes flicker on and off. It has also caught pulsars, supernova remnants, and other bizarre cosmic beasts.

    “This is our deepest look at two-thirds of the sky, as well as the highest energy photons we’ve ever seen from any source,” says Brenda Dingus of Los Alamos National Laboratory, who presented the map at the American Physical Society meeting in Salt Lake City, Utah on 18 April. “We’re at the high energy frontier.”

    HAWC has been operating from the top of a mountain in central Mexico for about a year, and has caught some of the highest-energy photons ever observed. It is sensitive to gamma rays between 0.1 and 100 teraelectronvolts (TeV) in energy – more than 7 times higher energy than the particles produced in the Large Hadron Collider. The most energetic photon they’ve picked up so far is 60 TeV.

    1
    HAWC’s map of the gamma ray sky. HAWC Collaboration

    But this is no normal telescope. “HAWC doesn’t look or work like any other observatory,” Dingus says. The detector is made up of 300 water tanks filled with 200,000 litres of purified water each (see main image, above). When high-energy particles go through the water, they emit a blue light called Cherenkov radiation. Physicists can use that light to reconstruct where the particles came from.

    HAWC doesn’t observe the extremely high-energy photons directly. They are blocked by our atmosphere – luckily for us, as they can damage living tissue. Instead the detector catches the spray of secondary particles that gamma rays produce when they strike the atmosphere, called air showers.

    “20,000 air shower particles per second hit our detector,” Dingus says. “In fact they’re hitting us right now.”

    In the first year of data, HAWC picked up 40 distinct sources of gamma rays, 10 of which had not been seen in gamma rays before. The team is now working to figure out if they were associated with any other known objects that have been seen in other wavelengths like visible or infrared light.

    One, for instance, was associated with a known supernova remnant from an energetic pulsar, says Michelle Hui of NASA’s Marshall Spaceflight Center in Huntsville, Alabama. When massive stars die as supernovas, they slough off material in a cloud called a supernova remnant. The shock wave from the explosion then sweeps through the cloud and accelerates particles in it to extremely high energies, where they radiate gamma rays.

    Another source is a known pulsar 26,000 light years away. A nearby third is still being identified, and might be related to the supernova remnant.

    2
    Three new sources of gamma rays spotted by HAWC. HAWC Collaboration

    Flickering black holes

    HAWC can also pick up gamma rays from galaxies outside the Milky Way, the sources of which are much more mysterious. We think they are caused by the black holes at galactic centres, but the details of how the photons gain so much energy are murky.

    Because it is watching 24 hours a day, the detector can pick up changes in gamma ray brightness more reliably than ever before. Just 10 days ago, HAWC spotted a flare in a galaxy called Markarian 501.

    “On April 5 we didn’t see it, on April 6 it got very bright, and by April 8 it had nearly disappeared again,” said Robert Lauer of the University of New Mexico. The team put out an announcement on the Astronomer’s Telegram network to alert other observatories to follow up in different wavelengths, although it has not had any responses yet.

    Previous telescopes that could catch such energetic photons could only look at one part of the sky at once, so they couldn’t measure the frequency of these flares. Over the next five years, HAWC will be able to make the first measurements of how often they happen. This level of flaring seems to happen about 5 to 10 times per year, Lauer says, but it seems to vary from galaxy to galaxy.

    HAWC will also be able to see such a flare from the black hole at the centre of our own galaxy.

    “We know these things happen, so we expect them to happen here,” Hui says. “We just don’t know how often.”

    See the full article here .

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