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  • richardmitnick 1:15 pm on December 3, 2017 Permalink | Reply
    Tags: , , , Cherenkov Telescope Array Namibia, , , Variable TeV emission from the most luminous gamma-ray binary   

    From H.E.S.S.: “Variable TeV emission from the most luminous gamma-ray binary” 

    Max Planck Gesellschaft

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

    December 2017

    Gamma-ray binaries are binary systems comprising a massive star (typically of spectral types O or B) and a compact object. Most of the radiated power of these systems is emitted in the gamma-ray energy regime (i.e. photon energies larger than 1 MeV), which sets them apart from other binary systems, like high-mass X-ray binaries, novae and colliding wind binaries.

    The compact object could either be a neutron star or a black hole. The emission can be powered by the rotational energy loss of a pulsar and produced by particle acceleration in the shock between the winds of the massive star and the pulsar wind nebula. This is the case for PSR B1259-63, the only system where the compact object could be identified [1]. Alternatively, the emission could be driven by accretion of the stellar wind onto the compact object and the formation of jets, in so-called microquasars.

    In the Milky Way, only 5 such objects are known: PSR B1259-63, LS 5039, LS I +61 303, HESS J0632+057 and 1FGL J1018.6-5856 (see [2] for a review). They have orbital periods between 3.9 days (LS 5039) and more than 3 years (PSR B1259-63). The gamma-ray emission is typically modulated with the orbital period of the system. The peak of the emission can be rather short compared to the orbital period (e.g. 1FGL J1018.6-5856) or spanning more than half of the orbit (e.g. LS 5039). The MeV/GeV and TeV emission may be in phase (e.g. 1FGL J1018.6-5856) or out of phase (LS 5039). The TeV emission may be aligned with periastron of the orbit (e.g. PSR B1259-63) or with inferior conjunction (e.g. LS 5039). In order to understand the gamma-ray production mechanisms, it is necessary to observe many different systems with varying orbital shapes and orientations.

    In order to identify new gamma-ray binaries, Corbet et al. [3] performed a blind search for periodic emission in the data obtained with the Fermi Large Area Telescope. These authors discovered that the gamma-ray emission of the known, but previously unidentified gamma-ray source LMC P3 is periodic with a period of 10.3 days. This source is located in the Large Magellanic Cloud, making it the first extra-galactic gamma-ray binary. The companion star is an O5 giant, and the binary system is located inside a visible supernova remnant. With this association the age of the binary system can be assumed to be the age of the supernova remnant of 100.000 years [4].

    Fig 1: Phase-folded H.E.S.S. gamma-ray light-curve of LMC P3. Phase 0 is defined at MJD 57410.25, the maximum of the MeV/GeV emission. The blue lines indicate the upper limit on the gamma-ray flux in the off-peak part of the orbit (orbital phase between 0.4 and 0.2).

    The Large Magellanic Cloud has been observed extensively with H.E.S.S. since 2004 for a total observation time of 277 hours.

    Large Magellanic Cloud. Adrian Pingstone December 2003

    At the position of the binary system the acceptance-corrected exposure is 100 hours. In this data set a TeV gamma-ray source has been detected with a statistical significance of 6.4 sigma which is positionally coincident with the location of the binary system. In order to test for variability and periodicity of the signal, the data set has been divided into 5 parts of roughly similar exposure and covering different parts of the orbit. This phase-folded light-curve is shown in Figure 1. The source is detected only in one phase bin, orbital phase 0.2 to 0.4, with a statistical significance of 7.1 sigma. Considering the 5 trials (the 5 different phase bins), this corresponds to a post-trial significance of 6.9 sigma. The phase-folded light-curve is clearly variable, and the gamma-ray emission can be attributed to the binary system. Figure 2 shows an animation of the significance maps in the individual orbital phase bins.

    Fig 2: Significance sky maps in the individual orbital phase bins of LMC P3. The cross indicates the test position, the grey circle illustrates the angular resolution of the instrument.

    Figure 3 shows the spectral energy distribution of the emission averaged over the entire orbit (green) and for phase bin with significant emission (“on-peak”, orbital phase 0.2 to 0.4, blue) only. The spectra can be fitted with a straight power law, without a high-energy cut-off. The spectral index of the on-peak flux is 2.1± 0.2, the orbit-averaged spectrum appears slightly, but not significantly softer. The integrated flux above 1 TeV of the on-peak emission is (5± 2)x10-13 cm-2s-1. For a distance of 50 kpc, this corresponds to a luminosity in the 1 to 10 TeV energy range of 5×1035 erg/s, which makes this object by far the most luminous gamma-ray binary.

    Fig 3: Spectral energy distribution for the full orbit (green) and the on-peak part of the orbit (blue).

    Two main scenarios are usually invoked to explain the gamma-ray emission from gamma-ray binaries: particle acceleration in a pulsar wind nebula (PWN) driven by the spin-down of the pulsar or accretion of the stellar wind on the compact object. The on-peak gamma-ray luminosity of 5×1035 erg/s is about the same as the luminosity of the PWN N 157B in the LMC. N 157B is driven by the spin-down of its central pulsar PSR J0537−6910 with a spin-down luminosity of 5×1038 erg/s. A putative pulsar in LMC P3 would need about the same spin-down luminosity. This would make it one of the four most luminous pulsars. That such a luminous pulsar remains undetected so far can be explained by absorption in the stellar photon field of the companion star. In the accretion scenario a luminosity of up to 1036 erg/s can be generated by the conversion of gravitational potential energy of in-falling material, assuming a typical mass accretion rate of 10-10 solar masses per year on a neutron star with 1.4 solar masses and a radius of 10 km. A significant amount of the accretion luminosity needs to be converted into TeV gamma rays in order to explain the observed emission.

    For the first time TeV gamma rays have been detected from an extra-galactic binary system. It is by far the most luminous gamma-ray binary. In order to provide for the observed gamma-ray luminosity either a pusar with a very high spin-down luminosity or a very efficient accretion of the stellar wind onto the compact object is needed.


    H. Abdalla et al. (H.E.S.S. Collaboration): Detection of variable VHE gamma-ray emission from the extra-galactic gamma-ray binary LMC P3, to be submitted.

    Sorry, on below references no links provided.

    [1] M. Chernyakova et al., MNRAS 439 (2014) 432
    [2] G. Dubus, Comptes Rendus Physique 16 (2015) 661
    [3] R.H.D. Corbet et al., Astrophys. J. 829 (2016) 105
    [4] F.D. Seward et al., Astrophys. J. 759 (2012) 123

    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 nebula

  • richardmitnick 4:52 pm on February 28, 2017 Permalink | Reply
    Tags: , , , Cherenkov Telescope Array Namibia, Cosmic gamma-rays, Cosmic particle accelerator blazar Markarian 421, , , ,   

    From DESY: “New eyes for the gamma-ray sky” 



    Final milestone for the upgraded H.E.S.S. telescopes in Namibia

    Cherenkov Telescope Array Namibia

    The newly refurbished cameras of the H.E.S.S. gamma-ray telescopes in Namibia have detected their first signals from a cosmic particle accelerator: The new cameras recorded Markarian 421 as their first target, a well-known blazar in the constellation of Ursa Major. The active galactic nucleus, 400 million light years away, was detected during an active state and at high significance. After four years of development, testing, production and deployment, this is the last big milestone of the H.E.S.S. I camera upgrade project, which was led by DESY. The success is also an important test for the next generation gamma-ray observatory, the Cherenkov Telescope Array CTA, which will use the same camera technology.

    When H.E.S.S. explores the mysteries of the high-energy sky, it actually does not look into the Universe, but at the upper atmosphere. Cosmic gamma-rays are absorbed there and produce short, faint, violet Cherenkov light flashes that can be detected from the ground using large mirrors and ultra-fast electronics. The exposure times per image are as short as 16 nanoseconds (billionths of a second), and H.E.S.S. is recording about 300 of such events per second. Since some images only consist of a few handfuls of light particles (photons), the technical requirements to build such cameras are very challenging.

    In the ten years for which the original H.E.S.S. I cameras have been operated, their fragile electronic components have suffered a natural level of ageing, which degraded their performance. In parallel, also the technologies available on the market have developed much further, like faster Ethernet solutions, and smaller and faster readout chips. One of these chips is the NECTAr chip, which has been developed for the next big experiment in the field, CTA. Therefore, in 2012 the H.E.S.S. collaboration placed an order with their new collaborators at DESY in Zeuthen to team up with colleagues from the Paris area and Universities of Leicester and Amsterdam to make use of this chip and design a new, modernised version of the four H.E.S.S. I cameras.

    The engineers lost no time and developed a holistic modernisation concept that foresaw not only the replacement of single electronics boards, but also a better cabling, pneumatics and ventilation scheme. On top of this, colleagues from LLR near Paris added a full renewal of the light collimators in front of the PMT pixels (“Winston cones”) to the list of things to improve, so more light is collected in the first place. The first of the cameras was installed in July 2015, the other three were brought to Namibia in September 2016. “The installation went extremely well. Although it’s a very isolated work situation, out there in the remote countryside of Namibia, the team was really performing great and the atmosphere was very good”, summarises Stefan Klepser, DESY project leader of the upgrade. “Also, I am happy to say that we stayed well within the budget and the time frame we were aiming at.”

    After the installation, software needed to be adjusted, network connections to be established, and real-life, unexpected issues needed to be trouble-shooted. Around Christmas 2016 the systems were all fit for observation, and as luck would have it, an old friend in the gamma-ray sky, the blazar Markarian 421 was reported to show increased activity. Despite being located in the Northern sky, in the constellation of Ursa Major, it was within reach for observations by H.E.S.S. The scientists turned the four telescopes at it and could record thousands of images.

    “The refurbished cameras delivered the first large scale demonstration that the NECTAr technology is fit for teraelectronvolt astronomy”, summarises Christian Stegmann, head of the DESY institute in Zeuthen. “This makes us look forward to the final years of H.E.S.S., where the new cameras will provide us with enhanced performance at both very low and very high energies. And it is a promising outlook at the next major gamma-ray observatory CTA, where DESY is an important partner.”

    See the full article here .

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    DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

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