42 research outputs found
Determining pitch-angle diffusion coefficients from test particle simulations
No full textThe transport and acceleration of charged particles in turbulent media are topics of great interest in space physics and interstellar astrophysics. These processes are dominated by the scattering of particles off magnetic irregularities. The scattering process itself is usually described by small-angle scattering, with the pitch-angle coefficient playing a major role. Since the diffusion coefficient can be determined analytically only for the approximation of quasilinear theory, the determination of this coefficient from numerical simulations has become more important. So far these simulations have yielded particle tracks for small-scale scattering, which can then be interpreted using the running diffusion coefficients. This method has a limited range of validity. This paper presents two new methods that allow for the calculation of the pitch-angle diffusion coefficient from numerical simulations. These methods no longer analyze particle trajectories and instead examine the change of particle distribution functions. It is shown that these methods provide better resolved results and allow for the analysis of strong turbulence. The application of these methods to Monte Carlo simulations of particle scattering and hybrid MHD-particle simulations is presented. Both analysis methods are able to recover the diffusion coefficients used as input for the Monte Carlo simulations and provide better results in MHD simulations, especially for stronger turbulenc
The nature of diffusive particle transport in turbulent magnetic fields
No full textPhD (Space Physics), North-West University, Potchefstroom Campus, 2017The transport of charged particles in turbulent plasma is a crucially important area of
research in astrophysics, since it directly impacts our ability to interpret observations
done with a major group of messenger particles. In order to understand these measurements
of highly energetic charged particles - or cosmic rays - a comprehension of
their interactions with the turbulent magnetic fields, which permeate the heliosphere,
the interstellar and the intergalactic medium, is equally important as the understanding
of their generation and acceleration at the sources, if not more so.
In this work, a numerical approach is taken to derive transport parameters for charged
energetic particles in the heliosphere. A spectral incompressible MHD code is used to
generate realistic turbulence in a self-consistent way. The properties of the turbulence
are then probed by injecting test particles and analysing their propagation. New numerical
analysis methods, that were developed to work especially well in strong turbulence
scenarios, where classical methods and analytical solutions fail, are presented, together
with their validation and transport parameter results obtained for various simulation setups.
In most astrophysical scenarios the magnetic field fluctuations can be assumed to
be much larger than the fluctuating electric fields δB >>δE, consequently the predominant
transport process is the change of the direction of the particle momentum relative
to the magnetic field - or pitch angle - as opposed to the change of the absolute value
of the momentum p, which is suppressed in comparison. Hence, the focus of the analysis
is on the pitch angle diffusion coefficient Dµµ. To demonstrate that the concept can also
be applied to other quantities, results for the perpendicular spacial diffusion coefficient
D┴ are derived and presented as well.
Additionally, an alternative method to generate turbulence in magnetised plasmas using
Perlin gradient noise is described and its characteristics concerning particle transport
are analysed and compared with the self-consistent MHD-turbulence in order to test its
validity. Although the properties of the Perlin noise turbulence are not in complete
agreement with MHD, the deviations can be neglected in specific cases (especially in
strong turbulence) and are offset somewhat by the much lower computational effort.Doctora
Long-term monitoring of PKS2155-304 with ATOM and H.E.S.S.: investigation of optical/y-ray correlations in different spectral states
No full textIn this paper we report on the analysis of all the available optical and very high-energy
y-ray (>200 GeV) data for the BL Lac object PKS 2155-304,
collected simultaneously with the ATOM and H.E.S.S. telescopes from 2007 until 2009. This study also includes X-ray (RXTE, Swift) and high-energy y-ray (Fermi-LAT) data. During the period analysed, the source was transitioning from its flaring to quiescent optical states, and was
characterized by only moderate flux changes at different wavelengths on the timescales of days and months. A flattening of the optical continuum
with an increasing optical flux can be noted in the collected dataset, but only occasionally and only at higher flux levels. We did not find any
universal relation between the very high-energy y-ray and optical flux changes on the timescales from days and weeks up to several years. On
the other hand, we noted that at higher flux levels the source can follow two distinct tracks in the optical flux–colour diagrams, which seem to be
related to distinct y-ray states of the blazar. The obtained results therefore indicate a complex scaling between the optical and y-ray emission of
PKS 2155-304, with different correlation patterns holding at different epochs, and a y-ray flux depending on the combination of an optical flux
and colour rather than a flux aloneThe support of the Namibian authorities and of the
University of Namibia in facilitating the construction and operation of H.E.S.S.
is gratefully acknowledged, as is the support by the German Ministry
for Education and Research (BMBF), the Max Planck Society, the French
Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary
Programme of the CNRS, the U.K. Science and Technology Facilities Council
(STFC), the IPNP of the Charles University, the Czech Science Foundation, the
Polish Ministry of Science and Higher Education, the South African Department
of Science and Technology and National Research Foundation, and by the
University of Namibia
The 2012 flare of PG 1553+113 seen with H.E.S.S. and Fermi-LAT
No full textVery high energy (VHE, E > 100 GeV) γ-ray flaring activity of the high-frequency peaked BL Lac object PG 1553+113 has been detected by the H.E.S.S. telescopes. The flux of the source increased by a factor of 3 during the nights of 2012 April 26 and 27 with respect to the archival measurements with a hint of intra-night variability. No counterpart of this event has been detected in the Fermi-Large Area Telescope data. This pattern is consistent with VHE γ-ray flaring being caused by the injection of ultrarelativistic particles, emitting γ-rays at the highest energies. The dataset offers a unique opportunity to constrain the redshift of this source at z = 0.49 ± 0.04 using a novel method based on Bayesian statistics. The indication of intra-night variability is used to introduce a novel method to probe for a possible Lorentz invariance violation (LIV), and to set limits on the energy scale at which Quantum Gravity (QG) effects causing LIV may arise. For the subluminal case, the derived limits are EQG,1 > 4.10 × 1017 GeV and EQG,2 > 2.10 × 1010 GeV for linear and quadratic LIV effects, respectivel
H.E.S.S. reveals a lack of TeV emission from the supernova remnant Puppis A
No full textContext: Puppis A is an interesting ~4 kyr-old supernova remnant (SNR) that shows strong evidence of interaction between the forward shock and a molecular cloud. It has been studied in detail from radio frequencies to high-energy (HE, 0.1−100 GeV) γ-rays. An analysis of the Fermi-LAT data has shown extended HE γ-ray emission with a 0.2−100 GeV spectrum exhibiting no significant deviation from a power law, unlike most of the GeV-emitting SNRs known to be interacting with molecular clouds. This makes it a promising target for imaging atmospheric Cherenkov telescopes (IACTs) to probe the γ-ray emission above 100 GeV.
Aims: Very-high-energy (VHE, E ≥ 0.1 TeV) γ-ray emission from Puppis A has been, for the first time, searched for with the High Energy Stereoscopic System (H.E.S.S.).
Methods: Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the direction and energy of the incident γ-rays in order to produce sky images and source spectra. The profile likelihood method is applied to find constraints on the existence of a potential break or cutoff in the photon spectrum.
Results: The analysis of the H.E.S.S. data does not reveal any significant emission towards Puppis A. The derived upper limits on the differential photon flux imply that its broadband γ-ray spectrum must exhibit a spectral break or cutoff. By combining Fermi-LAT and H.E.S.S. measurements, the 99% confidence-level upper limits on such a cutoff are found to be 450 and 280 GeV, assuming a power law with a simple exponential and a sub-exponential cutoff, respectively. It is concluded that none of the standard limitations (age, size, radiative losses) on the particle acceleration mechanism, assumed to be continuing at present, can explain the lack of VHE signal. The scenario in which particle acceleration has ceased some time ago is considered as an alternative explanation. The HE/VHE spectrum of Puppis A could then exhibit a break of non-radiative origin (as observed in several other interacting SNRs, albeit at somewhat higher energies), owing to the interaction with dense and neutral material, in particular towards the NE regio
Discovery of the hard spectrum VHE γ-ray source HESS J1641-463
No full textThis Letter reports the discovery of a remarkably hard spectrum source, HESS J1641–463, by the High Energy Stereoscopic System (H.E.S.S.) in the very high energy (VHE) domain. HESS J1641–463 remained unnoticed by the usual analysis techniques due to confusion with the bright nearby source HESS J1640–465. It emerged at a significance level of 8.5 standard deviations after restricting the analysis to events with energies above 4 TeV. It shows a moderate flux level of phgr(E>1 TeV) = (3.64 ± 0.44stat ± 0.73sys) × 10–13 cm–2 s–1, corresponding to 1.8% of the Crab Nebula flux above the same energy, and a hard spectrum with a photon index of Γ = 2.07 ± 0.11stat ± 0.20sys. It is a point-like source, although an extension up to a Gaussian width of σ = 3 arcmin cannot be discounted due to uncertainties in the H.E.S.S. point-spread function. The VHE γ-ray flux of HESS J1641–463 is found to be constant over the observed period when checking time binnings from the year-by-year to the 28 minute exposure timescales. HESS J1641–463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1. No X-ray candidate stands out as a clear association; however, Chandra and XMM-Newton data reveal some potential weak counterparts. Various VHE γ-ray production scenarios are discussed. If the emission from HESS J1641–463 is produced by cosmic ray protons colliding with the ambient gas, then their spectrum must extend close to 1 PeV. This object may represent a source population contributing significantly to the galactic cosmic ray flux around the kneeGerman Ministry for Education and Research (BMBF), the Max
Planck Society, the French Ministry for Research, the CNRSIN2P3,
and the Astroparticle Interdisciplinary Programme of
the CNRS, the U.K. Science and Technology Facilities Council
(STFC), the IPNP of the Charles University, the PolishMinistry
of Science and Higher Education, the SouthAfrican Department
of Science and Technology and National Research Foundation,
and by the University of Namibia
Constraints on an annihilation signal from a core of constant dark matter density around the Milky Way center with H.E.S.S.
No full textAn annihilation signal of dark matter is searched for from the central region of the Milky Way. Data acquired in dedicated on-off observations of the Galactic center region with H.E.S.S. are analyzed for this purpose. No significant signal is found in a total of ∼9 h of on-off observations. Upper limits on the velocity averaged cross section, ⟨σv⟩, for the annihilation of dark matter particles with masses in the range of ∼300 GeV to ∼10 TeV are derived. In contrast to previous constraints derived from observations of the Galactic center region, the constraints that are derived here apply also under the assumption of a central core of constant dark matter density around the center of the Galaxy. Values of ⟨σv⟩ that are larger than 3×10−24 cm3/s are excluded for dark matter particles with masses between ∼1 and ∼4 TeV at 95% C.L. if the radius of the central dark matter density core does not exceed 500 pc. This is the strongest constraint that is derived on ⟨σv⟩ for annihilating TeV mass dark matter without the assumption of a centrally cusped dark matter density distribution in the search regio
Detailed spectral and morphological analysis of the shell type supernova remnant RCW 86
No full textAims. We aim for an understanding of the morphological and spectral properties of the supernova remnant RCW 86 and for insights into the
production mechanism leading to the RCW 86 very high-energy γ-ray emission.
Methods. We analyzed High Energy Spectroscopic System (H.E.S.S.) data that had increased sensitivity compared to the observations presented in
the RCW 86 H.E.S.S. discovery publication. Studies of the morphological correlation between the 0.5–1 keV X-ray band, the 2–5 keV X-ray band,
radio, and γ-ray emissions have been performed as well as broadband modeling of the spectral energy distribution with two different emission
models.
Results. We present the first conclusive evidence that the TeV γ-ray emission region is shell-like based on our morphological studies. The
comparison with 2–5 keV X-ray data reveals a correlation with the 0.4–50 TeV γ-ray emission. The spectrum of RCW 86 is best described by
a power law with an exponential cutoff at Ecut = (3.5 ± 1.2stat) TeV and a spectral index of Γ ≈ 1.6 ± 0.2. A static leptonic one-zone model
adequately describes the measured spectral energy distribution of RCW 86, with the resultant total kinetic energy of the electrons above 1 GeV
being equivalent to ∼0.1% of the initial kinetic energy of a Type Ia supernova explosion (1051 erg). When using a hadronic model, a magnetic field
of B ≈ 100 µG is needed to represent the measured data. Although this is comparable to formerly published estimates, a standard E−2
spectrum for
the proton distribution cannot describe the γ-ray data. Instead, a spectral index of Γp ≈ 1.7 would be required, which implies that ∼7×1049/ncm−3 erg
has been transferred into high-energy protons with the effective density ncm−3 = n/1 cm−3
. This is about 10% of the kinetic energy of a typical
Type Ia supernova under the assumption of a density of 1 cm−
Discovery of variable VHE γ-ray emission from the binary system 1FGL J1018.6-5856
No full textRe-observations with the HESS telescope array of the very high-energy (VHE) source HESS J1018–589 A that is coincident with the Fermi-LAT γ-ray binary 1FGL J1018.6–5856 have resulted in a source detection significance of more than 9σ and the detection of variability (χ2/ν of 238.3/155) in the emitted γ-ray flux. This variability confirms the association of HESS J1018–589 A with the high-energy γ-ray binary detected by Fermi-LAT and also confirms the point-like source as a new VHE binary system. The spectrum of HESS J1018–589 A is best fit with a power-law function with photon index Γ = 2.20 ± 0.14stat ± 0.2sys. Emission is detected up to ~20 TeV. The mean differential flux level is (2.9 ± 0.4) × 10-13 TeV-1 cm-2 s-1 at 1 TeV, equivalent to ~1% of the flux from the Crab Nebula at the same energy. Variability is clearly detected in the night-by-night light curve. When folded on the orbital period of 16.58 days, the rebinned light curve peaks in phase with the observed X-ray and high-energy phaseograms. The fit of the HESS phaseogram to a constant flux provides evidence of periodicity at the level of Nσ> 3σ. The shape of the VHE phaseogram and measured spectrum suggest a low-inclination, low-eccentricity system with amodest impact from VHE γ-ray absorption due to pair production (τ ≲ 1 at 300 GeV
