ORCID Profile
0000-0001-6666-6866
Current Organisations
North-West University
,
University of Pisa
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Publisher: American Physical Society (APS)
Date: 15-05-2018
Publisher: Springer Science and Business Media LLC
Date: 12-2008
DOI: 10.1038/NATURE07632
Publisher: Public Library of Science (PLoS)
Date: 15-01-2009
Publisher: Public Library of Science (PLoS)
Date: 31-12-2008
Publisher: Public Library of Science (PLoS)
Date: 06-06-2011
Publisher: Public Library of Science (PLoS)
Date: 18-07-2011
Publisher: EDP Sciences
Date: 04-2018
DOI: 10.1051/0004-6361/201731169
Abstract: Context. The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes above the sensitivity limits of current Cherenkov telescopes have been predicted for both the central X-ray binary system and the interaction regions of SS 433 jets with the surrounding W50 nebula. Non-thermal emission at lower energies has been previously reported, indicating that efficient particle acceleration is taking place in the system. Aim. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses ( P orb ~ 13.1 days) and precession of the circumstellar disk ( P pre ~ 162 days) periodically covering the central binary system is expected to be at its minimum. The eastern and western SS 433/W50 interaction regions are also examined using the whole data set available. We aim to constrain some theoretical models previously developed for this system with our observations. Methods. We made use of dedicated observations from the Major Atmospheric Gamma Imaging Cherenkov telescopes (MAGIC) and High Energy Spectroscopic System (H.E.S.S.) of SS 433 taken from 2006 to 2011. These observation were combined for the first time and accounted for a total effective observation time of 16.5 h, which were scheduled considering the expected phases of minimum absorption of the putative VHE emission. Gamma-ray attenuation does not affect the jet/medium interaction regions. In this case, the analysis of a larger data set amounting to ~40–80 h, depending on the region, was employed. Results. No evidence of VHE gamma-ray emission either from the central binary system or from the eastern/western interaction regions was found. Upper limits were computed for the combined data set. Differential fluxes from the central system are found to be ≲ 10 −12 –10 −13 TeV −1 cm −2 s −1 in an energy interval ranging from ~few × 100 GeV to ~few TeV. Integral flux limits down to ~ 10 −12 –10 −13 ph cm −2 s −1 and ~ 10 −13 –10 −14 ph cm −2 s −1 are obtainedat 300 and 800 GeV, respectively. Our results are used to place constraints on the particle acceleration fraction at the inner jetregions and on the physics of the jet/medium interactions. Conclusions. Our findings suggest that the fraction of the jet kinetic power that is transferred to relativistic protons must be relatively small in SS 433, q p ≤ 2.5 × 10 −5 , to explain the lack of TeV and neutrino emission from the central system. At the SS 433/W50 interface, the presence of magnetic fields ≳10 μ G is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with E e − up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.
Publisher: EDP Sciences
Date: 27-01-2017
DOI: 10.1051/0004-6361/201629419
Abstract: Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155−304 is analyzed in the high (HE, 100 MeV E 300 GeV) and very high energy (VHE, E 200 GeV) γ -ray domain. Over the course of ~9 yr of H.E.S.S. observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index β VHE = 1.10 +0.10 -0.13 ) on timescales larger than one day. An analysis of ~5.5 yr of HE Fermi -LAT data gives consistent results (β HE = 1.20 +0.21 -0.23 , on timescales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior ( β ~ 2) seen on shorter timescales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.
Publisher: EDP Sciences
Date: 10-2017
DOI: 10.1051/0004-6361/201731200
Abstract: Very high-energy γ rays (VHE, E ≳ 100 GeV) propagating over cosmological distances can interact with the low-energy photons of the extragalactic background light (EBL) and produce electron-positron pairs. The transparency of the Universe to VHE γ rays is then directly related to the spectral energy distribution (SED) of the EBL. The observation of features in the VHE energy spectra of extragalactic sources allows the EBL to be measured, which otherwise is very difficult. An EBL model-independent measurement of the EBL SED with the H.E.S.S. array of Cherenkov telescopes is presented. It was obtained by extracting the EBL absorption signal from the reanalysis of high-quality spectra of blazars. From H.E.S.S. data alone the EBL signature is detected at a significance of 9.5 σ , and the intensity of the EBL obtained in different spectral bands is presented together with the associated γ -ray horizon.
Publisher: Public Library of Science (PLoS)
Date: 29-02-2012
Publisher: EDP Sciences
Date: 2017
Publisher: Springer Science and Business Media LLC
Date: 23-11-2011
Publisher: EDP Sciences
Date: 04-2018
DOI: 10.1051/0004-6361/201732098
Abstract: We present the results of the most comprehensive survey of the Galactic plane in very high-energy (VHE) γ -rays, including a public release of Galactic sky maps, a catalog of VHE sources, and the discovery of 16 new sources of VHE γ -rays. The High Energy Spectroscopic System (H.E.S.S.) Galactic plane survey (HGPS) was a decade-long observation program carried out by the H.E.S.S. I array of Cherenkov telescopes in Namibia from 2004 to 2013. The observations amount to nearly 2700 h of quality-selected data, covering the Galactic plane at longitudes from ℓ = 250° to 65° and latitudes | b |≤ 3°. In addition to the unprecedented spatial coverage, the HGPS also features a relatively high angular resolution (0.08° ≈ 5 arcmin mean point spread function 68% containment radius), sensitivity (≲1.5% Crab flux for point-like sources), and energy range (0.2–100 TeV). We constructed a catalog of VHE γ -ray sources from the HGPS data set with a systematic procedure for both source detection and characterization of morphology and spectrum. We present this likelihood-based method in detail, including the introduction of a model component to account for unresolved, large-scale emission along the Galactic plane. In total, the resulting HGPS catalog contains 78 VHE sources, of which 14 are not reanalyzed here, for ex le, due to their complex morphology, namely shell-like sources and the Galactic center region. Where possible, we provide a firm identification of the VHE source or plausible associations with sources in other astronomical catalogs. We also studied the characteristics of the VHE sources with source parameter distributions. 16 new sources were previously unknown or unpublished, and we in idually discuss their identifications or possible associations. We firmly identified 31 sources as pulsar wind nebulae (PWNe), supernova remnants (SNRs), composite SNRs, or gamma-ray binaries. Among the 47 sources not yet identified, most of them (36) have possible associations with cataloged objects, notably PWNe and energetic pulsars that could power VHE PWNe.
Publisher: EDP Sciences
Date: 12-2018
DOI: 10.1051/0004-6361/201732153
Abstract: Aims. We report on the measurement and investigation of pulsed high-energy γ -ray emission from the Vela pulsar, PSR B0833−45, based on observations with the largest telescope of H.E.S.S., CT5, in monoscopic mode, and on data obtained with the Fermi -LAT. Methods. Data from 40.3 h of observations carried out with the H.E.S.S. II array from 2013 to 2015 have been used. A dedicated very low-threshold event reconstruction and analysis pipeline was developed to achieve the lowest possible energy threshold. Eight years of Fermi -LAT data were analysed and also used as reference to validate the CT5 telescope response model and analysis methods. Results. A pulsed γ -ray signal at a significance level of more than 15 σ is detected from the P2 peak of the Vela pulsar light curve. Of a total of 15 835 events, more than 6000 lie at an energy below 20 GeV, implying a significant overlap between H.E.S.S. II-CT5 and the Fermi -LAT. While the investigation of the pulsar light curve with the LAT confirms characteristics previously known up to 20 GeV in the tens of GeV energy range, CT5 data show a change in the pulse morphology of P2, i.e. an extreme sharpening of its trailing edge, together with the possible onset of a new component at 3.4 σ significance level. Assuming a power-law model for the P2 spectrum, an excellent agreement is found for the photon indices (Γ ≃ 4.1) obtained with the two telescopes above 10 GeV and an upper bound of 8% is derived on the relative offset between their energy scales. Using data from both instruments, it is shown however that the spectrum of P2 in the 10–100 GeV has a pronounced curvature this is a confirmation of the sub-exponential cut-off form found at lower energies with the LAT. This is further supported by weak evidence of an emission above 100 GeV obtained with CT5. In contrast, converging indications are found from both CT5 and LAT data for the emergence of a hard component above 50 GeV in the leading wing (LW2) of P2, which possibly extends beyond 100 GeV. Conclusions. The detection demonstrates the performance and understanding of CT5 from 100 GeV down to the sub-20 GeV domain, i.e. unprecedented low energy for ground-based γ -ray astronomy. The extreme sharpening of the trailing edge of the P2 peak found in the H.E.S.S. II light curve of the Vela pulsar and the possible extension beyond 100 GeV of at least one of its features, LW2, provide further constraints to models of γ -Ray emission from pulsars.
Publisher: EDP Sciences
Date: 11-2018
DOI: 10.1051/0004-6361/201832640
Abstract: Centaurus A (Cen A) is the nearest radio galaxy discovered as a very-high-energy (VHE 100 GeV–100 TeV) γ -ray source by the High Energy Stereoscopic System (H.E.S.S.). It is a faint VHE γ -ray emitter, though its VHE flux exceeds both the extrapolation from early Fermi -LAT observations as well as expectations from a (misaligned) single-zone synchrotron-self Compton (SSC) description. The latter satisfactorily reproduces the emission from Cen A at lower energies up to a few GeV. New observations with H.E.S.S., comparable in exposure time to those previously reported, were performed and eight years of Fermi -LAT data were accumulated to clarify the spectral characteristics of the γ -ray emission from the core of Cen A. The results allow us for the first time to achieve the goal of constructing a representative, contemporaneous γ -ray core spectrum of Cen A over almost five orders of magnitude in energy. Advanced analysis methods, including the template fitting method, allow detection in the VHE range of the core with a statistical significance of 12 σ on the basis of 213 hours of total exposure time. The spectrum in the energy range of 250 GeV–6 TeV is compatible with a power-law function with a photon index Γ = 2.52 ± 0.13 stat ± 0.20 sys . An updated Fermi -LAT analysis provides evidence for spectral hardening by Δ Γ ≃ 0.4 ± 0.1 at γ -ray energies above 2.8 +1.0 −0.6 GeV at a level of 4.0 σ . The fact that the spectrum hardens at GeV energies and extends into the VHE regime disfavour a single-zone SSC interpretation for the overall spectral energy distribution (SED) of the core and is suggestive of a new γ -ray emitting component connecting the high-energy emission above the break energy to the one observed at VHE energies. The absence of significant variability at both GeV and TeV energies does not yet allow disentanglement of the physical nature of this component, though a jet-related origin is possible and a simple two-zone SED model fit is provided to this end.
Publisher: EDP Sciences
Date: 02-2018
DOI: 10.1051/0004-6361/201732426
Abstract: Context. Recently, the high-energy (HE, 0.1–100 GeV) γ -ray emission from the object LMC P3 in the Large Magellanic Cloud (LMC) has been discovered to be modulated with a 10.3-day period, making it the first extra-galactic γ -ray binary. Aim. This work aims at the detection of very-high-energy (VHE, GeV) γ -ray emission and the search for modulation of the VHE signal with the orbital period of the binary system. Methods. LMC P3 has been observed with the High Energy Stereoscopic System (H.E.S.S.) the acceptance-corrected exposure time is 100 h. The data set has been folded with the known orbital period of the system in order to test for variability of the emission. Results. VHE γ -ray emission is detected with a statistical significance of 6.4 σ . The data clearly show variability which is phase-locked to the orbital period of the system. Periodicity cannot be deduced from the H.E.S.S. data set alone. The orbit-averaged luminosity in the 1–10 TeV energy range is (1.4 ± 0.2) × 1035 erg s −1 . A luminosity of (5 ± 1) × 1035 erg s −1 is reached during 20% of the orbit. HE and VHE γ -ray emissions are anti-correlated. LMC P3 is the most luminous γ -ray binary known so far.
Publisher: Elsevier BV
Date: 10-2015
Publisher: Springer Science and Business Media LLC
Date: 30-09-2015
DOI: 10.1038/NATURE15390
Publisher: Oxford University Press (OUP)
Date: 20-02-2018
DOI: 10.1093/MNRAS/STY439
Publisher: EDP Sciences
Date: 04-2018
DOI: 10.1051/0004-6361/201526545
Abstract: Aim. 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 E cut = (3.5 ± 1.2 stat ) 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 (10 51 erg). When using a hadronic model, a magnetic field of B ≈ 100 μ G 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 Γ p ≈ 1.7 would be required, which implies that ∼7 × 10 49 / n cm −3 has been transferred into high-energy protons with the effective density n cm −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 −3 .
Publisher: EDP Sciences
Date: 05-2015
Publisher: Cambridge University Press (CUP)
Date: 03-2007
DOI: 10.1017/S1743921307012616
Abstract: General characteristics of methanol (CH 3 OH) maser emission are summarized. It is shown that methanol maser sources are concentrated in the spiral arms. Most of the methanol maser sources from the Perseus arm are associated with embedded stellar clusters and a considerable portion is situated close to compact H II regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically identified H II regions which means that massive star formation in the Perseus Arm is to a great extent triggered by local phenomena. A multiline analysis of the methanol masers allows us to determine the physical parameters in the regions of maser formation. Maser modelling shows that class II methanol masers can be pumped by the radiation of the warm dust as well as by free-free emission of a hypercompact region (hcH II ) with a turnover frequency exceeding 100 GHz. Methanol masers of both classes can reside in the vicinity of hcH II s. Modelling shows that periodic changes of maser fluxes can be reproduced by variations of the dust temperature by a few percent which may be caused by variations in the brightness of the central young stellar object reflecting the character of the accretion process. Sensitive observations have shown that the masers with low flux densities can still have considerable lification factors. The analysis of class I maser surveys allows us to identify four distinct regimes that differ by the series of their brightest lines.
Publisher: EDP Sciences
Date: 04-2018
DOI: 10.1051/0004-6361/201732125
Abstract: Shell-type supernova remnants (SNRs) are considered prime candidates for the acceleration of Galactic cosmic rays (CRs) up to the knee of the CR spectrum at E ≈ 3 × 10 15 eV. Our Milky Way galaxy hosts more than 350 SNRs discovered at radio wavelengths and at high energies, of which 220 fall into the H.E.S.S. Galactic Plane Survey (HGPS) region. Of those, only 50 SNRs are coincident with a H.E.S.S source and in 8 cases the very high-energy (VHE) emission is firmly identified as an SNR. The H.E.S.S. GPS provides us with a legacy for SNR population study in VHE γ -rays and we use this rich data set to extract VHE flux upper limits from all undetected SNRs. Overall, the derived flux upper limits are not in contradiction with the canonical CR paradigm. Assuming this paradigm holds true, we can constrain typical ambient density values around shell-type SNRs to n ≤ 7 cm −3 and electron-to-proton energy fractions above 10 TeV to ϵ ep ≤ 5 × 10 −3 . Furthermore, comparisons of VHE with radio luminosities in non-interacting SNRs reveal a behaviour that is in agreement with the theory of magnetic field lification at shell-type SNRs.
Publisher: EDP Sciences
Date: 04-2017
DOI: 10.1051/0004-6361/201629427
Abstract: Context. The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment’s sensitivityto lower energies. The lowest energy threshold is obtained using monoscopic analysis of data taken with CT5, providing access to gamma-ray energies below 100 GeV for small zenith angle observations. Such an extension of the instrument’s energy range is particularly beneficial for studies of active galactic nuclei with soft spectra, as expected for those at a redshift ≥0.5. The high-frequency peaked BL Lac objects PKS 2155−304 ( z = 0.116) and PG 1553+113 (0.43 z 0.58) are among the brightest objects in the gamma-ray sky, both showing clear signatures of gamma-ray absorption at E 100 GeV interpreted as being due to interactions with the extragalactic background light (EBL). Aims. The aims of this work are twofold: to demonstrate the monoscopic analysis of CT5 data with a low energy threshold, and to obtain accurate measurements of the spectral energy distributions (SED) of PKS 2155−304 and PG 1553+113 near their SED peaks at energies ≈100 GeV. Methods. Multiple observational c aigns of PKS 2155−304 and PG 1553+113 were conducted during 2013 and 2014 using the full H.E.S.S. II instrument (CT1–5). A monoscopic analysis of the data taken with the new CT5 telescope was developed along with an investigation into the systematic uncertainties on the spectral parameters which are derived from this analysis. Results. Using the data from CT5, the energy spectra of PKS 2155−304 and PG 1553+113 were reconstructed down to conservative threshold energies of 80 GeV for PKS 2155−304 , which transits near zenith, and 110 GeV for the more northern PG 1553+113 . The measured spectra, well fitted in both cases by a log-parabola spectral model (with a 5.0 σ statistical preference for non-zero curvature for PKS 2155−304 and 4.5 σ for PG 1553+113 ), were found consistent with spectra derived from contemporaneous Fermi -LAT data, indicating a sharp break in the observed spectra of both sources at E ≈ 100 GeV. When corrected for EBL absorption, the intrinsic H.E.S.S. II mono and Fermi -LAT spectrum of PKS 2155−304 was found to show significant curvature. For PG 1553+113 , however, no significant detection of curvature in the intrinsic spectrum could be found within statistical and systematic uncertainties.
Publisher: Springer Science and Business Media LLC
Date: 28-08-2015
Publisher: American Astronomical Society
Date: 22-11-2017
Publisher: EDP Sciences
Date: 09-2018
DOI: 10.1051/0004-6361/201833202
Abstract: Context. NGC 253 is one of only two starburst galaxies found to emit γ -rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE E 100 GeV) and high-energy (HE E 60 MeV) spectra are crucial to study the underlying particle accelerators, probe the dominant emission mechanism(s) and to study cosmic-ray interaction and transport. Aims. The measurement of the VHE γ -ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, the most up to date measurement of the γ -ray spectrum of NGC 253 is investigated in both HE and VHE γ -rays. Assuming a hadronic origin of the γ -ray emission, the measurement uncertainties are propagated into the interpretation of the accelerated particle population. Methods. The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The improved Fermi –LAT analysis employs more than 8 yr of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE–VHE γ -ray spectrum using NAIMA in the optically thin case. Results. The VHE γ -ray energy spectrum is best fit by a power-law distribution with a flux normalisation of (1.34 ± 0.14 stat ± 0.27 sys ) × 10 −13 cm −2 s −1 TeV 1 at 1 TeV – about 40% above, but compatible with the value obtained in Abramowski et al. (2012). The spectral index Γ = 2.39 ± 0.14 stat ± 0.25 sys is slightly softer than but consistent with the previous measurement within systematic errors. In the Fermi energy range an integral flux of F ( E 60 MeV) = (1.56 ± 0.28 stat ± 0.15 sys ) × 10 −8 cm −2 s −1 is obtained. At energies above ∼3 GeV the HE spectrum is consistent with a power-law ranging into the VHE part of the spectrum measured by H.E.S.S. with an overall spectral index Γ = 2.22 ± 0.06 stat . Conclusions. Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as either a thin or a thick target for hadronic cosmic rays accelerated by the in idual sources in the nucleus. In these two models, the level to which NGC 253 acts as a calorimeter is estimated to a range of f cal = 0.1 to 1 while accounting for the measurement uncertainties. The presented spectrum is likely to remain the most accurate measurements until the Cherenkov Telescope Array (CTA) has collected a substantial set of data towards NGC 253.
Publisher: Springer Science and Business Media LLC
Date: 16-12-2019
DOI: 10.1038/S41467-019-13480-Z
Abstract: The human genetic factors that affect resistance to infectious disease are poorly understood. Here we report a genome-wide association study in 17,000 severe malaria cases and population controls from 11 countries, informed by sequencing of family trios and by direct typing of candidate loci in an additional 15,000 s les. We identify five replicable associations with genome-wide levels of evidence including a newly implicated variant on chromosome 6. Jointly, these variants account for around one-tenth of the heritability of severe malaria, which we estimate as ~23% using genome-wide genotypes. We interrogate available functional data and discover an erythroid-specific transcription start site underlying the known association in ATP2B4 , but are unable to identify a likely causal mechanism at the chromosome 6 locus. Previously reported HLA associations do not replicate in these s les. This large dataset will provide a foundation for further research on the genetic determinants of malaria resistance in erse populations.
Publisher: IOP Publishing
Date: 23-11-2018
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Johan van der Walt.