Free Radical Copolymerization of Ethylene with Vinyl Acetate under Mild Conditions

This work highlights a medium pressure and temperature radical polymerization process in organic solvents for the synthesis of ethylene–vinyl acetate copolymers (EVA). Organic solvents play a crucial role in copolymerization yields, copolymer molar masses, chemical composition, and branching degree. THF and dimethylcarbonate (DMC) were selected, yielding EVA copolymers exhibiting molar masses from 1000 up to 35000 g mol^–1 and vinyl acetate contents between 4 and 32 mol %. In the case of DMC, an additional transfer agent (propanal) was required to reach the molar-mass range targeted for the specific application of EVA copolymers as cold flow improver (CFI): an essential diesel fuel additive

Discovery of very high energy γ-ray emission from the BL Lacertae object H 2356-309 with the HESS Cherenkov telescopes

The extreme synchrotron BL Lac object H 2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of $\approx$40 h live-time with the HESS (High Energy Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 $\gamma$-rays (10 standard deviations above background) from H 2356-309, corresponding to an observed integral flux above 200 GeV of I(>200 GeV) = (4.1 $\pm$ 0.5) $\times$ 10-12 cm-2 s-1 (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N0 = (4.1 $\pm$ 0.5) $\times$ 10-13 cm-2 s-1 TeV-1 and a photon index of $\Gamma$ = $3.09\,\pm\,0.24_\mathrm{stat}\,\pm\,0.10_\mathrm{sys}$. H 2356-309 is one of the most distant BL Lac objects detected at very-high-energy $\gamma$-rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the HESS data to constrain a single-zone homogeneous synchrotron self-Compton (SSC) model. This model provides an adequate fit to the HESS data when using a reasonable set of model parameters

Characterizing the γ-ray long-term variability of PKS 2155 - 304 with H.E.S.S. and Fermi-LAT

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 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 βVHE = 1.10-0.13+0.10) on timescales larger than one day. An analysis of ∼5.5 yr of HE Fermi-LAT data gives consistent results (βHE = 1.20-0.23+0.21), 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

Discovery of the two "wings" of the Kookaburra complex in VHE γ-rays with HESS

Aims.Search for Very High Energy $\gamma$-ray emission in the Kookaburra complex through observations with the HESS array. Methods.Stereoscopic imaging of Cherenkov light emission of the $\gamma$-ray showers in the atmosphere is used for the reconstruction and selection of the events to search for $\gamma$-ray signals. Their spectrum is derived by a forward-folding maximum likelihood fit. Results.Two extended $\gamma$-ray sources with an angular (68%) radius of 3.3-3.4´ are discovered at high (>13$\sigma$) statistical significance: HESS J1420-607 and HESS J1418-609. They exhibit a flux above 1 TeV of ( $2.97 \pm 0.18_{\rm stat} \pm 0.60_{\rm sys}) \times 10^{-12}$ and ( $2.17 \pm 0.17_{\rm stat} \pm 0.43_{\rm sys}) \times 10^{-12}$ cm-2 s-1, respectively, and similar hard photon indices ~2.2. Multi-wavelength comparisons show spatial coincidence with the wings of the Kookaburra. Two pulsar wind nebulæ candidates, K3/PSR J1420-6048 and the Rabbit, lie on the edge of the HESS sources. Conclusions. The two new sources confirm the non-thermal nature of at least parts of the two radio wings which overlap with the $\gamma$-ray emission and establish their connection with the two X-ray pulsar wind nebulæ candidates. Given the large point spread function of EGRET, the unidentified source(s) 3EG J1420-6038/GeV J1417-6100 could possibly be related to either or both HESS sources. The most likely explanation for the Very High Energy $\gamma$-rays discovered by HESS is inverse Compton emission of accelerated electrons on the Cosmic Microwave Background near the two candidate pulsar wind nebulæ, K3/PSR J1420-6048 and the Rabbit. Two scenarios which could lead to the observed large (~10 pc) offset-nebula type morphologies are briefly discussed

3.9 day orbital modulation in the TeV γ-ray flux and spectrum from the X-ray binary LS 5039

Aims. LS 5039 is a High Mass X-ray Binary (HMXRB) comprising a compact object in an eccentric 3.9 day orbit around a massive O6.5V star. Observations at energies above 0.1 TeV (1011 eV) by the High Energy Stereoscopic System (HESS) in 2004 revealed that LS 5039 is a source of Very High Energy (VHE) $\gamma$-rays and hence, is able to accelerate particles to multi-TeV energies. Deeper observations by HESS were carried out in 2005 in an effort to probe further the high energy astrophysics taking place. In particular, we have searched for orbital modulation of the VHE $\gamma$-ray flux, which if detected, would yield new information about the complex variation in $\gamma$-ray absorption and production within X-ray binary systems. Methods. Observations at energies above 0.1 TeV (1011 eV), were carried out with the High Energy Stereoscopic System (HESS) of Cherenkov Telescopes in 2005. A timing analysis was performed on the dataset employing the Lomb-Scargle and Normalised Rayleigh statistics, and orbital phase-resolved energy spectra were obtained. Results. The timing analysis reveals a highly significant (post-trial chance probability <10-15) peak in the TeV emission periodogram at a frequency matching that of the 3.9 day orbital motion of the compact object around the massive stellar companion. This is the first time in $\gamma$-ray astronomy that orbital modulation has been observed, and periodicity clearly established using ground-based $\gamma$-ray detectors. The $\gamma$-ray emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the compact object. Around this epoch, there is also a hardening of the energy spectrum in the energy range between 0.2 TeV and a few TeV. Conclusions. The $\gamma$-ray flux vs. orbital phase profile suggests the presence of $\gamma$-ray absorption via pair production, which would imply that a large fraction of the $\gamma$-ray production region is situated within ~1 AU of the compact object. This source size constraint can be compared to the collimated outflows or jets observed in LS 5039 resolved down to scales of a few AU. The spectral hardening is however not explained exclusively by the absorption effect, indicating that other effects are present, perhaps related to the $\gamma$-ray production mechanism(s). If the $\gamma$-ray emission arises from accelerated electrons, the hardening may arise from variations with phase in the maximum electron energies, the dominant radiative mechanism, and/or the angular dependence in the inverse-Compton scattering cross-section. Overall, these results provide new insights into the competing $\gamma$-ray absorption and production processes in X-ray binaries

Detection of extended very-high-energy γ-ray emission towards the young stellar cluster Westerlund 2

Aims. Results from $\gamma$-ray observations by the HESS telescope array in the direction of the young stellar cluster Westerlund 2 are presented. Methods. Stereoscopic imaging of Cherenkov light emission of $\gamma$-ray induced showers in the atmosphere is used to study the celestial region around the massive Wolf-Rayet (WR) binary WR 20a. Spectral and positional analysis is performed using standard event reconstruction techniques and parameter cuts. Results. The detection of a new $\gamma$-ray source is reported from HESS observations in 2006. HESS J1023-575 is found to be coincident with the young stellar cluster Westerlund 2 in the well-known HII complex RCW 49. The source is detected with a statistical significance of more than 9$\sigma$, and shows extension beyond a point-like object within the HESS point-spread function. The differential $\gamma$-ray spectrum of the emission region is measured over approximately two orders of magnitude in flux. Conclusions.The spatial coincidence between HESS J1023-575 and the young open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires one to look into a variety of potential models to account for the observed very-high-energy (VHE) $\gamma$-ray emission. Considered emission scenarios include emission from the colliding wind zone of WR 20a, collective stellar winds from the extraordinary ensemble of hot and massive stars in the stellar cluster Westerlund 2, diffusive shock acceleration in the wind-blown bubble itself, and supersonic winds breaking out into the interstellar medium (ISM). The observed source extension argues against a single star origin of the observed VHE emission

Discovery of a point-like very-high-energy γ-ray source in Monoceros

Aims.The complex Monoceros Loop SNR/Rosette Nebula region contains several potential sources of very-high-energy (VHE) $\gamma$-ray emission and two as yet unidentified high-energy EGRET sources. Sensitive VHE observations are required to probe acceleration processes in this region. Methods.The HESS telescope array has been used to search for very high-energy $\gamma$-ray sources in this region. CO data from the NANTEN telescope were used to map the molecular clouds in the region, which could act as target material for $\gamma$-ray production via hadronic interactions. Results.We announce the discovery of a new $\gamma$-ray source, HESS J0632+057, located close to the rim of the Monoceros SNR. This source is unresolved by HESS and has no clear counterpart at other wavelengths but is possibly associated with the weak X-ray source 1RXS J063258.3+054857, the Be-star MWC 148 and/or the lower energy $\gamma$-ray source 3EG J0634+0521. No evidence for an associated molecular cloud was found in the CO data

Exploring a SNR/molecular cloud association within HESS J1745-303

Aims. HESS J1745-303 is an extended, unidentified VHE (very high energy) gamma-ray source discovered using HESS in the Galactic Plane Survey. Since no obvious counterpart has previously been found in longer-wavelength data, the processes that power the VHE emission are not well understood. Methods. Combining the latest VHE data with recent XMM-Newton observations and a variety of source catalogs and lower-energy survey data, we attempt to match (from an energetic and positional standpoint) the various parts of the emission of HESS J1745-303 with possible candidates. Results. Though no single counterpart is found to fully explain the VHE emission, we postulate that at least a fraction of the VHE source may be explained by a supernova-remnant/molecular-cloud association and/or a high-spin-down-flux pulsar

HESS upper limits for Kepler's supernova remnant

Aims. Observations of Kepler's supernova remnant (G4.5+6.8) with the HESS telescope array in 2004 and 2005 with a total live time of 13 h are presented. Methods. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. Results. No evidence for a very high energy (VHE: >100 GeV) gamma-ray signal from the direction of the remnant is found. An upper limit (99% confidence level) on the energy flux in the range $230~{\rm GeV}{-}12.8~{\rm TeV}$ of 8.6 $\times$ $10^{-13}~{\rm erg}~{\rm cm}^{-2}~{\rm s}^{-1}$ is obtained. Conclusions. In the context of an existing theoretical model for the remnant, the lack of a detectable gamma-ray flux implies a distance of at least $6.4~{\rm kpc}$. A corresponding upper limit for the density of the ambient matter of $0.7~{\rm cm}^{-3}$ is derived. With this distance limit, and assuming a spectral index $\Gamma = 2$, the total energy in accelerated protons is limited to $E_{\rm p} < 8.6$ $\times$ $10^{49}~{\rm erg}$. In the synchrotron/inverse Compton framework, extrapolating the power law measured by RXTE between 10 and $20~{\rm keV}$ down in energy, the predicted gamma-ray flux from inverse Compton scattering is below the measured upper limit for magnetic field values greater than $52~\mu {\rm G}$

Simultaneous HESS and Chandra observations of Sagitarius A(star) during an X-ray flare

The rapidly varying (~10 min timescale) non-thermal X-ray emission observed from Sgr A$^{\star}$ implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV $\gamma$-ray source HESS J1745-290 is coincident with Sgr A$^{\star}$ and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint HESS/Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of $\approx$9 increase in the X-ray flux of Sgr A$^{\star}$, no evidence is found for an increase in the TeV $\gamma$-ray flux from this region. We find that an increase in the $\gamma$-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the $\gamma$-ray emission is produced within ~1014 cm (~100 $\,R_{\rm S}$) of the supermassive black hole