Isobutene is an important intermediate in the pyrolysis and oxidation of higher-order branched alkanes, and it is also a component of commercial gasolines. To better understand its combustion characteristics, a series of ignition delay time (IDT) and laminar flame speed (LFS) measurements have been performed. In addition, flow reactor speciation data recorded for the pyrolysis and oxidation of isobutene is also reported. Predictions of an updated kinetic model described herein are compared with each of these data sets, as well as with existing jet-stirred reactor (JSR) species measurements.IDTs of isobutene oxidation were measured in four different shock tubes and in two rapid compression machines (RCMs) under conditions of relevance to practical combustors. The combination of shock tube and RCM data greatly expands the range of available validation data for isobutene oxidation models to pressures of 50 atm and temperatures in the range 666-1715 K. Isobutene flame speeds were measured experimentally at 1 atm and at unburned gas temperatures of 298-398 K over a wide range of equivalence ratios. For the flame speed results, there was good agreement between different facilities and the current model in the fuel-rich region.Ab initio chemical kinetics calculations were carried out to calculate rate constants for important reactions such as H-atom abstraction by hydroxyl and hydroperoxyl radicals and the decomposition of 2-methylallyl radicals.A comprehensive chemical kinetic mechanism has been developed to describe the combustion of isobutene and is validated by comparison to the presently considered experimental measurements. Important reactions, highlighted via flux and sensitivity analyses, include: (a) hydrogen atom abstraction from isobutene by hydroxyl and hydroperoxyl radicals, and molecular oxygen; (b) radical-radical recombination reactions, including 2-methylallyl radical self-recombination, the recombination of 2-methylallyl radicals with hydroperoxyl radicals; and the recombination of 2-methylallyl radicals with methyl radicals; (c) addition reactions, including hydrogen atom and 2 hydroxyl radical addition to isobutene; and (d) 2-methylallyl radical decomposition reactions. The current mechanism accurately predicts the IDT and LFS measurements presented in this study, as well as the JSR and flow reactor speciation data already available in the literature.The differences in low-temperature chemistry between alkanes and alkenes are also highlighted in this work. In normal alkanes, the fuel radical Ṙ adds to molecular oxygen forming alkylperoxyl (RȮ 2 ) radicals followed by isomerization and chain branching reactions which promote low-temperature fuel reactivity. However, in alkenes, because of the relatively shallow well (~20 kcal mol -1 ) for RȮ 2 formation compared to ~35 kcal mol -1 in alkanes, the Ṙ + O 2 ⇌ RȮ 2 equilibrium lies more to the left favoring Ṙ + O 2 rather than RȮ 2 radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity fo...

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Polarisation is a powerful tool to study compact objects where high-energy particle acceleration processes and magnetic fields play a major role. Joining optical and high-energy polarisation data, whilst challenging, should develop a better understanding of the source acceleration processes and magnetic fields. A recent study of the polarisation from the Crab nebula and pulsar in hard Xray with the International Gamma-Ray Astrophysics Laboratory satellite (INTEGRAL) and in optical with the Galway Astronomical Stokes Polarimeter (GASP) indicated similar variations in the phase average polarisation signal observed between 2005 and 2012. This result was tentatively proposed to evoke that magnetic reconnection was at work in the inner part of the Crab nebula, a physical process suggested to explain the strong flaring activities observed in the past years with the high energy gamma-ray missions Agile and Fermi. We describe here new and higher quality GASP optical data obtained in December 2015 at the WHT telescope at La Palma during nearly simultaneous hard X-rays INTEGRAL/IBIS observations. Our result points to a polarisation angle value higher than that perviously measured, confirming that the inner core of the Crab nebula is variable and cannot be considered as a polarimetric standard. PoS(INTEGRAL2016)038Hard X-rays and optical polarisation of the Crab nebula and pulsar Christian Gouiffès

The Galway Astronomical Stokes Polarimeter (GASP) is a high time resolution, full-Stokes imaging polarimeter which utilises a Fresnel rhomb prism as a beam-splitter and quarter wave retarder. The ability to perform optical photometry and polarimetry at high frame rates enables more detailed studies of a multitude of compact objects including; optical pulsars, magnetars and Active Galactic Nuclei (AGN). We present a brief discussion of the instruments’ current capabilities and hardware design. GASP is currently configured with two Electron Multiplying Charge-Coupled Devices (EMCCDs), which offer sub-millisecond frame readout speeds over a defined region of interest on the sensor. We will report results from an observational campaign at the William Herschel Telescope (WHT) in December, 2015. During this campaign GASP was used to study the Crab Pulsar, V404 Cygni and polarisation standards. As a subset of our analysis we have identified a significant contribution from the Interstellar Medium (ISM) and the interaction of a varying polarised source with the ISM, the subsequent conversion of linearly polarised light to circular, and its dependence on the angle of the source emission electric field orientation. Further to the presentation of results we will discuss future observational work, which is planned for September 2017 and subsequent improvements to increase the temporal resolution of the detectors.

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