5,722 research outputs found
An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies
We present an XMM-Newton observation of the bright, narrow-line, ultrasoft
Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved,
showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless
excess of emission at lower energies. The spectrum can be adequately
parameterised using a simple double power-law model. The source is strongly
variable over the course of the observation but shows only weak spectral
variability, with the fractional variability amplitude remaining approximately
constant over more than a decade in energy. The curved continuum shape and weak
spectral variability are discussed in terms of various physical models for the
soft X-ray excess emission, including reflection off the surface of an ionised
accretion disc, inverse-Compton scattering of soft disc photons by thermal
electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal
distribution. We emphasise the possibility that the strong soft excess may be
produced by dissipation of accretion energy in the hot, upper atmosphere of the
putative accretion disc.Comment: 9 pages, accepted for publication in MNRA
DFT-D2 simulations of water adsorption and dissociation on the low-index surfaces of mackinawite (FeS)
The adsorption and dissociation of water on mackinawite (layered FeS) surfaces were studied using dispersion-corrected density functional theory (DFT-D2) calculations. The catalytically active sites for H2O and its dissociated products on the FeS {001}, {011}, {100}, and {111} surfaces were determined, and the reaction energetics and kinetics of water dissociation were calculated using the climbing image nudged elastic band technique. Water and its dissociation products are shown to adsorb more strongly onto the least stable FeS{111} surface, which presents low-coordinated cations in the surface, and weakest onto the most stable FeS{001} surface. The adsorption energies decrease in the order FeS{111} > FeS{100} > FeS{011} > FeS{001}. Consistent with the superior reactivity of the FeS{111} surface towards water and its dissociation products, our calculated thermochemical energies and activation barriers suggest that the water dissociation reaction will take place preferentially on the FeS nanoparticle surface with the {111} orientation. These findings improve our understanding of how the different FeS surface structures and the relative stabilities dictate their reactivity towards water adsorption and dissociation
XMM-Newton discovery of a sharp spectral feature at ~7 keV in the Narrow-Line Seyfert 1 galaxy 1H 0707-495
We report the first detection of a sharp spectral feature in a Narrow-Line
Seyfert 1 galaxy. Using XMM-Newton we have observed 1H0707-495 and find a drop
in flux by a factor of more than 2 at a rest-frame energy of ~7 keV without any
detectable narrow Fe K alpha line emission. The energy of this feature suggests
a connection with the neutral iron K photoelectric edge, but the lack of any
obvious absorption in the spectrum at lower energies makes the interpretation
challenging. We explore two alternative explanations for this unusual spectral
feature: (i) partial covering absorption by clouds of neutral material and (ii)
ionised disc reflection with lines and edges from different ionisation stages
of iron blurred together by relativistic effects. We note that both models
require an iron overabundance to explain the depth of the feature. The X-ray
light curve shows strong and rapid variability, changing by a factor of four
during the observation. The source displays modest spectral variability which
is uncorrelated with flux.Comment: 5 pages incl. 6 figures, accepted for publication in MNRA
Quantum limits to center-of-mass measurements
We discuss the issue of measuring the mean position (center-of-mass) of a
group of bosonic or fermionic quantum particles, including particle number
fluctuations. We introduce a standard quantum limit for these measurements at
ultra-low temperatures, and discuss this limit in the context of both photons
and ultra-cold atoms. In the case of fermions, we present evidence that the
Pauli exclusion principle has a strongly beneficial effect, giving rise to a
1/N scaling in the position standard-deviation -- as opposed to a
scaling for bosons. The difference between the actual mean-position fluctuation
and this limit is evidence for quantum wave-packet spreading in the
center-of-mass. This macroscopic quantum effect cannot be readily observed for
non-interacting particles, due to classical pulse broadening. For this reason,
we also study the evolution of photonic and matter-wave solitons, where
classical dispersion is suppressed. In the photonic case, we show that the
intrinsic quantum diffusion of the mean position can contribute significantly
to uncertainties in soliton pulse arrival times. We also discuss ways in which
the relatively long lifetimes of attractive bosons in matter-wave solitons may
be used to demonstrate quantum interference between massive objects composed of
thousands of particles.Comment: 12 pages, 6 figures. Submitted to PRA. Revised to include more
references as well as a discussion of fermionic center-of-mas
Variable iron-line emission near the black hole of Markarian 766
We investigate the link between ionised Fe X-ray line emission and continuum
emission in the bright nearby AGN, Mrk 766. A new long (433 ks) XMM-Newton
observation is analysed, together with archival data from 2000 and 2001. The
contribution from ionised line emission is measured and its time variations on
short (5-20 ks) timescales are correlated with the continuum emission. The
ionised line flux is found to be highly variable and to be strongly correlated
with the continuum flux, demonstrating an origin for the ionised line emission
that is co-located with the continuum emission. Most likely the emission is
ionised reflection from the accretion disc within a few A.U. of the central
black hole, and its detection marks the first time that such an origin has been
identified other than by fitting to spectral line profiles. Future observations
may be able to measure a time lag and hence achieve reverberation mapping of
AGN at X-ray energies.Comment: Accepted for publication, Astronomy and Astrophysics letter
A 100ks XMM-Newton view of the Seyfert 1.8 ESO113-G010. I. Discovery of large X-ray variability and study of the FeKalpha line complex
(Abridged) We present here a long (100ks) XMM-Newton follow-up of the Seyfert
1.8 galaxy ESO113-G010 performed in November 2005, in order to study over a
longer time-scale its main X-ray properties. The source was found in a
higher/softer time-averaged flux state, and timing analysis of this source
reveals strong, rapid variability. The Power Spectral Density (PSD) analysis
indicates (at 95% c.l.) a break at 3.7 x 10^-4 Hz. This cut-off frequency is
comparable to those measured in some other rapidly-variable Seyferts, such as
MCG-6-30-15 and NGC4051. From the mass-luminosity-time-scale, we infer that
M_BH ranges from 4 x 10^6 - 10^7 M_odot and the source is accreting at or close
to the Eddington rate (or even higher). The existing data cannot distinguish
between spectral pivoting of the continuum and a two-component origin for the
spectral softening, primarily because the data do not span a broad enough flux
range. In the case of the two-component model, the fractional offsets measured
in the flux-flux plots increase significantly toward higher energies (similar
to what is observed in MCG-6-30-15) as expected if there exists a constant
reflection component. Contrary to May 2001, no significant highly redshifted
emission line is observed (which might be related to the source flux level),
while two narrow emission lines at about 6.5keV and 7keV are observed. The S/N
is not high enough to establish if the lines are variable or constant. As
already suggested by the 2001 observation, no significant constant narrow
6.4keV FeK line (EW~32eV) is observed, hence excluding any dominant emission
from distant cold matter such as a torus in this Seyfert type 1.8 galaxy.Comment: Accepted for publication in A&A, 10 pages, 11 figures, 2 table
Matching of internal combustion engine characteristics for continuously variable transmissions
This work proposes to match the engine characteristics to the requirements of the Continuously Variable Transmission [CVT] powertrain. The normal process is to pair the transmission to the engine and modify its calibration without considering the full potential to modify the engine. On the one hand continuously variable transmissions offer the possibility to operate the engine closer to its best efficiency. They benefit from the high versatility of the effective speed ratio between the wheel and the engine to match a driver requested power. On the other hand, this concept demands slightly different qualities from the gasoline or diesel engine. For instance, a torque margin is necessary in most cases to allow for engine speed controllability and transients often involve speed and torque together. The necessity for an appropriate engine matching approach to the CVT powertrain is justified in this thesis and supported by a survey of the current engineering trends with particular emphasis on CVT prospects. The trends towards a more integrated powertrain control system are highlighted, as well as the requirements on the engine behaviour itself. Two separate research axes are taken to investigate low Brake Specific Fuel Consumption [BSFC] in the low speed region and torque transient respectively for a large V8 gasoline engine and a turbocharged diesel V6 engine. This work is based on suitable simulation environments established for both engines in the powertrain. The modelling exercises are aimed at supplying appropriate models that can be validated against experimental data. The simulation platforms developed then allow the investigation of CVT powertrain biased engine characteristics. The V8 engine model in particular benefited from engine and vehicle dynamometer data to validate the model behaviour and the accuracy of the prediction. It benefited from the parallel work conducted on the Electrically Assisted Infinitely Variable Transmission [EASIVT] project in Cranfield University. The EASIVT vehicle is a parallel mild hybrid aimed at demonstrating the combined fuel economy benefits of a CVT technology and hybridisation. From the CVT powertrain requirements for fuel economy, BSFC operation can be further promoted in the low speed region if Noise Vibration and Harshness [NVH] counter-measures are developed. A study of the combustion torque oscillations at the crankshaft led to the elaboration of an Active Vibration Control [AVC] strategy for the hybrid Integrated Motor Generator [IMG]. Successful implementation of the strategy in both simulation and in-vehicle helped quantify the benefits and short comings of engine operation for best fuel economy. The development in parallel of the hybrid control functions for torque assist and regenerative braking made it possible to implement the low speed AVC in the vehicle without a driveability penalty. The V6 TDI model yielded a realistic and representative simulation for the transient torque response improvement research to be undertaken. For that purpose, the model was tuned against full-load data and the air path control sub-systems were designed and calibrated similarly to a real application. The model was able to highlight the turbocharger lag issue associated with a large combined speed and torque transient inevitable in the fuel economy biased CVT powertrain. This study proposes a Manifold Air Injection [MAI] system in the intake of the engine to help breathing when the VGT operating conditions cannot be shifted rapidly enough for a manoeuvre. The system design constraints were analysed and a suitable strategy was elaborated and calibrated. A sensitivity analysis was also conducted to demonstrate the influence of the MAI design and control variables on the engine performance in the CVT powertrain In conclusion, the benefits of the engine characteristic matching were highlighted in both cases. A review of the work achieved is available in the last chapter, including prospects for further improvements and investigations. The ideal engine characteristics for gasoline and diesel engine technologies integrated in a CVT powertrain are derived from the experience gathered in the research and the results obtained from the tests in low speed operation and transient torque control respectively for the gasoline and the diesel engines. The engine characteristics can be altered toward a better match with a CVT by the use of specific hardware and control strategy. This work recommends that a direct injected, variable valve actuated gasoline engine provides the ideal starting point for low fuel consumption powertrain. When integrated within a mild hybrid CVT powertrain, the full benefits are obtained with the use of low speed operation and AVC. If no electrical machine is available to torque assist the engine, then existing supercharging concepts for a downsized engine can be applied. Diesel engines can also be downsized because of their high torque density. Increased turbocharging boost levels allow steady state torque levels to be maintained in the downsizing process. The CVT powertrain can optimise the fuel consumption and emission levels by appropriate selection of the engine steady state operating points. The torque response lag then becomes critical for the CVT to control the engine speed. This can be improved by the use of Manifold air Injection to assist the turbocharger.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Control of a hybrid electric vehicle with predictive journey estimation
Battery energy management plays a crucial role in fuel economy improvement of charge-sustaining parallel hybrid electric vehicles. Currently available control strategies consider battery state of charge (SOC) and driverâs request through the pedal input in decision-making. This method does not achieve an optimal performance for saving fuel or maintaining appropriate SOC level, especially during the operation in extreme driving conditions or hilly terrain. The objective of this thesis is to develop a control algorithm using forthcoming traffic condition and road elevation, which could be fed from navigation systems. This would enable the controller to predict potential of regenerative charging to capture cost-free energy and intentionally depleting battery energy to assist an engine at high power demand. The starting point for this research is the modelling of a small sport-utility vehicle by the analysis of the vehicles currently available in the market. The result of the analysis is used in order to establish a generic mild hybrid powertrain model, which is subsequently examined to compare the performance of controllers. A baseline is established with a conventional powertrain equipped with a spark ignition direct injection engine and a continuously variable transmission. Hybridisation of this vehicle with an integrated starter alternator and a traditional rule-based control strategy is presented. Parameter optimisation in four standard driving cycles is explained, followed by a detailed energy flow analysis. An additional potential improvement is presented by dynamic programming (DP), which shows a benefit of a predictive control. Based on these results, a predictive control algorithm using fuzzy logic is introduced. The main tools of the controller design are the DP, adaptive-network-based fuzzy inference system with subtractive clustering and design of experiment. Using a quasi-static backward simulation model, the performance of the controller is compared with the result from the instantaneous control and the DP. The focus is fuel saving and SOC control at the end of journeys, especially in aggressive driving conditions and a hilly road. The controller shows a good potential to improve fuel economy and tight SOC control in long journey and hilly terrain. Fuel economy improvement and SOC correction are close to the optimal solution by the DP, especially in long trips on steep road where there is a large gap between the baseline controller and the DP. However, there is little benefit in short trips and flat road. It is caused by the low improvement margin of the mild hybrid powertrain and the limited future journey information. To provide a further step to implementation, a software-in-the-loop simulation model is developed. A fully dynamic model of the powertrain and the control algorithm are implemented in AMESim-Simulink co-simulation environment. This shows small deterioration of the control performance by driverâs pedal action, powertrain dynamics and limited computational precision on the controller performance.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Age-Depth Stratigraphy of Pine Island Glacier Inferred from Airborne Radar and Ice-Core Chronology
Understanding the contribution of the West Antarctic Ice Sheet (WAIS) to past and future sea level has been a major scientific priority over the last three decades. In recent years, observed thinning and iceâflow acceleration of the marineâbased Pine Island Glacier has highlighted that understanding dynamic changes is critical to predicting the longâterm stability of the WAIS. However, relatively little is known about the evolution of the catchment during the Holocene. Internal Reflecting Horizons (IRHs) provide a cumulative record of accumulation, basal melt and ice dynamics that, if dated, can be used to constrain iceâflow models. Here, we use airborne radars to trace four spatiallyâextensive IRHs deposited in the late Quaternary across the Pine Island Glacier catchment. We use the WAIS Divide iceâcore chronology to assign ages to three IRHs: 4.72 ± 0.28, 6.94 ± 0.31, and 16.50 ± 0.79 ka. We use a 1âD model, constrained by observational and modelled accumulation rates, to produce an independent validation of our iceâcoreâderived ages and provide an age estimate for our shallowest IRH (2.31â2.92 ka). We find that our upper three IRHs correspond to three large peaks in sulphate concentrations in the WAIS Divide iceâcore record and hypothesise that the origin of these spatiallyâextensive IRHs is from past volcanic activity. The clear correspondence between our IRHs and the ones previously identified over the Weddell Sea Sector, altogether representing âŒ20% of the WAIS, indicates that a unique set of stratigraphic markers spanning the Holocene exists over a large part of West Antarctica
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