953 research outputs found
Constraints on a strong X-ray flare in the Seyfert galaxy MCG-6-30-15
We discuss implications of a strong flare event observed in the Seyfert
galaxy MCG-6-30-15 assuming that the emission is due to localized magnetic
reconnection. We conduct detailed radiative transfer modeling of the
reprocessed radiation for a primary source that is elevated above the disk. The
model includes relativistic effects and Keplerian motion around the black hole.
We show that for such a model setup the observed time-modulation must be
intrinsic to the primary source. Using a simple analytical model we then
investigate time delays between hard and soft X-rays during the flare. The
model considers an intrinsic delay between primary and reprocessed radiation,
which measures the geometrical distance of the flare source to the reprocessing
sites. The observed time delays are well reproduced if one assumes that the
reprocessing happens in magnetically confined, cold clouds.Comment: 4 pages, 2 figures, proceedings of a talk given at the symposium 238
at the IAU General Assembly 200
Modeling the X-ray fractional variability spectrum of Active Galactic Nuclei using multiple flares
Using Monte-Carlo simulations of X-ray flare distributions across the
accretion disk of active galactic nuclei (AGN), we obtain modeling results for
the energy-dependent fractional variability amplitude. Referring to previous
results of this model, we illustrate the relation between the shape of the
point-to-point fractional variability spectrum, F_pp, and the time-integrated
spectral energy distribution, F_E. The results confirm that the spectral shape
and variability of the iron Kalpha line are dominated by the flares closest to
the disk center.Comment: 2 pages, 1 figure, conference proceedings of the AGN meeting held in
October 2006 in Xi'an, China. To appear in "The Central Engine of Active
Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP
Constraining global parameters of accreting black holes by modeling magnetic flares
We present modeling results for the reprocessed radiation expected from
magnetic flares above AGN accretion disks. Relativistic corrections for the
orbital motion of the flare and for the curved space-time in the vicinity of
the black hole are taken into account. We investigate the local emission
spectra, as seen in a frame co-orbiting with the disk, and the observed spectra
at infinity. We investigate long-term flares at different orbital phases and
short-term flares for various global parameters of the accreting black hole.
Particular emphasis is put on the relation between the iron Kalpha line and the
Compton hump as these two features can be simultaneously observed by the Suzaku
satellite and later by Simbol-X.Comment: 4 pages, 1 figure, 1 table, proceedings for a poster at the
international conference "The Extreme Universe in the Suzaku Era" held in
Kyoto, Japan, December 4-8, 200
Variation of the X-ray non-thermal emission in the Arches cloud
The origin of the iron fluorescent line at 6.4 keV from an extended region
surrounding the Arches cluster is debated and the non-variability of this
emission up to 2009 has favored the low-energy cosmic-ray origin over a
possible irradiation by hard X-rays. By probing the variability of the Arches
cloud non-thermal emission in the most recent years, including a deep
observation in 2012, we intend to discriminate between the two competing
scenarios. We perform a spectral fit of XMM-Newton observations collected from
2000 to 2013 in order to build the Arches cloud lightcurve corresponding to
both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a
30% flux drop in 2012, detected with more than 4 sigma significance for both
components. This implies that a large fraction of the studied non-thermal
emission is due to the reflection of an X-ray transient source.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Investment incentives in auctions: An experiment
We experimentally analyze first- and second-price procurement auctions where one bidder can achieve a comparative cost advantage by investment prior to the auction. Theory predicts that bidders invest more often prior to second-price auctions than prior to first-price auctions, which is clearly confirmed by our experimental data. Bidding in the auction (after investment) is more aggressive than the equilibrium prediction in both auction formats. Introduction Different market institutions provide different incentives for firms to engage in activities that affect their competitive positions. For example, prior to a procurement auction investments can be made either to reduce a firm’s own cost of production, or even to raise the cost of possible competitors. Empirical evidence indicates that companies make use of this possibility extensively.1Thus, both auction rules and investment incentives have to be accounted for when it comes to comparing revenue and efficiency of selling (or buying) institutions. A number of papers theoretically analyze investment incentives in procurement auctions. Most of them assume that investment decisions are not observable prior to a competition.2Then, a typical finding is that investment is symmetric so that revenue equivalence between market institutions is preserved also in a model that allows for investment. This is not necessarily true if investment is observable. If firms strategically react to the decisions made at the investment stage, it is not immediately clear whether ex ante symmetry implies symmetric investment
Development of a Control-Oriented Ignition Delay Model for GCI Combustion
Increasingly stringent pollutant emission limits and CO2 reduction policies are forcing the automotive industry toward cleaner and decarbonized mobility. The goal is to achieve carbon neutrality within 2050 and limit global warming to 2 degrees C (possibly 1.5 degrees C) with respect to pre-industrial levels as stated in both the European Green Deal and the Paris Agreement and further reiterated at the COP26. With the aim of simultaneously reducing both pollutants and CO2 emissions, a large amount of research is currently carried out on low-temperature highly efficient combustions (LTC). Among these advanced combustions, one of the most promising is Gasoline Compression Ignition (GCI), based on the spontaneous ignition of a gasoline-like fuel. Nevertheless, despite GCI proving to be effective in reducing both pollutants and CO2 emissions, GCI combustion controllability represents the main challenge that hinders the diffusion of this methodology for transportation. Several works in the literature demonstrated that to properly control GCI combustion, a multiple injections strategy is needed. The rise of pressure and temperature generated by the spontaneous ignition of small amounts of early-injected fuel reduces the ignition delay of the following main injection, responsible for the torque production of the engine. Since the combustion of the pre-injections is chemically driven, the ignition delay might be strongly affected by a slight variation in the engine control parameters and, consequently, lead to misfire or knocking. The goal of this work was to develop a control-oriented ignition delay model suitable to improve the GCI combustion stability through the proper management of the pilot injections. After a thorough analysis of the quantities affecting the ignition delay, this quantity was modeled as a function of both a thermodynamic and a chemical-physical index. The comparison between the measured and modeled ignition delay shows an accuracy compatible with the requirements for control purposes (the average root mean squared error between the measured and estimated start of combustion is close to 1.3 deg), over a wide range of operating conditions. As a result, the presented approach proved to be appropriate for the development of a model-based feed-forward contribution for a closed-loop combustion control strategy
Long-term variability of AGN at hard X-rays
Variability at all observed wavelengths is a distinctive property of AGN.
Hard X-rays provide us with a view of the innermost regions of AGN, mostly
unbiased by absorption along the line of sight. Swift/BAT offers the unique
opportunity to follow, on time scales of days to years and with a regular
sampling, the 14-195 keV emission of the largest AGN sample available up to
date for this kind of investigation. We study the amplitude of the variations,
and their dependence on sub-class and on energy, for a sample of 110 radio
quiet and radio loud AGN selected from the BAT 58-month survey. About 80% of
the AGN in the sample are found to exhibit significant variability on months to
years time scales, radio loud sources being the most variable. The amplitude of
the variations and their energy dependence are incompatible with variability
being driven at hard X-rays by changes of the absorption column density. In
general, the variations in the 14-24 and 35-100 keV bands are well correlated,
suggesting a common origin of the variability across the BAT energy band.
However, radio quiet AGN display on average 10% larger variations at 14-24 keV
than at 35-100 keV and a softer-when-brighter behavior for most of the Seyfert
galaxies with detectable spectral variability on month time scale. In addition,
sources with harder spectra are found to be more variable than softer ones.
These properties are generally consistent with a variable power law continuum,
in flux and shape, pivoting at energies >~ 50 keV, to which a constant
reflection component is superposed. When the same time scales are considered,
the timing properties of AGN at hard X-rays are comparable to those at lower
energies, with at least some of the differences possibly ascribable to
components contributing differently in the two energy domains (e.g.,
reflection, absorption).Comment: 17 pages, 11 figures, accepted for publication in A&
Zero-Dimensional Model for Dynamic Behavior of Engineered Rubber in Automotive Applications
Abstract This paper presents a zero-dimensional model for the simulation of the mechanical behavior of automotive engineered rubber components, such as flexible couplings. The objective is to develop a real-time-capable model, able to simulate the behavior of a driveline containing elastomer components: the engineered rubber model has to correlate stretch to stress, the mechanical behavior being represented by means of a hysteresis cycle. The study presents the implementation of Maxwell and Voigt models, showing their limits in the representation of the material behavior: elastomers present a nonlinear response in the relationship stress-strain. A combination of Maxwell and Voigt models, with stiffness and damping variable according to the stress and strain rate, to represent nonlinear material responses, is coupled to a relaxation model, in order to represent the Mullins effect (the rubber mechanical behavior also depends on load history). Experimental tests have been carried out with different pre-load settings, stress amplitudes and stress frequencies. Tests results have been used to calibrate the parameters defining the simulation model, comparing the model outputs to experimental data: an optimization algorithm has been applied, with the aim of minimizing the results discrepancy with respect to experimental results. The optimization tool has been also used to reduce the number of parameters defining the model, in order to simplify the required computational power, avoiding at the same time over-parametrization. In the second section of the paper, the model is used for the simulation of a different rubber component, whose behavior is identified using quasi-static load ramps, frequency and amplitude sweeps, steps and random cycles. An alternative model formulation, minimizing the degrees of freedom is then applied to the new dataset. The model parameters are separately optimized using different tests, in order to capture the specific mechanical behavior. Finally, the identified parameters are used to simulate the elastomer response in random tests, comparing the results to experimental data, to evaluate the simulation quality in terms of RMSE
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