Continuum Properties Research Articles

Polarization properties of bremsstrahlung continuum and line emission of multicharged ions in a micropinch

The polarization properties of X-ray emission from micropinch discharge plasma in the spectral range corresponding to the K-lines of iron were studied experimentally. It is shown that emissions of different nature differ in the preferred orientation of polarization. The mechanisms responsible for the emission polarization are discussed.

UNCOVERING THE PUTATIVE B-STAR BINARY COMPANION OF THE SN 1993J PROGENITOR

The Type IIb supernova (SN) 1993J is one of only a few stripped-envelope supernovae with a progenitor star identified in pre-explosion images. SN IIb models typically invoke H envelope stripping by mass transfer in a binary system. For the case of SN 1993J, the models suggest that the companion grew to 22 M_solar and became a source of ultraviolet (UV) excess. Located in M81, at a distance of only 3.6 Mpc, SN 1993J offers one of the best opportunities to detect the putative companion and test the progenitor model. Previously published near-UV spectra in 2004 showed evidence for absorption lines consistent with a hot (B2 Ia) star, but the field was crowded and dominated by flux from the SN. Here we present Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) and Wide-Field Camera 3 (WFC3) observations of SN 1993J from 2012, at which point the flux from the SN had faded sufficiently to potentially measure the UV continuum properties from the putative companion. The resulting UV spectrum is consistent with contributions from both a hot B star and the SN, although we cannot rule out line-of-sight coincidences.

Optical and near-infrared spectroscopy of the black hole GX 339−4 – II. The spectroscopic content in the low/hard and high/soft states★

As a complement to our optical and near-infrared study of the continuum properties of GX 339-4 in the two hard and one soft state observations made by the ESO/VLT FORS2 and ISAAC in early 2010, we report here on the results of our spectral line analysis for the same observations. In the soft state, the presence of strong Balmer, Paschen and Brackett emission lines points to the optical and near-infrared spectra stemming from the irradiated chromosphere of the optically thick and geometrically thin accretion disc. Most of these HI features are still detected in emission in both hard states but are veiled by the compact jets continuum. We also confirm the presence of a broad Hbeta absorption feature, prominent in the soft state and shallower in the first hard state, which we argue forms in the deep layers of the optically thick accretion disc. However, this trough is absent in the second hard state, a likely consequence of the formation of a geometrically thick extended envelope that arises above the disc plane and eventually enshrouds the region where the Hbeta absorption feature forms. We detect this envelope through the presence of a broad Pabeta emission line, which is constant during the first hard state but correlates with the underlying continuum during the second hard state, pointing to changing physical properties. We consider that this behaviour may be consistent with the launch of a thermally-driven accretion disc wind during the second hard state.

Quantifying correlations between galaxy emission lines and stellar continua using a PCA-based technique

AbstractWe analyse the correlations between continuum properties and emission line equivalent widths of star-forming and narrow-line active galaxies from SDSS. We show that emission line strengths can be predicted reasonably well from PCA coefficients of the stellar continuum using local multiple linear regression. Since upcoming sky surveys will make broadband observations only, theoretical modelling of spectra will be essential to estimate physical properties of galaxies. Combined with stellar population synthesis models, our technique will help generate more accurate model spectra and mock catalogues of galaxies to be used to fit data from new surveys. We also show that, by combining PCA coefficients from the pure continuum and the emission lines, a plausible distinction can be made between weak AGNs and quiescent star-forming galaxies. Our method uses a support vector machine, and allows a more refined separation of active and star-forming galaxies than the empirical curve of Kauffmann et al. (2003).

The Galactic plane at faint X-ray fluxes – II. Stacked X-ray spectra of a sample of serendipitous XMM–Newton sources

We have investigated the X-ray spectral properties of a sample of 138 X-ray sources detected serendipitously in $XMM-Newton$ observations of the Galactic plane, at an intermediate to faint flux level. We divide our sample into 5 subgroups according to the spectral hardness of the sources, and stack (i.e. co-add) the individual source spectra within each subgroup. As expected these stacked spectra show a softening trend from the hardest to the softest subgroups, which is reflected in the inferred line-of-sight column density. The spectra of the three hardest subgroups are characterized by a hard continuum plus superimpose Fe-line emission in the 6--7 keV bandpass. The average equivalent width (EW) of the 6.7-keV He-like Fe-K$\alpha$ line is 170$^{+35}_{-32}$ eV, whereas the 6.4-keV Fe-K fluorescence line from neutral iron and the 6.9-keV H-like Fe-Ly$\alpha$ line have EWs of 89$^{+26}_{-25}$ eV and 81$^{+30}_{-29}$ eV respectively, i.e. roughly half that of the 6.7-keV line. The remaining subgroups exhibit soft thermal spectra. Virtually all of the spectrally-soft X-ray sources can be associated with relatively nearby coronally-active late-type stars, which are evident as bright near-infrared (NIR) objects within the X-ray error circles. On a similar basis only a minority of the spectrally-hard X-ray sources have likely NIR identifications. The average continuum and Fe-line properties of the spectrally-hard sources are consistent with those of magnetic cataclysmic variables but the direct identification of large numbers of such systems in Galactic X-ray surveys, probing intermediate to faint flux levels, remains challenging.

Transitions Between Rotational Nuclear Few-Body States in the Continuum

Continuum to continuum transitions are used to investigate the continuum properties of 8Be. The dependence of the cross section on the α−α potential and the rotational character of the 8Be energy spectrum are investigated.

The optical emission lines of type 1 X-ray bright Active Galactic Nuclei

A strong X-ray emission is one of the defining signatures of nuclear activity in galaxies. According to the Unified Model for Active Galactic Nuclei (AGN), both the X-ray radiation and the prominent broad emission lines, characterizing the optical and UV spectra of Type 1 AGNs, are originated in the innermost regions of the sources, close to the Super Massive Black Holes (SMBH), which power the central engine. Since the emission is concentrated in a very compact region (with typical size r⩽0.1 pc) and it is not possible to obtain resolved images of the source, spectroscopic studies of this radiation represent the only valuable key to constrain the physical properties of matter and its structure in the center of active galaxies. Based on previous studies on the physics of the Broad Line Region (BLR) and on the X-ray spectra of broad (FWHMHβ⩾2000kms−1) and narrow line (1000 km s−1 ⩽FWHMHβ⩽2000kms−1) emitting objects, it has been observed that the kinematic and ionization properties of matter close to the SMBHs are related together, and, in particular, that ionization is higher in narrow line sources. Here we report on the study of the optical and X-ray spectra of a sample of Type 1 AGNs, selected from the Sloan Digital Sky Survey (SDSS) database, within an upper redshift limit of z=0.35, and detected at X-ray energies. We present analysis of the broad emission line fluxes and profiles, as well as the properties of the X-ray continuum and Fe Kα emission and we use these parameters to assess the consistency of our current AGN understanding.

An effective Cosserat continuum model for waves in a material with microstructure

The effective acoustic properties of a medium can be tailored by appropriately designing its microstructural components. The effective properties of a material may be generally defined as those that describe the limiting case where the wavelength of propagation is infinite compared to the characteristic scale of the microstructure. Generally, the limit of vanishingly small microstructural scale in an elastic medium results in an effective medium that is again elastic. In this contribution, we give an example for which the above limit results not in an effective elastic medium, but rather, an effective Cosserat medium. We briefly review the properties of Cosserat continua, provide a basic derivation of their fundamental equations of motion, and interpret the continuum properties in terms of the microstructural elements.

Evaluation of representative elementary volume for a vuggy carbonate rock—Part: Porosity, permeability, and dispersivity

Abstract This work highlights variation in petrophysical properties of a macroporous vugular carbonate rock. The macroporosity consists of vugs and moldic pores with sizes ranging from mm to cm scale. Carbonate rocks exhibit multi-scale heterogeneities that make the understanding of fluid flow and recovery a challenge. Properties such as porosity and absolute permeability often vary within a volume and sampling at the representative elementary volume (REV) is needed to determine the appropriate continuum properties for reservoir characterization and prediction. A classical approach to examine the variation with rock sample size and the REV is through analysis at different scales. Measuring petrophysical properties at various scales has therefore been a part of the REV investigation for the studied material. To study the variation with sample size and the effect of heterogeneities on fluid flow, laboratory tests were conducted at different scales, ranging from cm to m scale. Totally 37 sub-samples of a large outcrop block with different sizes and shapes have been included in this work. The main objective has been to determine porosity, effective permeability and dispersivity as function of rock sample size. Porosity–permeability correlations for heterogeneous material can be inadequate and strongly scale dependant. Thus, we also investigate the variation of the porosity–permeability ratio with sample size. The experimental results show that the variance and coefficient of variation decreases with increase in sample size for all investigated properties. Based on the reduction of variation with increased rock sample size a representative volume for measurement is proposed for the studied material. The results also indicate less variation for the porosity–permeability relationship with increased sample volume. It is shown that a single measurement at traditional laboratory scale can lead to erroneous interpretation of a measured property for the vuggy carbonate material. However, multiple measurements at laboratory scale can be used to determine the representative properties.

Effect of Ionic Strength and Layer Number on Swelling of Polyelectrolyte Multilayers in Water Vapour

The swelling behavior of polyelectrolyte multilayers (PEMs) of poly(sodium-4 styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared from aqueous solution of 0.1 M and 0.5 M NaCl are investigated by ellipsometry and Atomic Force Microscopy (AFM). From 1 double-layer up to 4 double-layers of 0.1 M NaCl, the amount of swelling water in the PEMs decreases with increasing number of adsorbed double layers due to an increase in polyelectrolyte density as a result of the attraction between the positively charged outermost PDADMAC layer and the Si substrate. From 6 double layers to 30 double layers, the attraction is reduced due to a much larger distance between substrate and outermost layer leading to a much lower polyelectrolyte density and higher swelling water. In PEMs prepared from aqueous solution of 0.5 M NaCl, the amount of water constantly increases which is related to a monotonically decreasing polyelectrolyte density with increasing number of polyelectrolyte layers. Studies of the surface topology also indicate a transition from a more substrate affected interphase behavior to a continuum properties of the polyelectrolyte multilayers. The threshold for the transition from interphase to continuum behavior depends on the preparation conditions of the PEM.

Understanding coupled stress, flow and transport processes in fractured rocks

This paper presents a review of a systematic research program for understanding the scale and stress effects on the transport behaviors of fractured crystalline rocks, using a hybrid discrete element and particle tracking approach. The motivation is the importance of understanding the stress effects on the behaviors of contaminant transport in fractured crystalline rocks, which is an important issue of rock mechanics for environmental safety assessments of many rock engineering projects. The study is divided into three steps: the first step is a basic study that established the mathematical platform for deriving the conditions, criteria, basic approaches, and test case results for investigating the stress and scale effects on the hydraulic behavior of the fractured rock concerned. At the second step, based on outstanding issues drawn from the first step, the study was extended to consider the effects of the correlation between the fracture aperture and size (represented by trace length) on the permeability of the fractured rock and uncertainties in deriving equivalent continuum properties of fractured rocks. The third step added the particle/solute transport processes to the mathematical platform, including different retardation mechanisms, so that the impact of stress on safety can be directly evaluated, even though it can only be done conceptually. The obtained results show that stress, scale, and inter-parameter correlations of the fracture system geometry are dominant issues for the understanding and characterization of coupled hydro-mechanical processes of fractured rocks and play a significant role for understanding the mass transport behavior in them, with direct impact on geo-environmental safety.

Modeling of loudspeaker using hp-adaptive methods

Numerical modeling of harmonic acoustic field produced by a common commercial loudspeaker is carried out. The goal is to obtain the directional characteristic of the device. So far, these characteristics could only be obtained by measurements, which was very complicated and also expensive. Another benefit of the model consists in obtaining a reference mathematical model for other acoustic devices, such as diffusors or resonators. The continuous mathematical model is derived from the physical laws and is considered for the ideal fluids with properties of a perfect continuum. The model of the loudspeaker in the axial symmetry (2D) is described by the Helmholtz differential equation for harmonic acoustic field and for the correct solution the appropriate boundary conditions and material properties are also defined. The task is solved numerically by a fully adaptive higher-order finite element method developed by the hp-FEM group. The convergence of results is discussed for the different adaptive techniques and the advantages of the use of \(hp\)-adaptivity are demonstrated. Selected results of the computations are then compared with experiments performed in an anechoic chamber at the University of West Bohemia and based on the standard CSN EN 60268-5.

Parameter optimisation of a tyre brush model based on test bench measurements

Identification of tyre parameters is essential to create an accurate tyre model. This paper describes a measurement and optimisation work as part of a multibody dynamical tyre model development. The tyre model is a modified brush model, which is simplified for planar motions, and extended by tyre mass point to take into account the continuum property of the tyre. In a brush model the most important parameter is the stiffness of the tyre elements. This stiffness should be identified with static tests using a special test bench. With this test bench it was measured both in longitudinal and lateral direction with different inflation pressures and wheel loads. The measured stiffness cannot be used directly in the tyre model; optimisation should be performed as part of the identification process. Finally, validation was performed to check the results of the identification.

MULTI-EPOCH OBSERVATIONS OF THE RED WING EXCESS IN THE SPECTRUM OF 3C 279

It has been previously determined that there is a highly significant correlation between the spectral index from 10 GHz to 1350 $\AA$ and the amount of excess luminosity in the red wing of quasar CIV $\lambda$1549 broad emission lines (BELs). Ostensibly, the prominence of the red excess is associated with the radio jet emission mechanism and is most pronounced for lines of sight close to the jet axis. Studying the scant significant differences in the UV spectra of radio loud and radio quiet quasars might provide vital clues to the origin of the unknown process that creates powerful relativistic jets that appear in only about ten percent of quasars. In this study, the phenomenon is explored with multi-epoch observations of the MgII $\lambda$2798 broad line in 3C 279 which has one of the largest known redwing excesses in a quasar spectrum. The amount of excess that is detected appears to be independent of all directly observed optical continuum, radio or submm properties (fluxes or polarizations). The only trend that occurs in this sparse data is: the stronger the BEL, the larger the fraction of flux that resides in the redwing. It is concluded that more monitoring is needed and spectropolarimetry with a large telescope is essential during low states to understand more.

A blurred reflection interpretation for the intermediate flux state in Mrk 335

As part of a long term monitoring campaign of Mrk 335, deep XMM-Newton observations catch the narrow-line Seyfert 1 galaxy (NLS1) in a complex, intermediate flux interval as the active galaxy is transiting from low- to high-flux. Other works on these same data examined the general behaviour of the NLS1 (Grupe et al.) and the conditions of its warm absorber (Longinotti et al.). The analysis presented here demonstrates the X-ray continuum and timing properties can be described in a self-consistent manner adopting a blurred reflection model with no need to invoke partial covering. The rapid spectral variability appears to be driven by changes in the shape of the primary emitter that is illuminating the inner accretion disc around a rapidly spinning black hole (a > 0.7). While light bending is certainly prominent, the rather constant emissivity profile and break radius obtained in our spectral fitting suggest that the blurring parameters are not changing as would be expected if the primary source is varying its distance from the disc. Instead changes could be intrinsic to the power law component. One possibility is that material in an unresolved jet above the disc falls to combine with material at the base of the jet producing the changes in the primary emitter (spectral slope and flux) without changing its distance from the disc.

INVISIBLE ACTIVE GALACTIC NUCLEI. I. SAMPLE SELECTION AND OPTICAL/NEAR-IR SPECTRAL ENERGY DISTRIBUTIONS

In order to find more examples of the elusive high-redshift molecular absorbers, we have embarked on a systematic discovery program for highly obscured, radio-loud "invisible AGN" using the VLA Faint Images of the Radio Sky at Twenty centimeters (FIRST) radio survey in conjunction with Sloan Digital Sky Survey (SDSS) to identify 82 strong (> 300 mJy) radio sources positionally coincident with late-type, presumably gas-rich galaxies. In this first paper, the basic properties of this sample are described including the selection process and the analysis of the spectral-energydistributions (SEDs) derived from the optical (SDSS) + near-IR (NIR) photometry obtained by us at the Apache Point Observatory 3.5m. The NIR images confirm the late-type galaxy morphologies found by SDSS for these sources in all but a few (6 of 70) cases (12 previously well-studied or misclassified sources were culled). Among 70 sources in the final sample, 33 show galaxy type SEDs, 17 have galaxy components to their SEDs, and 20 have quasar power-law continua. At least 9 sources with galaxy SEDs have K-band flux densities too faint to be giant ellipticals if placed at their photometric redshifts. Photometric redshifts for this sample are analyzed and found to be too inaccurate for an efficient radio-frequency absorption line search; spectroscopic redshifts are required. A few new spectroscopic redshifts for these sources are presented here but more will be needed to make significant progress in this field. Subsequent papers will describe the radio continuum properties of the sample and the search for redshifted H I 21 cm absorption.

Discovery of six high-redshift quasars with the Lijiang 2.4 m telescope and the Multiple Mirror Telescope

Quasars with redshifts greater than 4 are rare, and can be used to probe the structure and evolution of the early universe. Here we report the discovery of six new quasars with i-band magnitudes brighter than 19.5 and redshifts between 2.4 and 4.6 from spectroscopy with the Yunnan Faint Object Spectrograph and Camera (YFOSC) at the Lijiang 2.4 m telescope in February, 2012. These quasars are in the list of z > 3.6 quasar candidates selected by using our proposed J — K/i — Y criterion and the photometric redshift estimations from the SDSS optical and UKIDSS near-IR photometric data. Nine candidates were observed by YFOSC, and five among six new quasars were identified as z > 3.6 quasars. One of the other three objects was identified as a star and the other two were unidentified due to the lower signal-to-noise ratio of their spectra. This is the first time that z > 4 quasars have been discovered using a telescope in China. Thanks to the Chinese Telescope Access Program (TAP), the redshift of 4.6 for one of these quasars was confirmed by the Multiple Mirror Telescope (MMT) Red Channel spectroscopy. The continuum and emission line properties of these six quasars, as well as their central black hole masses and Eddington ratios, were obtained.

ON THE EXISTENCE OF “RADIO THERMALLY ACTIVE” GALACTIC SUPERNOVA REMNANTS

In this paper, we investigate the possibility of significant production of thermal bremsstrahlung radiation at radio continuum frequencies that could be linked to some Galactic supernova remnants (SNRs). The main targets for this investigation are SNRs expanding in high density environments. There are several indicators of radio thermal bremsstrahlung radiation from SNRs, such as a flattening at higher frequencies and thermal absorption at lower frequencies intrinsic to an SNR. In this work we discuss the radio continuum properties of 3 SNRs that are the best candidates for testing our hypothesis of significant thermal emission. In the case of SNRs IC443 and 3C391, thermal absorption has been previously detected. For IC443, the contribution of thermal emission at 1 GHz, from our model fit is 3-57%. It is similar to the estimate obtained from the thermal absorption properties (10-40% at 1 GHz). In the case of the 3C391 the conclusions are not so clear. The results from our model fit (thermal emission contribution of 10-25% at 1 GHz) and results obtained from the low frequency absorption (thermal contribution of 0.15-7% at 1 GHz) do not overlap. For the SNR 3C396 we suggest that if previously detected thermal absorption could be intrinsic to the SNR then the thermal emission (<47% at 1 GHz from our model fit) could be significant enough to shape the radio continuum spectrum at high frequencies. Polarization observations for these SNRs can constrain the strength of a thermal component. Reliable observations at low frequencies (<100 MHz) are needed as well as more data at high radio frequencies (>1 GHz), in order to make stronger conclusions about the existence of "radio thermally active" SNRs.

Correlation space of narrow-line Seyfert 1 galaxies

AbstractNarrow-line Seyfert 1 (NLS1) galaxies, as active galactic nuclei with low-mass black holes and high accretion rates, are important targets when addressing questions related to black hole growth and accretion physics. We have studied the correlations among the optical emission-line and continuum properties of a sample of NLS1 galaxies, in comparison with a sample of broad-line Seyfert 1 galaxies. We have shown that the density of the narrow-line region is a key component of Eigenvector 1 space. Density turned out to be as important as the Eddington ratio L/LEdd. This result therefore establishes a close link between central engine and host properties, and places new constraints on host - black hole co-evolution.

Modeling yarn slip in woven fabric at the continuum level: Simulations of ballistic impact

Woven fabric is used in a wide variety of military and commercial products—both in neat form and as the reinforcement phase of composites. In many applications, yarn slip, the relative sliding of the yarns composing the weave, is an important mode of deformation or failure. Yarn slip can significantly change the energy absorption capacity and yarn density of the fabric and also cause yarns to unravel from the weave. Virtually all existing models for woven fabric that allow yarn slip are discrete in nature. They simulate every yarn in the weave and are therefore computationally expensive and difficult to integrate with other material models. A promising alternative to discrete models is the mesostructure-based continuum technique. With this technique, homogenized continuum properties are determined from a deforming analytic model of the fabric mesostructure at each material point. Yarn-level mechanisms of deformation are thus captured without the computational cost of simulating every yarn in the fabric. However, existing mesostructure-based continuum models treat the yarns as pinned together at the cross-over points of the weave, and an operative model that allows yarn slip has not been published. Here, we introduce a mesostructure-based continuum model that permits yarn slip and use the model to simulate the ballistic impact of woven fabric. In our approach, the weave is the continuum substrate on which the model is anchored, and slip of the yarns occurs relative to the weave continuum. The cross-over points of the weave act as the material points of the continuum, and the evolution of the local weave mesostructure at each point of the continuum is represented by state variables. At the same time, slip velocity fields simulate the slip of each yarn family relative to the weave continuum and therefore control the evolution of the yarn pitch. We found that simulating yarn slip significantly improves finite element predictions of the ballistic impact of a Kevlar® woven fabric, in particular by increasing the energy absorbed at high initial projectile velocities. Further simulations elucidate the micromechanisms of deformation of ballistic impact of woven fabric with yarn slip. Our findings suggest ways to improve the performance of flexible armor and indicate that this approach has the potential to simulate many other types of woven fabric in applications in which yarn slip occurs.