Absorption Opacity Research Articles

XMM-Newton observations of the nitrogen-rich Wolf-Rayet star WR 1

We present XMM-Newton results for the X-ray spectrum from the N-rich Wolf-Rayet (WR) star WR 1. The EPIC instrument was used to obtain a medium-resolution spectrum. The following features characterize this spectrum: (a) significant emission “bumps” appear that are coincident with the wavelengths of typical strong lines, such as Mgxi, Si xiii, and Sxv; (b) little emission is detected above 4 keV, in contrast to recent reports of a hard component in the stars WR 6 and WR 110 which are of similar subtype; and (c) evidence for sulfur K-edge absorption at about 2.6 keV, which could only arise from absorption of X-rays by the ambient stellar wind. The lack of hard emission in our dataset is suggestive that WR 1 may truly be a single star, thus representing the first detailed X-ray spectrum that isolates the WR wind alone (in contrast to colliding wind zones). Although the properties of the S-edge are not well-constrained by our data, it does appear to be real, and its detection indicates that at least some of the hot gas in WR 1 must reside interior to the radius of optical depth unity for the total absorptive opacity at the energy of the edge.

The Effect of Multiple Scattering on the Polarization from Binary Star Envelopes. I. Self‐ and Externally Illuminated Disks

We present the results of a Monte Carlo radiative transfer code that calculates the polarization produced by multiple Thomson scattering and variable absorptive opacity in a circumstellar disk around one component of a close detached binary system. We consider in detail the polarization variations over the binary cycle that result from the disk's illumination both by the external star and by its own volume emission. We identify key features of these polarization phase curves and investigate their behavior as functions of optical depth, albedo, and inclination for geometrically thin and thick disks. The polarization due to disk self-illumination is sensitive to the internal optical characteristics of the disk, while the polarization arising from external illumination is sensitive mainly to the disk's geometrical thickness. With appropriate flux weighting, these results, combined with those for an internally illuminated disk, allow simulation of the polarization signature from an arbitrary binary-disk system.

Free–Free Absorption and the Unified Scheme

AbstractWe report very long baseline array (VLBA) observations at 2.3, 8.4, and 15.4 GHz towards nine gigahertz peaked spectrum (GPS) sources. One Seyfert 1 galaxy, one Seyfert 2 galaxy, three radio galaxies, and four quasars were included in our survey. We obtained spatial distributions of the free–free absorption (FFA) opacity with milliarcsecond resolution for all sources. It is found that type 1 (Seyfert 1 and quasars) and type 2 (Seyfert 2 and radio galaxies) sources showed different distributions of the FFA opacities. The type 1 sources tend to show more asymmetric opacity distributions towards a double lobe, while those of the type 2 sources are rather symmetric. Our results imply that the different viewing angle of the jet causes the difference of FFA opacity along the external absorber. This idea supports the unified scheme between quasars and radio galaxies, proposed by Barthel (1989).

A Module for Radiation Hydrodynamic Calculations with ZEUS‐2D Using Flux‐limited Diffusion

A module for the ZEUS-2D code is described which may be used to solve the equations of radiation hydrodynamics to order unity in v/c, in the flux-limited diffusion (FLD) approximation. In this approximation, the tensor Eddington factor f which closes the radiation moment equations is chosen to be an empirical function of radiation energy density. This is easier to implement and faster than full-transport techniques, in which f is computed by solving the transfer equation. However, FLD is less accurate when the flux has a component perpendicular to the gradient in radiation energy density, and in optically thin regions when the radiation field depends strongly on angle. The material component of the fluid is here assumed to be in local thermodynamic equilibrium. The energy equations are operator-split, with transport terms, radiation diffusion term, and other source terms evolved separately. Transport terms are applied using the same consistent transport algorithm as in ZEUS-2D. The radiation diffusion term is updated using an alternating-direction implicit method with convergence checking. Remaining source terms are advanced together implicitly using numerical root-finding. However when absorption opacity is zero, accuracy is improved by treating compression and expansion source terms using time-centered differencing. Results are discussed for test problems including radiation-damped linear waves, radiation fronts propagating in optically-thin media, subcritical and supercritical radiating shocks, and an optically-thick shock in which radiation dominates downstream pressure.

Time-dependent photoionization and fluorescence line emission in gamma-ray burst environments

If γ -ray bursts (GRBs) are associated with dense star-forming regions, photoelectric absorption by the circumburster material (CBM) will occur. As the burst evolves, the surrounding material is photoionized, leading to fluorescence line emission and reduced photoelectric absorption opacity. We have numerically simulated this problem, accounting for all relevant microscopic processes. We find that if GRBs are hosted in a quasi-isotropic surrounding medium, photoionization of the CBM leads to a constant but weak level of delayed fluorescence line emission on timescales of weeks to months after the GRB. A temporally evolving Fe K edge absorption feature can serve as diagnostic tool for redshift measurements.

Neutrino-nucleon interactions in magnetized neutron-star matter: The effects of parity violation

We study neutrino-nucleon scattering and absorption in a dense, magnetized nuclear medium. These are the most important sources of neutrino opacity governing the cooling of a proto-neutron star in the first tens of seconds after its formation. Because the weak interaction is parity violating, the absorption and scattering cross sections depend asymmetrically on the directions of the neutrino momenta with respect to the magnetic field. We develop the moment formalism of neutrino transport in the presence of such asymmetric opacities and derive explicit expressions for the neutrino flux and other angular moments of the Boltzmann transport equation. For a given neutrino species, there is a drift flux of neutrinos along the magnetic field in addition to the usual diffusive flux. This drift flux depends on the deviation of the neutrino distribution function from thermal equilibrium. Hence, despite the fact that the neutrino cross sections are asymmetric throughout the star, the asymmetric neutrino flux can be generated only in the outer region of the proto-neutron star where the neutrino distribution deviates significantly from thermal equilibrium. The deviation from equilibrium is similarly altered by the asymmetric scattering and absorption, although its magnitude will still be quite small in the interior of themore » star. We clarify two reasons why previous studies have led to misleading results. First, inelasticity must be included in the phase space integrals in order to satisfy detail balance. Second, nucleon recoil must be included in order to find the leading order asymmetric cross sections correctly, even though it can be ignored to leading order to get the zero field opacities. In addition to the asymmetric absorption opacity arising from nucleon polarization, we also derive the contribution of the electron (or positron) ground state Landau level. For neutrinos of energy less than a few times the temperature, this is the dominant source of asymmetric opacity. Last, we discuss the implication of our result to the origin of pulsar kicks: in order to generate kick velocity of a few hundred kmhs{sup {minus}1} from asymmetric neutrino emission using the parity violation effect, the proto-neutron star must have a dipole magnetic field of at least 10{sup 15}{minus}10{sup 16} G. {copyright} {ital 1999} {ital The American Physical Society}« less

LS-forbidden strength of atomic transition arrays

In a transition array (i.e. the totality of the lines linking the levels of two electronic configurations), some lines are forbidden in pure LS coupling, and very weak in its vicinity. The total strength of this set of lines is evaluated statistically in terms of the parameters defining the coupling. The numerical application is made to the Fe V - array. Such an evaluation of the LS-forbidden strength is important for the computations of absorption coefficients and opacities.

Neutrino Transport in Strongly Magnetized Proto–Neutron Stars and the Origin of Pulsar Kicks: The Effect of Asymmetric Magnetic Field Topology

In proto-neutron stars with strong magnetic fields, the cross section for νe (${u{{ν}}{7016}}$ --> -->e) absorption on neutrons (protons) depends on the local magnetic field strength resulting from the quantization of energy levels for the e- (e+) produced in the final state. If the neutron star possesses an asymmetric magnetic field topology in the sense that the magnitude of magnetic field in the north pole is different from that in the south pole, then asymmetric neutrino emission may be generated. We calculate the absorption cross sections of νe and ${u{{ν}}{7016}}$ --> -->e in strong magnetic fields as a function of the neutrino energy. These cross sections exhibit oscillatory behaviors that occur because new Landau levels for the e- (e+) become accessible as the neutrino energy increases. By evaluating the appropriately averaged neutrino opacities, we demonstrate that the change in the local neutrino flux caused by the modified opacities is rather small. To generate appreciable kick velocity (~300 km s-1) to the newly formed neutron star, the difference between the field strengths at the two opposite poles of the star must be at least 1016 G. We also consider the magnetic field effect on the spectral neutrino energy fluxes. The oscillatory features in the absorption opacities give rise to modulations in the emergent spectra of νe and ${u{{ν}}{7016}}$ --> -->e.

Below the Lyman Edge: Ultraviolet Polarimetry of Quasars

The Lyman edge at 912 A is an important diagnostic region for studying quasi-stellar objects (QSOs). In particular, it reveals a great deal about the physical conditions within the atmospheres of accretion disks, a ubiquitous component of QSO theories. A robust prediction of accretion disk models is a significant polarization due to electron scattering just longward (in wavelength) of the Lyman edge, because of the wavelength dependence of the hydrogen absorption opacity. Observations of the Lyman edge regions of QSOs have shown scant evidence for the predicted features; few QSOs show the broad, partial Lyman edges expected to be common according to most theories, and none show the high polarizations expected longward of the Lyman edge. Still, polarization spectra of a small number of QSOs have shown a rising polarization (up to 20%) at wavelengths shortward of the Lyman edge. We have now doubled our sample of intermediate-redshift QSOs observed with the Hubble Space Telescope Faint Object Spectrograph spectropolarimeter to determine the amount of polarization on both sides of the Lyman limit. For this new sample of six objects, polarizations are low and mostly consistent with zero below the Lyman edge. Another important result of the new data is that it strengthens the conclusion that quasars are generally not polarized significantly just longward of the Lyman edge at ~1000 A. There is no significant statistical wavelength dependence of the polarization longward of the Lyman edge, indicating that simple plane-parallel atmospheres with scattering-dominated opacity are not significant sources of UV flux in quasars.

X‐Ray Transmission in Cold Matter: Nonrelativistic Corrections for Compton Scattering

Obscuration by cold matter is frequently observed in a variety of X-ray sources. With the improved sensitivity and energy resolution of the new generation of X-ray detectors, such as those aboard ASCA, it may not always be possible to neglect electron scattering for column densities much greater than ~1023 cm-2. We derive simple, approximate, analytic expressions for a spherical geometry, which enable the transmitted flux to be corrected for nonrelativistic Compton scattering. The method allows the use of arbitrary element abundances, photoelectric absorption cross sections, and input spectra and is computationally fast enough for use in a spectral fitting code. The approximations can be used for column densities up to ~5 × 1024 cm-2. The maximum observed energy for which the approximations are valid depends on the column density and is ~18.5 keV for NH = 1023 cm-2 and ~10 keV for NH = 5 × 1024 cm-2. The model provides a significantly improved description of the transmitted spectrum compared to the usual model, in which only attenuation due to the absorption opacity is considered, and scattering is neglected altogether. The latter leads to errors in the transmitted spectrum that can exceed 1 order of magnitude, especially around the Fe K edge.

Polarization from magnetized accretion discs - II. The effects of absorption opacity on Faraday rotation

Equipartition magnetic fields can dramatically affect the polarization of radiation emerging from accretion disk atmospheres in active galactic nuclei. We extend our previous work on this subject by exploring the interaction between Faraday rotation and absorption opacity in local, plane-parallel atmospheres with parameters appropriate for accretion discs. Faraday rotation in pure scattering atmospheres acts to depolarize the radiation field by rotating the polarization planes of photons after last scattering. Absorption opacity in an unmagnetized atmosphere can increase or decrease the polarization compared to the pure scattering case, depending on the thermal source function gradient. Combining both Faraday rotation and absorption opacity, we find the following results. If absorption opacity is much larger than scattering opacity throughout the atmosphere, then Faraday rotation generally has only a small effect on the emerging polarization because of the small electron column density along a photon mean free path. However, if the absorption opacity is not too large and it acts alone to increase the polarization, then the effects of Faraday rotation can be enhanced over those in a pure scattering atmosphere. Finally, while Faraday rotation often depolarizes the radiation field, it can in some cases increase the polarization when the thermal source function does not rise too steeply with optical depth. We confirm the correctness of the Silant'ev (1979) analytic calculation of the high magnetic field limit of the pure scattering atmosphere, which we incorrectly disputed in our previous paper.

The Spectropolarimetric Test of the Quasar Emission Mechanism

The spectral energy distribution of ordinary quiescent quasars is dominated by the big blue bump continuum component in the optical/UV region. This has been modeled as a viscous dissipative thin disk, though fundamental problems with that model are known.Here we test the dramatic polarization predictions for one such model. Pure scattering atmospheres are ruled out by existing data. However, the models of Laor, Netzer, & Piran have significant absorption opacity. They indicate that the optical light and that shortward of the Lyman edge may have low polarization but that the 1000-2000 A region may be significantly polarized. The position angle should be perpendicular to the radio axis where available. We do not see the predicted behavior.The polarization data also constrain reprocessing models for the disk.

Modeling the Violet Spectral Region of Cool Stars: Line and Continuous Opacity

view Abstract Citations (15) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Modeling the Violet Spectral Region of Cool Stars: Line and Continuous Opacity Short, C. Ian ; Lester, John B. Abstract The calculated flux distribution in the 300-400 nm region only fits the observed distribution closely for both the Sun and Arcturus when the complete ATLAS9 line list is used. However, a good fit to the detailed shape of the observed high-resolution spectrum in the Sun may only be obtained by using a subset of this line list that only includes lines with the highest quality atomic data. Moreover, for Arcturus, even the subset of high-quality lines can only provide a close fit to the observed high-resolution spectrum if we include, ad hoc, an extra amount of continuous absorption opacity. We suggest that the complete ATLAS9 line list contains many lines that either do not exist or have oscillator strengths that are too strong, and that the inclusion of a hitherto undiscovered source of continuous absorption opacity is necessary to fit the violet flux in late-type stars. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177837 Bibcode: 1996ApJ...469..898S Keywords: ATOMIC DATA; STARS: ATMOSPHERES; STARS: INDIVIDUAL NAME: ARCTURUS; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (1)

Polarization near the Lyman Edge in Accretion Disk Atmosphere Models of Quasars

We calculate self-consistently the structure, spectrum, and polarization of optically thick atmospheres under the conditions expected in quasar accretion disks. We consider a pure-hydrogen composition and neglect the effects of external illumination. We show that, blueward of the Lyman edge, the polarization rises above the value expected from an optically thick, pure electron-scattering atmosphere. This arises because of the wavelength dependence of the absorption opacity, combined with the fact that the source function in this region of the spectrum has a steep variation with optical depth. We suggest that this effect may help explain the recent Hubble Space Telescope observations of steep rises in polarization blueward of the Lyman edge in several quasars. We predict that the polarization should continue to rise blueward of the wavelength at which the current observations end, that cooler spectra should be more polarized, and that the polarization angle in this region should be parallel to the disk plane.

The Effect of Multiple Scattering on the Polarization from Axisymmetric Circumstellar Envelopes. II. Thomson Scattering in the Presence of Absorptive Opacity Sources

The Effect of Multiple Scattering on the Polarization from Axisymmetric Circumstellar Envelopes. II. Thomson Scattering in the Presence of Absorptive Opacity Sources

Semianalytic continuum spectra of Type 2 supernovae

We extend the approximate radiative transfer analysis of Hershkowitz, Linder, & Wagoner (1986) to a more general class of supernova model atmospheres, using a simple fit to the effective continuum opacity produced by lines (Wagoner, Perez, & Vasu 1991). At the low densities considered, the populations of the excited states of hydrogen are governed mainly by photoionization and recombination, and scattering dominates absorptive opacity. We match the asymptotic expressions for the spectral energy density J(sub nu) at the photosphere, whose location at each frequency is determined by a first-order calculation of the deviation of J(sub nu) from the Planck function B(sub nu). The emergent spectral luminosity then assumes the form L(sub nu) = 4 pi(squared)r(squared)(sub *) zeta(squared)B(sub nu)(T(sub p)), where T(sub p)(nu) is the photospheric temperature zeta is the dilution factor, and r(sub *) is a fiducial radius (ultimately taken to be the photospheric radius r(sub p)(nu)). The atmosphere is characterized by an effective temperature T(sub e) (varies as L(sup 1/4)r(sup -1/2)(sub *)) and hydrogen density n(sub H) = dependence of zeta on frequency nu and the parameters T(sub p), r(sub p), and alpha. The resulting understanding of the dependence of the spectral luminosity on observable parameters which characterize the relevant physical conditions will be of particular use in assessing the reliability of the expanding photosphere method of distance determination. This is particularly important at cosmological distances, where no information about the progenitor star will be available. This technique can also be applied to other low-density photosphere.

Missing opacity in the atmospheric models of red giants

Synthetic spectra of Arcturus computed with the latest theoretical stellar atmospheres disagree strongly with both the spectral energy distribution and with the spectroscopic data for $\lambda < 4000$ \AA , even though the stellar parameters are well constrained. We find that the discrepancy can be removed by adding a continuous absorption opacity that is approximately equal to that which is already included in the {\sc atlas9} code. Spectroscopy of other K2 III stars shows that the spectrum of Arcturus in not anomalous. Therefore, missing opacity may be a common feature of the models for stars in this temperature and luminosity range.

An iron absorption model of gamma-ray burst spectra

view Abstract Citations (7) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Iron Absorption Model of Gamma-Ray Burst Spectra Liang, Edison P. ; Kargatis, Vincent E. Abstract Most gamma-ray bursts (GRBs) exhibit deficits of X-rays below approximately 200 keV. Here we consider a spectral model in which the burst source is shielded by an optically thick layer of circumburster material (CBM) rich in iron-group elements whose photoelectric absorption opacity exceeds the Thomson opacity below approximately 120 keV. For power-law distributions of absorption depths along the lines of sight the absorbed spectrum can indeed mimic the typical GRB spectrum. This model predicts that (a) the spectrum should evolve monotonically from hard to soft during each energy release, which is observed in most bursts, especially in fast rise exponential decay bursts; (b) Fe spectral features near 7 keV may be present in some bursts; and (c) the ratio of burst distances to the CBM and to Earth should be approximately 10-11 if the spectral evolution is purely due to Fe stripping by the photons. Publication: The Astrophysical Journal Pub Date: September 1994 DOI: 10.1086/187524 Bibcode: 1994ApJ...432L.111L Keywords: Absorption Spectra; Astronomical Models; Gamma Ray Bursts; Iron; X Rays; Evolution (Development); Opacity; Optical Thickness; Space Radiation; GAMMA RAYS: BURSTS; GAMMA RAYS: THEORY full text sources ADS | data products SIMBAD (1)

Ultraviolet continuum variability and visual flickering in the peculiar object MWC 560

High-speed U-band photometry of the peculiar emission object MWC 560 obtained with the ground-based instrumentation, and V-band photometry obtained with the International Ultraviolet Explorer-Fine Error Sensor indicates irregular brightness variations are quasi-periodic. Multiple peaks of relative brightness power indicate statistically significant quasi periods existing in a range of 3-35 minutes, that are superposed on slower hourly varying components. We present a preliminary model that explains the minute and hourly time-scale variations in MWC 560 in terms of a velocity-shear instability that arises because a white dwarf magnetosphere impinges on an accretion disk. We also find evidence for Fe II multiplet pseudocontinuum absorption opacity in far-UV spectra of CH Cygni which is also present in MWC 560. Both CH Cyg and MWC 560 may be in an evolutionary stage that is characterized by strong UV continuum opacity which changes significantly during outburst, occurring before they permanently enter the symbiotic nebular emission phase.

Radiometric determination of water vapor and liquid water and its validation with other techniques

For remote sensing applications in the field of atmospheric physics it is necessary to know the highly variable part of the tropospheric water vapor absorption. With a dual‐channel microwave radiometer, operating at 21.3 and 31.5 GHz, the estimation of the water vapor and liquid water content is possible. The retrieval is performed iteratively with an algorithm using simplified model profiles of the troposphere and a millimeter wave absorption model. This site‐independent profile algorithm changes a model atmosphere until the calculated and measured sky brightness temperatures do agree. Integrated precipitable water vapor (IWV) and integrated liquid water (ILW) are obtained by integrating over the corresponding profiles. By the use of the absorption model sky brightness temperatures, opacities or propagation delays at other frequencies up to 1000 GHz can be calculated. Simulations with a few thousand test profiles showed that the rms error, excluding that of the absorption model, is about 3% for the IWV and between 10% and 20% for the ILW (ILW &gt; 0.1 mm), while for the predicted opacities at 90, 142, and 204 GHz the error ranges between 15% and 25%. The performance of the algorithm and the water vapor radiometer has been checked with independent brightness temperature measurements at 142 and 204 GHz, respectively, IR spectroscopic measurements and radiosonde data. The independent measurements showed all a good agreement with the retrieved quantities of the profile algorithm.