scholarly journals Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds

2018 ◽  
Vol 619 ◽  
pp. A59 ◽  
Author(s):  
J. O. Sundqvist ◽  
J. Puls
Keyword(s):  
Mass Loss Rate ◽  
Mean Free Path ◽  
Model Atmosphere ◽  
High Energy ◽  
Equal Mass ◽  
Velocity Space ◽  
Electromagnetic Spectrum ◽  
Marginal Effect ◽  
X Rays ◽  
Ionization Balance

Context. Clumping in the radiation-driven winds of hot, massive stars severly affects the derivation of synthetic observables across the electromagnetic spectrum. Aims. We implement a formalism for treating wind clumping – focusing in particular on the light-leakage effects associated with a medium that is porous in physical and velocity space – into the global (photosphere + wind) NLTE model atmosphere and spectrum synthesis code FASTWIND. Methods. The basic method presented here assumes a stochastic, two-component wind consisting of a mixture of optically thick and thin clumps embedded in a rarefied inter-clump medium. We have accounted fully for the reductions in opacity associated with porosity in physical and velocity-space (the latter due to Doppler shifts in an accelerating medium), as well as for the well-known effect that opacities depending on ⟨ρ2⟩ are higher in clumpy winds than in smooth ones of equal mass-loss rate. By formulating our method in terms of suitable mean and effective opacities for the clumpy wind, we are able to compute atmospheric models with the same speed (∼15 min on a modern laptop or desktop) as in previous generations of FASTWIND. Results. After verifying important analytic limits (smooth, optically thin, completely optically thick), we present some first, generic results of the new models. These include: i) Confirming earlier results that velocity-space porosity is critical for analysis of UV wind resonance lines in O-stars; ii) for the optical Hα line, we show that optically thick clumping effects are small for O-stars, but potentially very important for late B and A-supergiants; iii) in agreement with previous work, we show that spatial porosity is a marginal effect for absorption of high-energy X-rays in O-stars, as long as the mean-free path between clumps are kept at realistic values ≲R*; iv) whereas radio absorption in O-stars shows strong spatial porosity effects in near photospheric layers, it is negligible at their typical radio-photosphere radii ∼100R*; v) regarding the wind ionization balance, a general trend is that increased rates of recombination in simulations with optically thin clumps lead to overall lower degrees of ionization than in corresponding smooth models, but that this effect now is counteracted by the increased levels of light-leakage associated with porosity in physical and velocity space (i.e., by an increase of ionization rates). We conclude by discussing future work and some planned applications for this new generation of FASTWIND models.

scholarly journals Centaurus A: Hard X-ray and High-Energy Gamma-Ray Light Curve Correlation

Galaxies ◽  
2019 ◽  
Vol 7 (2) ◽  
pp. 44 ◽  
Author(s):  
Isak Davids ◽  
Markus Böttcher ◽  
Michael Backes
Keyword(s):  
Light Curve ◽  
High Energy ◽  
Electromagnetic Spectrum ◽  
X Rays ◽  
X Ray ◽  
Correlation Studies ◽  
High Energy Gamma ◽  
Energy Gamma ◽  
Γ Rays ◽  
Centaurus A

Centaurus A, powered by a 55 million solar mass supermassive black hole, has been intensively monitored in all accessible wavelength ranges of the electromagnetic spectrum. However, its very-high energy gamma ( γ ) ray flux (TeV photons), obtained from H.E.S.S. is relatively faint, hampering detailed light curve analyses in the most energetic energy band. Yet, the extensive long-term light curve data from Fermi-LAT and Swift-BAT (hard X-rays) allows for cross-correlation studies. We find a hint that X-ray emission from Centaurus A precedes the γ rays by 25 ± 125 days. If this lag is real and related to a γ γ absorption effect in the broad-line region (BLR) around the central source, we can constrain the size of the BLR using light-travel time arguments. These are first results of extended light curve correlation studies between high-energy γ rays and X-rays from Centaurus A.

scholarly journals Theγ-ray emitting region in low synchrotron peak blazars

2018 ◽  
Vol 616 ◽  
pp. A63 ◽  
Author(s):  
B. Arsioli ◽  
Y-L. Chang
Keyword(s):  
Spectral Energy Distribution ◽  
Thomson Scattering ◽  
High Energy ◽  
Spectral Energy ◽  
Electromagnetic Spectrum ◽  
X Rays ◽  
Complete Sample ◽  
Central Engine ◽  
Line Region ◽  
Open Question

Aims.From the early days inγ-ray astronomy, locating the origin of GeV emission within the core of an active galactic nucleus (AGN) persisted as an open question; the problem is to discern between near- and far-site scenarios with respect to the distance from the super massive central engine. We investigate this question under the light of a complete sample of low synchrotron peak (LSP) blazars which is fully characterized along many decades in the electromagnetic spectrum, from radio up to tens of GeV. We consider the high-energy emission from bright radio blazars and test for synchrotron self-Compton (SSC) and external Compton (EC) scenarios in the framework of localizing theγ-ray emission sites. Given that the inverse Compton (IC) process under the EC regime is driven by the abundance of external seed photons, these photons could be mainly ultraviolet (UV) to X-rays coming from the accretion disk region and the broad-line region (BLR), therefore close to the jet launch base; or infrared (IR) seed photons from the dust torus and molecular cloud spine-sheath, therefore far from jet launch base. We investigate both scenarios, and try to reveal the physics behind the production ofγ-ray radiation in AGNs which is crucial in order to locate the production site.Methods.Based on a complete sample of 104 radio-selected LSP blazars, with 37 GHz flux density higher than 1 Jy, we study broadband population properties associated with the nonthermal jet emission process, and test the capability of SSC and EC scenarios to explain the overall spectral energy distribution (SED) features. We use SEDs well characterized from radio toγrays, considering all currently available data. The enhanced available information from recent works allows us to refine the study of Syn to IC peak correlations, which points to a particularγ-ray emission site.Results.We show that SSC alone is not enough to account for the observed SEDs. Our analysis favors an EC scenario under the Thomson scattering regime, with a dominant IR external photon field. Therefore, the far-site (i.e., far from the jet launch) is probably the most reasonable scenario to account for the population properties of bright LSP blazars in cases modeled with a pure leptonic component. We calculate the photon energy density associated with the external field at the jet comoving frame to beU′ext= 1.69 × 10−2erg cm−3, finding good agreement to other correlated works.

scholarly journals Blazar variability power spectra from radio up to TeV photon energies: Mrk 421 and PKS 2155−304

2020 ◽  
Vol 494 (3) ◽  
pp. 3432-3448
Author(s):  
Arti Goyal
Keyword(s):  
Time Scales ◽  
Power Law ◽  
Thermal Emission ◽  
Flicker Noise ◽  
Power Spectra ◽  
High Energy ◽  
Electromagnetic Spectrum ◽  
X Rays ◽  
Owens Valley ◽  
Γ Rays

ABSTRACT We present the results of the power spectral density (PSD) analysis for the blazars Mrk 421 and PKS 2155−304, using good-quality, densely sampled light curves at multiple frequencies, covering 17 decades of the electromagnetic spectrum, and variability time-scales from weeks up to a decade. The data were collected from publicly available archives of observatories at radio from Owens Valley Radio Observatory, optical and infrared (B, V, R, I, J, H, and Kbands), X-rays from the Swift and the Rossi X-ray Timing Explorer, high and very high energy (VHE) γ-rays from the Fermi and Very Energetic Radiation Imaging Telescope Array System as well as the High Energy Stereoscopic System. Our results are: (1) the power-law form of the variability power spectra at radio, infrared, and optical frequencies have slopes ∼1.8, indicative of random-walk-type noise processes; (2) the power-law form of the variability power spectra at higher frequencies, from X-rays to VHE  γ-rays, however, have slopes ∼1.2, suggesting a flicker noise-type process; and (3) there is significantly more variability power at X-rays, high and VHE γ-rays on time-scales ≲ 100 d, as compared to lower energies. Our results do not easily fit into a simple model, in which a single compact emission zone is dominating the radiative output of the blazars across all the time-scales probed in our analysis. Instead, we argue that the frequency-dependent shape of the variability power spectra points out a more complex picture, with highly inhomogeneous outflow producing non-thermal emission over an extended, stratified volume.

scholarly journals The X-ray properties of magnetic massive stars

2014 ◽  
Vol 9 (S307) ◽  
pp. 437-442
Author(s):  
Yaël Nazé ◽  
Véronique Petit ◽  
Melanie Rinbrand ◽  
David Cohen ◽  
Stan Owocki ◽  
...  
Keyword(s):  
Massive Stars ◽  
Mass Loss Rate ◽  
High Energy ◽  
Strongly Correlated ◽  
X Rays ◽  
X Ray ◽  
Magnetically Confined ◽  
Higher Temperature ◽  
Early Type Stars ◽  
High Energy Emission

AbstractEarly-type stars are well-known to be sources of soft X-rays. However, this high-energy emission can be supplemented by bright and hard X-rays when magnetically confined winds are present. In an attempt to clarify the systematics of the observed X-ray properties of this phenomenon, a large series of Chandra and XMM observations was analyzed, over 100 exposures of 60% of the known magnetic massive stars listed recently by Petit et al. (2013). It is found that the X-ray luminosity is strongly correlated with mass-loss rate, in agreement with predictions of magnetically confined wind models, though the predictions of higher temperature are not always verified. We also investigated the behaviour of other X-ray properties (absorption, variability), yielding additional constraints on models. This work not only advances our knowledge of the X-ray emission of massive stars, but also suggests new observational and theoretical avenues to further explore magnetically confined winds.

scholarly journals Soft X-ray heating as a mechanism of optical continuum generation in solar-type star superflares

2019 ◽  
Vol 489 (3) ◽  
pp. 4338-4345
Author(s):  
Bulat A Nizamov
Keyword(s):  
Emission Measure ◽  
Model Atmosphere ◽  
Stellar Atmosphere ◽  
X Rays ◽  
Large Area ◽  
Flare Loop ◽  
Continuum Generation ◽  
Solar Type ◽  
Optical Continuum ◽  
Ionization Balance

ABSTRACT Superflares on the solar-type stars observed by Kepler demonstrate the contrast in the optical continuum of the order 0.1–1 per cent. The mechanism of formation of this radiation is not firmly established. We consider a model where the stellar atmosphere is irradiated by the soft X-rays emitted from the flaring loop filled with the hot plasma. This radiation heats a large area beneath the loop. Subsequent cooling due to h− and hydrogen free–bound emission can contribute to the observed enhanced continuum. We solve the equations of radiative transfer, statistical equilibrium, ionization balance, and radiative equilibrium in the model atmosphere illuminated by the soft X-rays; compute the temperature and the electron density in the atmosphere; and find the emergent radiation. We found that a flare loop of the length ∼1010 cm and plasma density 1012 cm−3 at the temperature T = 20 MK can provide the contrast in the Kepler bandpass of 0.1 and 0.8 per cent if the heated region covers ∼1 and 10 per cent of the visible stellar surface, respectively. The required emission measure is of the order of 1055 cm−3.

The Basic Physical Principles of Ion, Electron, and X-Ray Detectors and Their Application to Microanalysis

1985 ◽  
Vol 43 ◽  
pp. 154-157
Author(s):  
A.J. Tousimis
Keyword(s):  
Ion Beam ◽  
Depth Resolution ◽  
Sample Surface ◽  
High Energy ◽  
X Rays ◽  
X Ray ◽  
Electrons And Ions ◽  
Spatial Depth ◽  
Minimum Surface Area ◽  
Physical Principles

An integral and of prime importance of any microtopography and microanalysis instrument system is its electron, x-ray and ion detector(s). The resolution and sensitivity of the electron microscope (TEM, SEM, STEM) and microanalyzers (SIMS and electron probe x-ray microanalyzers) are closely related to those of the sensing and recording devices incorporated with them.Table I lists characteristic sensitivities, minimum surface area and depth analyzed by various methods. Smaller ion, electron and x-ray beam diameters than those listed, are possible with currently available electromagnetic or electrostatic columns. Therefore, improvements in sensitivity and spatial/depth resolution of microanalysis will follow that of the detectors. In most of these methods, the sample surface is subjected to a stationary, line or raster scanning photon, electron or ion beam. The resultant radiation: photons (low energy) or high energy (x-rays), electrons and ions are detected and analyzed.

Monte Carlo Modelling of Secondary Electron Yield from Rough Surfaces

1990 ◽  
Vol 48 (1) ◽  
pp. 422-423
Author(s):  
John C. Russ
Keyword(s):  
Monte Carlo ◽  
Energy Loss ◽  
Secondary Electron ◽  
Secondary Electron Emission ◽  
Analytical Calculation ◽  
Mean Free Path ◽  
Single Scattering ◽  
High Energy ◽  
Secondary Electron Yield ◽  
Electron Yield

Monte-Carlo programs are well recognized for their ability to model electron beam interactions with samples, and to incorporate boundary conditions such as compositional or surface variations which are difficult to handle analytically. This success has been especially powerful for modelling X-ray emission and the backscattering of high energy electrons. Secondary electron emission has proven to be somewhat more difficult, since the diffusion of the generated secondaries to the surface is strongly geometry dependent, and requires analytical calculations as well as material parameters. Modelling of secondary electron yield within a Monte-Carlo framework has been done using multiple scattering programs, but is not readily adapted to the moderately complex geometries associated with samples such as microelectronic devices, etc.This paper reports results using a different approach in which simplifying assumptions are made to permit direct and easy estimation of the secondary electron signal from samples of arbitrary complexity. The single-scattering program which performs the basic Monte-Carlo simulation (and is also used for backscattered electron and EBIC simulation) allows multiple regions to be defined within the sample, each with boundaries formed by a polygon of any number of sides. Each region may be given any elemental composition in atomic percent. In addition to the regions comprising the primary structure of the sample, a series of thin regions are defined along the surface(s) in which the total energy loss of the primary electrons is summed. This energy loss is assumed to be proportional to the generated secondary electron signal which would be emitted from the sample. The only adjustable variable is the thickness of the region, which plays the same role as the mean free path of the secondary electrons in an analytical calculation. This is treated as an empirical factor, similar in many respects to the λ and ε parameters in the Joy model.

“Cartography” in 7-Dimensions at CHESS: Mapping of Structure in Real Space, Reciprocal Space, and Time Using High-Energy X-rays

2020 ◽  
Vol 33 (6) ◽  
pp. 11-16
Author(s):  
K. E. Nygren, ◽  
D. C. Pagan, ◽  
J. P. C. Ruff ◽  
E. Arenholz ◽  
J. D. Brock
Keyword(s):  
High Energy ◽  
Real Space ◽  
Reciprocal Space ◽  
X Rays ◽  
Space And Time

scholarly journals The interstellar medium in young supernova remnants: key to the production of cosmic X-rays and $\gamma $-rays

2021 ◽  
Vol 366 (6) ◽  
Author(s):  
Hidetoshi Sano ◽  
Yasuo Fukui
Keyword(s):  
Interstellar Medium ◽  
Supernova Remnants ◽  
Cosmic Ray ◽  
High Energy ◽  
X Rays ◽  
Interstellar Gas ◽  
High Energy Radiation ◽  
Dense Cores ◽  
The Relationship ◽  
Turbulent Velocity Field

AbstractWe review recent progress in elucidating the relationship between high-energy radiation and the interstellar medium (ISM) in young supernova remnants (SNRs) with ages of ∼2000 yr, focusing in particular on RX J1713.7−3946 and RCW 86. Both SNRs emit strong nonthermal X-rays and TeV $\gamma $ γ -rays, and they contain clumpy distributions of interstellar gas that includes both atomic and molecular hydrogen. We find that shock–cloud interactions provide a viable explanation for the spatial correlation between the X-rays and ISM. In these interactions, the supernova shocks hit the typically pc-scale dense cores, generating a highly turbulent velocity field that amplifies the magnetic field up to 0.1–1 mG. This amplification leads to enhanced nonthermal synchrotron emission around the clumps, whereas the cosmic-ray electrons do not penetrate the clumps. Accordingly, the nonthermal X-rays exhibit a spatial distribution similar to that of the ISM on the pc scale, while they are anticorrelated at sub-pc scales. These results predict that hadronic $\gamma $ γ -rays can be emitted from the dense cores, resulting in a spatial correspondence between the $\gamma $ γ -rays and the ISM. The current pc-scale resolution of $\gamma $ γ -ray observations is too low to resolve this correspondence. Future $\gamma $ γ -ray observations with the Cherenkov Telescope Array will be able to resolve the sub-pc-scale $\gamma $ γ -ray distribution and provide clues to the origin of these cosmic $\gamma $ γ -rays.

scholarly journals Introducing the HD+B model for pulsar wind nebulae: a hybrid hydrodynamics/radiative approach

2020 ◽  
Vol 494 (3) ◽  
pp. 4357-4370
Author(s):  
B Olmi ◽  
D F Torres
Keyword(s):  
Gamma Ray ◽  
High Energy ◽  
Theoretical Modelling ◽  
X Rays ◽  
Pulsar Wind Nebulae ◽  
New Approach ◽  
Energy Gamma ◽  
Radiative Model ◽  
Pulsar Wind

ABSTRACT Identification and characterization of a rapidly increasing number of pulsar wind nebulae is, and will continue to be, a challenge of high-energy gamma-ray astrophysics. Given that such systems constitute -by far- the most numerous expected population in the TeV regime, such characterization is important not only to learn about the sources per se from an individual and population perspective, but also to be able to connect them with observations at other frequencies, especially in radio and X-rays. Also, we need to remove the emission from nebulae in highly confused regions of the sky for revealing other underlying emitters. In this paper, we present a new approach for theoretical modelling of pulsar wind nebulae: a hybrid hydrodynamic-radiative model able to reproduce morphological features and spectra of the sources, with relatively limited numerical cost.

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