External Radiation Field Research Articles

Resonantly enhanced photoionization in correlated three-atomic systems

Modifications of photoionization arising from resonant electron–electron correlations between neighbouring atoms in an atomic sample are studied. The sample contains atomic species A and B, with the ionization potential of A being smaller than the energy of a dipole-allowed transition in B. The atoms are subject to an external radiation field which is near resonant with the dipole transition in B. Photoionization of an atom A may thus proceed via a two-step mechanism: photoexcitation in the subsystem of species B, followed by interatomic Coulombic decay. As a basic atomic configuration, we investigate resonant photoionization in a three-atomic system A–B–B consisting of an atom A and two neighbouring atoms B. It is found that, under suitable conditions, the presence of neighbouring atoms can strongly affect the photoionization process, including its total probability, time development and photoelectron spectra. In particular, comparing our results with those for photoionization of an isolated atom A and a two-atomic system A–B, respectively, we reveal the characteristic impact made by the third atom.

VARIABLE GAMMA-RAY EMISSION INDUCED BY ULTRA-HIGH ENERGY NEUTRAL BEAMS: APPLICATION TO 4C +21.35

The flat spectrum radio quasar (FSRQ) 4C +21.35 (PKS 1222+216) displays prominent nuclear infrared emission from ~1200 K dust. A 70 -- 400 GeV flare with ~10 min variations during half an hour of observations was found by the MAGIC telescopes, and GeV variability was observed on sub-day timescales with the Large Area Telescope on Fermi. We examine 4C +21.35, assuming that it is a source of ultra-high energy cosmic rays (UHECRs). UHECR proton acceleration in the inner jet powers a neutral beam of neutrinos, neutrons and gamma rays from photopion production. The radiative efficiency and production spectra of neutrals formed through photohadronic processes with isotropic external target photons of the broad line region and torus are calculated. Secondary radiations made by this process have a beaming factor ~\delta^5, where \delta is the Doppler factor. The pair-production optical depth for gamma rays and the photopion efficiency for UHECR neutrons as they pass through external isotropic radiation fields are calculated. If target photons come from the broad line region and dust torus, large Doppler factors, \delta >~100 are required to produce rapidly variable secondary radiation with isotropic luminosity >~1e47 erg/s at the pc scale. The \gamma-ray spectra from leptonic secondaries are calculated from cascades initiated by the UHECR neutron beam at the pc-scale region and fit to the flaring spectrum of 4C +21.35. Detection of >~100 TeV neutrinos from 4C +21.35 or other VHE blazars with IceCube or KM3NeT would confirm this scenario.

SIMULATIONS OF HIGH-VELOCITY CLOUDS. II. ABLATION FROM HIGH-VELOCITY CLOUDS AS A SOURCE OF LOW-VELOCITY HIGH IONS

In order to determine if the material ablated from high-velocity clouds (HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI) such as those found in the Galactic halo, we simulate the hydrodynamics of the gas and the time-dependent ionization evolution of its carbon, nitrogen, and oxygen ions. Our suite of simulations examines the ablation of warm material from clouds of various sizes, densities, and velocities as they pass through the hot Galactic halo. The ablated material mixes with the environmental gas, producing an intermediate-temperature mixture that is rich in high ions and that slows to the speed of the surrounding gas. We find that the slow mixed material is a significant source of the low-velocity O VI that is observed in the halo, as it can account for at least ~1/3 of the observed O VI column density. Hence, any complete model of the high ions in the halo should include the contribution to the O VI from ablated HVC material. However, such material is unlikely to be a major source of the observed C IV, presumably because the observed C IV is affected by photoionization, which our models do not include. We discuss a composite model that includes contributions from HVCs, supernova remnants, a cooling Galactic fountain, and photoionization by an external radiation field. By design, this model matches the observed O VI column density. This model can also account for most or all of the observed C IV, but only half of the observed N V.

Classical study of ultrastrong nonperturbative-field interactions with a one-electron atom: Validity of the dipole approximation for the bound-state interaction

We present a classical, relativistic Monte Carlo calculation investigating the role of the magnetic field in bound-state dynamics and ionization for atoms in ultrastrong external radiation fields. Atoms with atomic numbers $1\ensuremath{\leqslant}Z\ensuremath{\leqslant}20$ in external fields from 0.01 to ${10}^{3}$ a.u. are studied. We show that, when calculating ionization rates, the dipole approximation yields accurate values compared to a full classical electromagnetic laser field for intensities up to ${10}^{23}$ W/cm${}^{2}$. The calculations indicate that the quasistatic approximation is valid for external-field frequencies less than 1/20 of the Kepler-orbit time for the ionizing state. For ionization at fields above 100 a.u., the magnetic field affects the atom by altering the portion of the bound state that ionizes and deflecting the mean emission angle of the photoelectron angular distributions, although no change in the width of the final-state angular emission about this mean occurs. The photoelectron distribution and change in the ionizing bound state are seen as an indication of nonperturbative magnetic field effects for atoms in ultrastrong radiation fields.

The effect of temperature mixing on the observable (T,β)-relation of interstellar dust clouds

Detailed studies of the shape of dust emission spectra are possible thanks to the current instruments capable of observations in several sub-millimetre bands (e.g., Herschel and Planck). However, some controversy remains even on the basic effects resulting from the mixing of temperatures along the line-of-sight. Studies have suggested either a positive or a negative correlation between the colour temperature T_C and the observed spectral index beta_Obs. Our aim is to show that both cases are possible and to determine the factors leading to either behaviour. We start by studying the sum of two or three modified black bodies of different temperature. With radiative transfer modelling, we examine the probability distributions of the dust mass as a function of the physical dust temperature. With these results as a guideline, we examine the (T_C, beta_Obs) relations for different sets of clouds. Even in the case of modified blackbodies at temperatures T_0 and T_0+ Delta T_0, the correlation between T_C and beta_Obs can be either positive or negative. If one compares models where Delta T_0 is varied, the correlation is negative. If the models differ in their mean temperature T_0 rather than in Delta T_0, the correlation remains positive. Radiative transfer models show that externally heated clouds have different mean temperatures but the widths of their temperature distributions are rather similar. Thus, the correlation between T_C and beta_Obs is expected to be positive. The same result applies to clouds illuminated by external radiation fields of different intensity. For internally heated clouds a negative correlation is the more likely alternative. If the signal-to-noise ratio is high, the observed negative correlation could be explained by the temperature dependence of the dust optical properties but that intrinsic dependence could be even steeper than the observed one.

On Seeking the Worst Case of Human Exposure to the External EMF Radiation

Electromagnetic field (EMF) radiation is very controversial issue. For one hand, many therapeutic and diagnostic applications are proved effective; on the other hand, excessive exposure may cause adverse biological effects. In order to prevent the over-exposure, the worst results are frequently discussed for the safety limits. The basis of the issue is to establish the relationship between the external radiation field and the dose (SAR) distribution in the whole body and the specified tissues. External sources, dosimetric methods and the digital human models are the key elements which could determine that relationship. Because of lack of the non-invasive measurement techniques, numerical methods such as FDTD, FEM and MoM are frequently utilized to qualify the in-body dose distribution. Variability introduced for the numerical methods are insignificant. Given the external radiation field are fixed; the influential factors were include the tissue dielectric properties, postures/statures, anatomical structures, physique features and etc. Practically, they are often interwoven to each other. Many studies were focusing on the surrogate models and the morphing was available in order to find out the worst case with a certain external exposure.

THE COMPLEXITY THAT THE FIRST STARS BROUGHT TO THE UNIVERSE: FRAGILITY OF METAL-ENRICHED GAS IN A RADIATION FIELD

The initial mass function (IMF) of the first (Population III) stars and Population II (Pop II) stars is poorly known due to a lack of observations of the period between recombination and reionization. In simulations of the formation of the first stars, it has been shown that, due to the limited ability of metal-free primordial gas to cool, the IMF of the first stars is a few orders of magnitude more massive than the current IMF. The transition from a high-mass IMF of the first stars to a lower-mass current IMF is thus important to understand. To study the underlying physics of this transition, we performed several simulations using the cosmological hydrodynamic adaptive mesh refinement code Enzo for metallicities of 10^{-4}, 10^{-3}, 10^{-2}, and 10^{-1} Z_{\odot}. In our simulations we include a star formation prescription that is derived from a metallicity dependent multi-phase ISM structure, an external UV radiation field, and a mechanical feedback algorithm. We also implement cosmic ray heating, photoelectric heating and gas-dust heating/cooling, and follow the metal enrichment of the ISM. It is found that the interplay between metallicity and UV radiation leads to the co-existence of Pop III and Pop II star formation in non-zero metallicity (Z/Z_{\odot} \geq10^{-2}) gas. A cold (T<100 K) and dense (\rho>10^{-22} g cm^{-3}) gas phase is fragile to ambient UV radiation. In a metal-poor (Z/Z_{\odot} \leq10^{-3}) gas, the cold and dense gas phase does not form in the presence of a radiation field of F_{0}\sim10^{-5}-10^{-4} erg cm^{-2} s^{-1}. Therefore, metallicity by itself is not a good indicator of the Pop III-Pop II transition. Metal-rich (Z/Z_{\odot}\geq10^{-2}) gas dynamically evolves two to three orders of magnitude faster than metal poor gas (Z/Z_{\odot}\leq10^{-3}). The simulations including SNe show that pre-enrichment of the halo does not affect the mixing of metals.

The beaming of external Compton emission

We consider a relativistically moving blob consisting of an isotropic electron distribution that Compton-scatters photons from an external isotropic radiation field. We compute the resulting beaming pattern, i.e. the distribution of the scattered photons, in the blob frame as well as in the observer’s frame by using the full Klein–Nishina cross section and the exact incident photon distribution. In the Thomson regime the comparison of our approach with Dermer (1995) results in concurrent characteristics but different absolute number of the scattered photons by a factor of f corr = 3.09. Additionally, our calculation yields a slightly lower boost factor which varies the more from the corresponding value in Dermer (1995) the higher the spectral index p of the electron distribution gets.

Non-equilibrium ionization states and cooling rates of photoionized enriched gas

Non-equilibrium (time-dependent) cooling rates and ionization state calculations are presented for low-density gas enriched with heavy elements (metals) and photoionized by external ultraviolet/X-ray radiation. We consider a wide range of gas densities and metallicities and also two types of external radiation field: a power-law and an extragalactic background spectra. We have found that both cooling efficiencies and ionic composition of enriched photoionized gas depend significantly on the gas metallicity and density, the flux amplitude and the shape of ionizing radiation spectrum. The cooling rates and ionic composition of the gas in non-equilibrium photoionization models differ strongly (by a factor of several) from those in photoequilibrium due to overionization of the ionic states in the non-equilibrium case. The difference is maximal at low values of the ionization parameter and similar in magnitude to that between the equilibrium and non-equilibrium cooling rates in the collisionally controlled gas. In general, the non-equilibrium effects are notable at T≲ 106 K. In this temperature range, the extent of mismatch between the two ionic states and their ratios between the photoequilibrium and the photo-non-equilibrium models reach a factor of several. The net result is that the time-dependent energy losses due to each chemical element (i.e. the contributions to the total cooling rate) differ significantly from the photoequilibrium ones. We advocate the use of non-equilibrium cooling rates and ionic states for gas with near-solar (and above) metallicity exposed to an arbitrary ionizing radiation flux. We provide a parameter space (in terms of temperature, density, metallicity and ionizing radiation flux), where the non-equilibrium cooling rates are to be used. More quantitatively, the non-equilibrium collisional cooling rates and ionization states are a better choice for the ionization parameter log U≲−5. The difference between the photoequilibrium and the photo-non-equilibrium decreases with the ionization parameter growth, and the photoequilibrium can be used for ionization parameter as high as log U≳−2 for Z≲ 10−2 Z⊙ and log U≳ 0 for Z∼ Z⊙. Thus, the non-equilibrium calculations should be used for the ionization parameter range between the above-mentioned values. In general, where the physical conditions favour collisional ionization, the non-equilibrium (photo)ionization calculations should be conducted.

Dynamics of electron–positron pairs in a vacuum polarized by an external radiation field

The polarization of the vacuum under the action of an external classical field of electromagnetic radiation is investigated in the stationary regime. The electron–positron pairs interact both with the external field and with their own polarization field. For a macroscopic piece of vacuum the pairs are condensed on the low-momenta states and tend to form a quasi-localized electron–positron plasma of pairs, with single-particle states labeled by the position vector. In the polarization process under the action of a classical field of radiation the electron–positron and photon dynamics can be treated by means of classical fields. Under these circumstances, the corresponding coupled non-linear equations of motion are solved. It is shown that the pair dynamics consists of quasi-stationary single-particle states, while the polarization field reduces to a static magnetic field. The single-particle ‘energy’ (temporal phase) due to a monochromatic external field exhibits a spatial distribution characteristic of a stationary wave. Both the pair energy and the polarization energy are computed. Their values are extremely small, even for highly focused, reasonably high, external fields. The number of pairs is determined by the external energy. Under the action of a classical field the polarized vacuum is magnetized, and the corresponding (very low) magnetic susceptibility (the refractive index of the vacuum) is computed.

MULTI-WAVELENGTH OBSERVATIONS OF THE FLARING GAMMA-RAY BLAZAR 3C 66A IN 2008 OCTOBER

The BL Lacertae object 3C 66A was detected in a flaring state by the Fermi Large Area Telescope (LAT) and VERITAS in 2008 October. In addition to these gamma-ray observations, F-GAMMA, GASP-WEBT, PAIRITEL, MDM, ATOM, Swift, and Chandra provided radio to X-ray coverage. The available light curves show variability and, in particular, correlated flares are observed in the optical and Fermi-LAT gamma-ray band. The resulting spectral energy distribution can be well fit using standard leptonic models with and without an external radiation field for inverse-Compton scattering. It is found, however, that only the model with an external radiation field can accommodate the intra-night variability observed at optical wavelengths.

Image-guided tumor ablation for the treatment of recurrent non-small cell lung cancer within the radiation field

The treatment options for non-small-cell lung cancer (NSCLC) that recurs after irradiation are limited. Image-guided percutaneous thermal ablation is an effective option in treating NSCLC that may provide an alternative to reirradiation. The purpose of this paper is to determine the survival and palliative benefit of image-guided percutaneous thermal ablation in the treatment of NSCLC that recurred within the treatment field of prior external beam radiation therapy. Twenty patients, median age 70, who had NSCLC recurrences following irradiation were treated with image-guided thermal ablation. Kaplan-Meier analysis was used to assess survival benefit and subjective pain reports were used to investigate pain relief. The median survival time was 13.1±SE 1.4 months and the median survival time without local recurrence was 8.5±1.6 months. Eight patients (40%) recurred locally after a median of 3.3 months. Seven out of ten patients (70%) presenting with significant pain had decreased pain at initial post-ablation evaluation. Following the 25 ablations, there were no Grade IV or V, 1 Grade III, 3 Grade II, and 23 Grade I complications. Thermal ablation offers a potential survival benefit compared with other available modalities for the treatment of NSCLC recurring within a previously irradiated field. This promising technique has a good safety profile and may also be useful in providing symptomatic relief.

The relativistic operator of interaction of two quasimolecular electrons as a third-order effect of quantum electrodynamics

We solve the problem of interaction two quasimolecular electrons located at an arbitrary separation near different atoms (nuclei). We consider third-order effects in quantum electrodynamics, which include the virtual photon exchange between electrons with emission (absorption) of a real photon. We obtain the general expression for matrix elements of the operator of the effective interaction energy of two quasimolecular electrons with the external radiation field, which allows calculating probabilities of inelastic processes with rearrangement at slow collisions of multicharge ions with relativistic atoms. We demonstrate that consistently taking the natural condition of the interaction symmetry with respect to the two electrons into account results in the appearance of additional terms in the operators of spin-orbit, spin-spin, and retarded interactions compared with the previously obtained expressions for these operators. We construct the operator of the dipole-dipole interaction of two neutral atoms located at an arbitrary separation.

A conjecture on the use of quantum algebras in the treatment of discrete systems

The interactions between atomic spin-states, and between them and an external radiation field, can be described in terms of quantum algebras by a trade-off of bosonic and fermionic degrees of freedom and q-deformed schemes. In this Letter we discuss the use of this concept concerning the calculation of a spin observable, like the spin squeezing.

Properties of dust in the high-latitude translucent cloud L1780

Context. Lynds 1780 is a high-latitude cloud where, based on 2MASS, the maximum visual extinction is A max V = 4 mag at a resolution of 3'. In LDN1780, increased far-infrared (FIR) emissivity of dust grains has been observed, and the infrared emission is found to peak at different locations at different wavelengths. Aims. By modelling the FIR observations, we try to quantify spatial variations of dust properties and to determine to what extent the observations could be affected by the asymmetry of the heating radiation field. Methods. We have constructed a three-dimensional cloud model and, with the help of radiative transfer calculations, compare its predictions with the FIR surface brightness measurements of LDN1780 performed with the ISO satellite. The effects of anisotropic radiation, its attenuation in a diffuse extinction layer around the cloud, and variations in the dust properties are investigated. Results. Asymmetry of the radiation field is found to have only a small effect on the morphology of mid- and far-infrared surface brightness. The general agreement between observations and the model predictions is improved by assuming the presence of a low extinction external layer with A V ~ 0.25 mag. However, to explain the changes in the relative intensity of mid- and far-infrared bands, one has to assume strong variations in the relative abundance of small and large grain components and, at the very centre of the cloud, enhanced emissivity of large grains. Conclusions. The separate emission maxima at different wavelengths in LDN1780 result from real variations in spatial distributions of dust components. Modifications to standard dust models, including a 30% increase in the FIR emissivity, are needed to explain the far-infrared observations towards the centre of LDN1780. The relative abundances of dust components are found to be very sensitive to the strength of the external radiation field.

The generalized Breit operator of the interaction between two quasimolecular electrons in the framework of the third-order effects of quantum electrodynamics

The problem of the interaction of two quasimolecular electrons located at an arbitrary distance from each other and near different atoms (nuclei) is solved. Effects of the third order of quantum electrodynamics, which include the exchange of a virtual photon between the electrons and emission (absorption) of a real photon, are considered. The general expression for the matrix elements of the operator of the effective interaction energy of the two quasimolecular electrons with the external radiation field, which allows us to calculate the probabilities of inelastic processes with rearrangement in slow collisions of multiply-charged ions with relativistic atoms, is obtained. Carrying out consistently the procedure of symmetrization of the retardation factor with respect to both the electrons results in the appearance of additional terms in the relativistic operator of the interaction of two quasimolecular electrons in comparison with both the standard and generalized Breit operators known previously.

TIMING SIGNATURES OF THE INTERNAL-SHOCK MODEL FOR BLAZARS

We investigate the spectral and timing signatures of the internal-shock model for blazars. For this purpose, we develop a semi-analytical model for the time-dependent radiative output from internal shocks arising from colliding relativistic shells in a blazar jet. The emission through synchrotron and synchrotron-self Compton (SSC) radiation as well as Comptonization of an isotropic external radiation field are taken into account. We evaluate the discrete correlation function (DCF) of the model light curves in order to evaluate features of photon-energy dependent time lags and the quality of the correlation, represented by the peak value of the DCF. The almost completely analytic nature of our approach allows us to study in detail the influence of various model parameters on the resulting spectral and timing features. This paper focuses on a range of parameters in which the gamma-ray production is dominated by Comptonization of external radiation, most likely appropriate for gamma-ray bright flat-spectrum radio quasars (FSRQs) or low-frequency peaked BL Lac objects (LBLs). In most cases relevant for FSRQs and LBLs, the variability of the optical emission is highly correlated with the X-ray and high-energy (HE: > 100 MeV) gamma-ray emission. Our baseline model predicts a lead of the optical variability with respect to the higher-energy bands by 1 - 2 hours and of the HE gamma-rays before the X-rays by about 1 hour. We show that variations of certain parameters may lead to changing signs of inter-band time lags, potentially explaining the lack of persistent trends of time lags in most blazars.

2d QUANTUM DOTS IN POLYCHROMATIC RADIATION FIELDS: EFFECTS OF FREQUENCY MIXING, PHASE AND ANHARMONICITY ON THE FREEZING OF DYNAMICS

We explore the pattern of time evolution of a harmonically confined single carrier 2-d quantum dot when an external time varying polychromatic radiation field is switched on. The radiation field is composed of four mutually prime "nonresonant" frequencies. For given strengths of the confining field, cyclotron frequency, intensities, and oscillation frequencies of the external field, the system reveals a counter-intuitive long-time dynamics leading to a kind of localization in the unperturbed state space. The presence of cubic and quartic anharmonicity in the confining fields, and phase shifts in the external radiation fields, bring in new features in the dynamics. Time dependent Hellman–Feynmann theorem is invoked to gain insight into the frequency resolved absorption spectrum.

Ion Plasma Responses to External Electromagnetic Fields

The response of ion plasmas to external radiation fields is investigated in a quantum mechanical formalism.We focus on the total electric field within the plasma. For general bandpass signals three frequency regions can be distinguished in terms of the plasma frequency. For low frequencies, the external field is shielded. For high frequencies, the field is not modified. Resonant behavior of the plasma appears for frequencies near the plasma frequency: large internal electric fields and induced currents are present. These effects may be relevant for biological systems. The model is therefore extended to a two-species plasma and additional interactions are studied. The response is not essentially altered. To make the models more realistic, a so-called bath is included. In the weak coupling approximation the resonance frequency is shifted and some damping occurs. Finite temperature effects on the electric field are absent. The energy of the system, however, depends on temperature.

KLEIN–NISHINA EFFECTS ON OPTICALLY THIN SYNCHROTRON AND SYNCHROTRON SELF-COMPTON SPECTRUM

We present analytic approximations to the optically thin synchrotron and synchrotron self-Compton (SSC) spectra when Klein-Nishina (KN) effects are important and pair production and external radiation fields can be neglected. This theory is useful for analytical treatment of radiation from astrophysical sources, such as gamma-ray bursts (GRBs), active galactic nuclei, and pulsar wind nebula, where KN effects may be important. We consider a source with continuous injection of relativistic electrons with a power-law energy distribution above some typical injection energy. We find that the synchrotron-SSC spectra can be described by a broken power law, and provide analytic estimates for the break frequencies and power-law indices. In general, we show that the dependence of the KN cross section on the energy of the upscattering electron results in a hardening of the energy distribution of fast cooling electrons and therefore in a hardening of the observed synchrotron spectrum. As a result the synchrotron spectrum of fast cooling electrons, below the typical injection energy, can be as hard as F_ν α ν^0, instead of the classical ν^(–1/2) when KN effects are neglected. The synchrotron energy output can be dominated by electrons with energy above the typical injection energy. We solve self-consistently for the cooling frequency and find that the transition between synchrotron and SSC cooling can result in discontinuous variations of the cooling frequency and the synchrotron and SSC spectra. We demonstrate the application of our results to theory by applying them to prompt and afterglow emission models of GRBs.