Narrow Results

The radiative transfer equations for multiple inverse Compton scattering of the Cosmic Microwave Background Radiation (CMBR) by the hot intra-cluster electrons are solved numerically. The spherical isothermal and inhomogeneous β model has been considered for the electron distribution. The anisotropy of the CMBR caused by scattering, known as thermal Sunyaev–Zel'dovich effect, along the radial axis of the medium is compared with the analytical solution of Kompaneets equation. The X-ray data of several clusters of galaxies at low redshifts provide an estimation of the central electron density n₀ to be of the order 10^-3. It is found that for this value of n₀ the effect of multiple scattering is negligible. The numerically calculated anisotropy along the radial axis matches well with the analytical solution that describes single scattering. The result incorporating multiple scattering is fitted with the recent observation of Sunyaev–Zel'dovich effect in the cluster Abell 2163. It is shown that if n₀ is greater by an order of magnitude, which could be possible for cluster of galaxies at comparatively higher redshift, multiple scattering would play a significant role at the Wien region of the anisotropy spectrum. A fitting formula for the correction to the Sunyaev–Zel'dovich effect due to multiple scattering is provided.

Based on new radiative transfer numerical evaluations, we reconsider an argument presented by Schack in 1972 that says that saturation of the absorption of infrared radiation by carbon dioxide in the atmosphere sets in as soon as the relative concentration of carbon dioxide exceeds a lower limit of approximately 300ppm. We provide a concise brief and explicit representation of the greenhouse effect of the earth’s atmosphere. We find an equilibrium climate sensitivity (temperature increase $\mathrm{\Delta }T$ due to doubling of atmospheric ${\mathrm{\text{CO}}}_2$ concentration) of $\mathrm{\Delta }T\simeq 0.5^{\circ }$C. We elaborate on the consistency of these results on $\mathrm{\Delta }T$ with results observationally obtained by satellite-based measurements of short-time radiation-flux versus surface-temperature changes.

A Boltzmann transport model for dose calculation in radiation therapy is considered. We formulate an optimal control problem for the desired dose. We prove existence and uniqueness of a minimizer. Based on this model, we derive optimality conditions. The P_N discretization in angle of the full model is considered. We show that the P_N approximation of the optimality system is in fact the optimality system of the P_N approximation, provided that, instead of the usually used Marshak boundary conditions, Mark's boundary conditions are used. Numerical results in one and two dimensions are presented.

In this paper we study a problem in radiotherapy treatment planning. This problem is formulated as an optimization problem of a functional of the radiative flux. It is constrained by the condition that the radiative flux, which depends on position, energy and direction of the particles, is governed by a Boltzmann integro-differential equation. We show the existence, uniqueness and regularity of solutions to this constrained optimization problem in an appropriate function space. The main new difficulty is the treatment of the energy loss term. Furthermore, we characterize optimal controls by deriving first-order optimality conditions.

We consider the imaging of objects buried in unknown heterogeneous media. The medium is probed by using classical (e.g. acoustic or electromagnetic) waves. When heterogeneities in the medium become too strong, inversion methodologies based on a microscopic description of wave propagation (e.g. a wave equation or Maxwell's equations) become strongly dependent on the unknown details of the heterogeneous medium. In some situations, it is preferable to use a macroscopic model for a quantity that is quadratic in the wave fields. Here, such macroscopic models take the form of radiative transfer equations also referred to as transport equations. They can model either the energy density of the propagating wave fields or more generally the correlation of two wave fields propagating in possibly different media. In particular, we consider the correlation of the two fields propagating in the heterogeneous medium when the inclusion is absent and present, respectively. We present theoretical and numerical results showing that reconstructions based on this correlation are more accurate than reconstructions based on measurements of the energy density.

Spectral line profiles produced in an outflow near a neutron star or a black hole can be strongly influenced by gravitational redshifting and by Doppler shifting due to a global motion of plasma. We consider a scenario in which a resonant absorption in a spectral line takes place in the outflowing plasma within several tens of Schwarzschild radii from a compact object. The main goal of this work is to show that under certain conditions a combination of the gravitational redshifting and Doppler blue/redshifting may produce line profiles which can be considered as "fingerprints" of the gravitational field of the compact object, much as P-Cygni profiles are "fingerprints" of stellar winds.

The aim of this paper is to determine how the gravitational field inside a massive star affects the propagation of photons, and to investigate the consequences for the radiation field. The chosen approach consists in modeling the curved metric by an effective refractive medium: from this very simple perspective, the energy transport problem is investigated and the new form of the radiative transfer is derived. Then, the main effects of curvature on the energy balance are discussed and, in particular, it is shown that these effects influence the radiative flux emitted by the star.

The Galactic microquasar GRS 1915 + 105 exhibits various types of light curves. There is, however, no understanding of when a certain type of light curve will be exhibited and only in a handful of cases, the transitions from one type to another have actually been observed. We study the detailed spectral properties in these cases to show that different classes have different ratio of the power-law photon and the blackbody photon. Since the power-law photons are from the Compton cloud, and the intensity of the power-law photon component depends on the degree of interception of the soft photons by the Compton cloud, we conclude that not only the accretion rate, but the accretion flow geometry must also change during a class transition.

Understanding how massive stars die as supernovae (SNe) is a crucial question in modern astrophysics. SNe are powerful stellar explosions and key drivers in the cosmic baryonic cycles by injecting their explosion energy and heavy elements to the interstellar medium that forms new stars. After decades of effort, astrophysicists have built up a stand model for the explosion mechanism of massive stars. However, this model is challenged by new kinds of stellar explosions discovered in the recent transit surveys. In particular, the new population called superluminous SNe, which are a hundred times brighter than typical SNe, is revolutionizing our understanding of SNe. New studies suggest the superluminous SNe are associated with the unusual demise of very massive stars and their extreme SNe powered by the radioactive isotopes or compact objects formed after the explosion. Studying these SNe fills a gap of knowledge between the death of massive stars and their explosions; furthermore, we may apply their intense luminosity to light up the distant universe. This paper aims to provide a timely review of superluminous SNe physics, focusing on the latest development of their theoretical models.

We study the large-time behavior of the solution of an initial-boundary value problem for the equations of 1D motions of a compressible viscous heat-conducting gas coupled with radiation through a radiative transfer equation.

Assuming only scattering processes between matter and photons (neglecting absorption and emission) and suitable hypotheses on the transport coefficients, we prove that the unique weak solution of the problem converges toward the static state.

The Discrete Ordinates Radiation Element Method (DOREM), which is radiative transfer code, is applied for solving phonon transport of nano/microscale materials. The DOREM allows phonon simulation with multi-dimensional complex geometries. The objective of this study is to apply the DOREM to the nano/microstructured materials. It is confirmed that significant changes of the heat transport phenomena with different characteristic length scales and geometries are observed. This study also discusses further variations for understanding of heat transport mechanisms.

We present some results on the radiative signatures of the one zone hadronic model. For this we have solved five spatially averaged, time-dependent coupled kinetic equations which describe the evolution of relativistic protons, electrons, photons, neutrons and neutrinos in a spherical volume containing a magnetic field. Protons are injected and lose energy by synchrotron, photopair and photopion production. We model photopair and photopion using the results of relevant MC codes, like the SOPHIA code in the case of photopion, which give accurate description for the injection of secondaries which then become source functions in their respective equations. This approach allows us to calculate the expected photon and neutrino spectra simultaneously in addition to examining questions like the efficiency and the temporal behaviour of the hadronic models.

We present examples of rapid changes in spectral and timing properties in accretion flows around compact objects and discuss what could be going on in these systems. We find a new way of quantifying the variation of the flow geometry. We show the evolution of the variation of the Comptonization efficiency computed from the ratio of the Comptonized photons and the injected seed photons. The time evolution is a direct consequence of the variation of the accretion rates which changes the hydrodynamic and radiative properties of the flow and therefore the flow geometry.

Although the observed spectra for gamma-ray burst (GRB) prompt emission is well constrained, the underlying radiation mechanism is still not very well understood. We explore photospheric emission in GRB jets by modelling the Comptonization of fast cooled synchrotron photons whilst the electrons and protons are accelerated to highly relativistic energies by repeated energy dissipation events as well as Coulomb collisions. In contrast to the previous simulations, we implement realistic photon-to-particle number ratios of N_γ/N_e∼ 10⁵ or higher, that are consistent with the observed radiation efficiency of relativistic jets. Using our Monte Carlo radiation transfer (MCRaT) code, we can successfully model the prompt emission spectra when the electrons are momentarily accelerated to highly relativistic energies (Lorentz factor ∼ 50 − 100) after getting powered by ∼ 30 − 50 episodic dissipation events in addition to their Coulomb coupling with the jet protons, and for baryonic outflows that originate from moderate optical depths ∼ 20 − 30. We also show that the resultant shape of the photon spectrum is practically independent of the initial photon energy distribution and the jet baryonic energy content, and hence independent of the emission mechanism.

The photospheric emission in the prompt phase is the natural prediction of the original fireball model for gamma-ray burst (GRB) due to the large optical depth (τ > 1) at the base of the outflow, which is supported by the quasi-thermal components detected in several Fermi GRBs. However, which radiation mechanism (photosphere or synchrotron) dominates in most GRB spectra is still under hot debate. The shape of the observed photosphere spectrum from a pure hot fireball or a pure Poynting-flux-dominated outflow has been investigated before. In this work, we further study the photosphere spectrum from a hybrid outflow containing both a thermal component and a magnetic component with moderate magnetization (σ₀ = L_P /L_Th ∼ 1 − 10), by invoking the probability photosphere model. The high-energy spectrum from such a hybrid outflow is a power law rather than an exponential cutoff, which is compatible with the observed Band function in large amounts of GRBs. Also, the distribution of the low-energy indices (corresponding to the peak-flux spectra) is found to be quite consistent with the statistical result for the peak-flux spectra of GRBs best-fitted by the Band function, with similar angular profiles of structured jet in our previous works. Finally, the observed distribution of the high-energy indices can be well understood after considering the different magnetic acceleration (due to magnetic reconnection and kink instability) and the angular profiles of dimensionless entropy with the narrower core.

Over the last decade or so, we have been developing the possible existence of highly magnetized white dwarfs with analytical stellar structure models. While the primary aim was to explain the nature of the peculiar overluminous type Ia supernovae, later on, these magnetized stars were found to have even wider ranging implications including those for white dwarf pulsars, soft gamma-ray repeaters and anomalous X-ray pulsars, as well as gravitational radiation. In particular, we have explored in detail the mass-radius relations for these magnetized stars and showed that they can be significantly different from the Chandrasekhar predictions which essentially leads to a new super-Chandrasekhar mass-limit. Recently, using the stellar evolution code STARS, we have successfully modelled their formation and cooling evolution directly from the magnetized main sequence progenitor stars. Here we briefly discuss all these findings and conclude with their current status in the scientific community.

In this paper, we prove existence and uniqueness of solutions to the coupling between the radiative transfer equation and equations for the population of atoms in a certain state. We also prove the validity of the quasi-static approximation in this context.

In this paper we consider the radiative transfer equations in a bounded domain with non-homogeneous boundary conditions, when the opacity does not depend on the frequency of the photons. We discuss the existence of weak solutions of the radiative transfer system and show that the corresponding Rosseland approximation is robust even when the target equation is parabolic degenerate and the flux on the boundary is non vanishing.

Very Long Baseline Interferometry (VLBI) at sub-millimeter/millimeter wavelengths shows promise at resolving the silhouette of the supermassive black hole at the Galactic Center, Sagittarius A* (Sgr A*), in the near future. In order to accurately test theoretical models of Sgr A* using these observations, a direct comparison of VLBI data to numerical models must be made. We present calculated images and spectra of Sgr A* using accretion disk simulation data from general relativistic magnetohydrodynamics (GRMHD) evolutions. Synchrotron and bremsstrahlung emission models are considered in the optically thin limit, which allows us to solve the radiative transfer equations using only simulation and geodesic data in a post-processing step. We show predictions of millimeter observations at the expected angular resolution limit and the spectrum's variability.

Direct imaging of the “shadow” cast by the black hole event horizon and the test of physics in strong gravity is one of the important goals in modern astronomy. The ongoing Greenland Telescope project in Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) is devoting itself to this exciting area. ASIAA has officially acquired the 12m ALMA (Atacama Large Millimeter/submillimeter Array) Vertex Prototype Telescope in 2011. This telescope, now renamed as the “Greenland Telescope (GLT)”, is going to be moved to the Summit Station in Greenland after upgrades. The first light of GLT at the Summit Station is expected to be available in 2018/2019. Together with SMA (Submillimeter Array) in Hawaii and ALMA (Atacama Large Millimeter/submillimeter Array) in Chile, the angular resolution at sub-mm will reach ∼20 µarcsec, providing an unique opportunity to observe the shadow of the supermassive black hole at the center of M87, and the accretion/jet structure near the event horizon. The GLT project is a collaborative project between ASIAA, Smithsonian Astrophysical Observatory, MIT Haystack Observatory, and National Radio Astronomy Observatory.