Relativistic Code Research Articles

POLARIZED EMISSION OF SAGITTARIUS A*

We explore the parameter space of the two temperature pseudo-Newtonian Keplerian accretion flow model for the millimeter and shorter wavelength emission from Sagittarius A*. A general relativistic ray-tracing code is used to treat the radiative transfer of polarized synchrotron emission from the flow. The synchrotron self-Comptonization and bremsstrahlung emission components are also included. It is shown that the model can readily account for the millimeter to sub-millimeter emission characteristics with an accretion rate of ~6x10^17g.s^-1 and an inclination angle of ~40 deg. However, the corresponding model predicted near-infrared and X-ray fluxes are more than one order of magnitude lower than the observed 'quiescent' state values. While the extended quiescent-state X-ray emission has been attributed to thermal emission from the large-scale accretion flow, the NIR emission and flares are likely dominated by emission regions either within the last stable orbit of a Schwarzschild black hole or associated with outflows. With the viscous parameter derived from numerical simulations, there is still a degeneracy between the electron heating rate and the magnetic parameter. A fully general relativistic treatment with the black hole spin incorporated will resolve these issues.

Resonant X-ray Enhancement of the Auger Effect in High-Z Atoms, Molecules, and Nanoparticles: Potential Biomedical Applications

It is shown that X-ray absorption can be considerably enhanced at resonant energies corresponding to K-shell excitation into higher shells with electron vacancies following Auger emissions in high-Z elements and compounds employed in biomedical applications. We calculate Auger resonant probabilities and cross sections to obtain total mass attenuation coefficients with resonant cross sections and detailed resonance structures corresponding to Kalpha, Kbeta, Kgamma, Kdelta, and Keta complexes lying between 6.4-7.1 keV in iron and 67-80 keV in gold. The basic parameters were computed using the relativistic atomic structure codes and the R-matrix codes. It is found that the average enhancement at resonant energies is up to a factor of 1000 or more for associated K --> L, M, N, O, P transitions. The resonant energies in high-Z elements such as gold are sufficiently high to ensure significant penetration in body tissue, and hence the possibility of achieving X-radiation dose reduction commensurate with resonant enhancements for cancer theranostics using high-Z nanoparticles and molecular radiosensitizing agents embedded in malignant tumors. The in situ deposition of X-ray energy, followed by secondary photon and electron emission, will be localized at the tumor site. We also note the relevance of this work to the development of novel monochromatic or narrow-band X-ray emission sources for medical diagnostics and therapeutics.

Electronic structure and Fermi surface of strongly ferromagneticU2N2Z-type(Z=Sb,Te,Bi)compounds from first principles

The electronic structures of the uranium ternaries of the ${\text{U}}_{2}{\text{N}}_{2}Z$-type $(Z=\text{Sb},\text{Te},\text{Bi})$, crystallizing in the tetragonal $I4/mmm$ structure and having among uranium systems relatively high values of both their ferromagnetic (FM) transition temperatures and ordered magnetic moments (mms), have been investigated ab initio. They were calculated employing the local-spin density-functional theory with different versions of the orbital polarization correction (OPC) applied within the fully relativistic and full potential local-orbital band-structure code. The obtained results predict all these materials to be metallic both in the ferromagnetically ordered and nonmagnetically ordered (NMO) states. However, the bottoms of their conduction bands in the NMO states occur merely about 0.3 eV below the Fermi level, ${\text{E}}_{\text{F}}$, and a pronounced gap in ${\text{U}}_{2}{\text{N}}_{2}\text{Te}$ or pseudogaps in the two remaining compounds are opening just below these bands, which makes such a metallic state unstable. The considerable reduction in these gap or pseudogaps in the FM states cause a stabilization of a metallic behavior. A covalently metallic bonding character of these ternaries has been obtained in all calculations. It appeared that the only uranium atoms create predominantly metallic bonding due to a fairly strong hybridization between the U $5f$ and U $6d$ electrons. The U $5f$ electrons contributing also to the covalent bonding have somewhat dual character, i.e., partly localized and itinerant. Contrary to the Sb- and Bi-based ternaries ordered along the $c$ axis, as deduced from previous neutron powder diffraction studies, ${\text{U}}_{2}{\text{N}}_{2}\text{Te}$ has a canted FM structure with its mms tilted from the $c$ axis by about $70\ifmmode^\circ\else\textdegree\fi{}$. This fact has also been revealed by the present theoretical results. These data also point to the fact that the telluride, unlike the two other compounds, exhibits an almost half-metallic behavior. For all three ternaries, using the OPXC version of OPC the computed mms are in accord with the published experimental data. In turn, the Fermi surfaces of investigated ternaries have quasi-two-dimensional properties and quite unique nesting features along directions of their easy magnetization axes.

Allowed and forbidden transition parameters for Fe XV

A comprehensive set of fine structure energy levels, oscillator strengths ( f ), line strengths ( S ), and radiative decay rates ( A ) for bound–bound transitions in Fe XV is presented. The allowed electric dipole (E1) transitions were obtained from the relativistic Breit–Pauli R-matrix method which is based on the close coupling approximation. A total of 507 fine structure energy levels with n ⩽ 10, l ⩽ 9, and 0 ⩽ J ⩽ 10 are found. They agree within 1% with the available observed energies. These energy levels yield a total of 27,812 E1, same-spin multiplets and intercombination transitions. The A values are in good agreement with those compiled by NIST and other existing values for most transitions. Forbidden transitions are obtained from a set of 20 configurations with orbitals ranging from 1s to 5f using the relativistic code SUPERSTRUCTURE (SS) in the Breit–Pauli approximation. From a set of 123 fine structure levels, a total of 6962 S and A values are presented for forbidden electric quadrupole (E2), electric octupole (E3), magnetic dipole (M1), and magnetic quadrupole (M2) transitions. The energies from SS calculations agree with observed energies to within 1–3%. A values for E2, M1 transitions agree very well with the available values for most transitions while those for M2 transitions show variable agreement. The large set of transition parameters presented should be applicable for both diagnostics and spectral modeling in the X-ray, ultraviolet, and optical regions of astrophysical plasmas.

X-ray spectrum in the range (6-12) Å emitted by laser-produced plasma of samarium

A detailed analysis of the x-ray spectrum emitted by laser-produced plasma of samarium (6-12 A) is presented, using ab initio calculations with the HULLAC relativistic code and isoelectronic considerations. Resonance 3d-nf (n=4 to 7), 3p-4d, 3d-4p, and 3p-4s transitions in Ni samarium ions and in neighboring ionization states (from Mn to Zn ions) were identified. The experiment results show changes in the fine details of the plasma spectrum for different laser intensities.

DIRECT CALCULATION OF THE RADIATIVE EFFICIENCY OF AN ACCRETION DISK AROUND A BLACK HOLE

Numerical simulation of magnetohydrodynamic (MHD) turbulence makes it possible to study accretion dynamics in detail. However, special effort is required to connect inflow dynamics (dependent largely on angular momentum transport) to radiation (dependent largely on thermodynamics and photon diffusion). To this end we extend the flux-conservative, general relativistic MHD code HARM from axisymmetry to full 3D. The use of an energy conserving algorithm allows the energy dissipated in the course of relativistic accretion to be captured as heat. The inclusion of a simple optically thin cooling function permits explicit control of the simulated disk's geometric thickness as well as a direct calculation of both the amplitude and location of the radiative cooling associated with the accretion stresses. Fully relativistic ray-tracing is used to compute the luminosity received by distant observers. For a disk with aspect ratio H/r ~ 0.1 accreting onto a black hole with spin parameter a/M = 0.9, we find that there is significant dissipation beyond that predicted by the classical Novikov-Thorne model. However, much of it occurs deep in the potential, where photon capture and gravitational redshifting can strongly limit the net photon energy escaping to infinity. In addition, with these parameters and this radiation model, significant thermal and magnetic energy remains with the gas and is accreted by the black hole. In our model, the net luminosi ty reaching infinity is 6% greater than the Novikov-Thorne prediction. If the accreted thermal energy were wholly radiated, the total luminosity of the accretion flow would be ~20% greater than the Novikov-Thorne value.

Auger transition rates for Ar-like ions

The Auger transition rates for Ar-like ions have been calculated by the relativistic configuration interaction code and flexible atomic code. The calculations have been carried out for the atomic numbers from 18 to 54, that is, for argon to xenon. The calculated data for argon is shown to be in a good agreement with experimental data and other calculated data.

One-to-one direct modeling of experiments and astrophysical scenarios: pushing the envelope on kinetic plasma simulations

There are many astrophysical and laboratory scenarios where kinetic effects play an important role. These range from astrophysical shocks and plasma shell collisions to high intensity laser–plasma interactions, with applications to fast ignition and particle acceleration. Further understanding of these scenarios requires detailed numerical modeling, but fully relativistic kinetic codes are computationally intensive, and the goal of one-to-one direct modeling of such scenarios and direct comparison with experimental results is still difficult to achieve. In this paper we discuss the issues involved in performing kinetic plasma simulations of experiments and astrophysical scenarios, focusing on what needs to be achieved for one-to-one direct modeling and the computational requirements involved. We focus on code efficiency and new algorithms, specifically on parallel scalability issues, namely, on dynamic load balancing, and on high-order interpolation and boosted frame simulations to optimize simulation performance. We also discuss the new visualization and data mining tools required for these numerical experiments and recent simulation work illustrating these techniques is also presented.

MAGNETIC RECONNECTION AT THE TERMINATION SHOCK OF A STRIPED PULSAR WIND

Most of the rotational luminosity of a pulsar is carried away by a relativistic magnetized wind in which the matter energy flux is negligible compared to the Poynting flux. However, observations indicate that most of the Poynting flux is eventually converted into ultrarelativistic particles. The mechanism responsible for this transformation remains poorly understood. Near the equatorial plane of an obliquely rotating pulsar magnetosphere, the magnetic field reverses polarity with the pulsar period, forming a wind with oppositely directed field lines, called a striped wind. We study the conditions required for magnetic energy release at the termination shock of the striped pulsar wind. Magnetic reconnection is considered via analytical methods and 1D relativistic PIC simulations. An analytical condition on the upstream parameters for partial and full magnetic reconnection is derived from the conservation laws of energy, momentum and particle number density across the relativistic shock. Furthermore, by using a 1D relativistic PIC code, we study in detail the reconnection process at the termination shock for different upstream Lorentz factors and magnetizations, and find good agreement with our analytical criterion. Thus, alternating magnetic fields annihilate easily at relativistic highly magnetized shocks. We apply our criterion for dissipation to deduce bounds on the pair multiplicity, κ, in the pulsar wind.

Relativistic radiation magnetohydrodynamics in dynamical spacetimes: Numerical methods and tests

Many systems of current interest in relativistic astrophysics require a knowledge of radiative transfer in a magnetized gas flowing in a strongly curved, dynamical spacetime. Such systems include coalescing compact binaries containing neutron stars or white dwarfs, disks around merging black holes, core-collapse supernovae, collapsars, and gamma-ray burst sources. To model these phenomena, all of which involve general relativity, radiation (photon and/or neutrino), and magnetohydrodynamics (MHD), we have developed a general relativistic code capable of evolving MHD fluids and radiation in dynamical spacetimes. Our code solves the coupled Einstein-Maxwell-MHD-radiation system of equations both in axisymmetry and in full $3+1$ dimensions. We evolve the metric by integrating the BSSN (Baumgarte-Shapiro-Shibata-Nakamura) equations, and use a conservative, high-resolution shock-capturing scheme to evolve both the MHD and radiation moment equations. In this paper, we implement our scheme for optically thick gases and gray-body opacities. Our code gives accurate results in a suite of tests involving radiating shocks and nonlinear waves propagating in Minkowski spacetime. In addition, to test our code's ability to evolve the relativistic radiation-MHD equations in strong-field dynamical spacetimes, we study ``thermal Oppenheimer-Snyder collapse'' to a black hole and find good agreement between analytic and numerical solutions.

Relativistic multi-configuration calculations of Kα and Kβ X-ray transitions for highly ionized Mo ions

Abstract A relativistic multi-configuration Dirac–Fock technique has been used for computing the transition wavelengths, transition probabilities, absorption oscillator strengths, and line strengths for the Kα and Kβ line transitions of He-like to Ne-like molybdenum ions. The contributions from the Breit interaction, quantum electrodynamic corrections, and nuclear mass corrections to the initial and final levels have been taken into account. Transitions from the ground state to the n = 2 and 3 states of He-like and Li-like molybdenum have been calculated using two sets of configuration-interaction wavefunctions. One set of wavefunctions was generated using the fully relativistic GRASPVU code and the other was obtained using GRASP 2 , the calculated transition wavelength, transition probabilities, and absorption strengths obtained by these two independent methods are in very good agreement and there is good agreement between these results and recent theoretical and experimental results. These data provide reference values for the level lifetimes, charge state distributions, and average charge of molybdenum plasmas.

Enhanced laser ion acceleration from mass-limited targets

AbstractLaser interactions with mass-limited targets are studied here via numerical simulations using our relativistic electromagnetic two-dimensional particle-in cell code including all three-velocity components. Analytical estimates are derived to clarify the simulation results. Mass-limited targets preclude the undesirable spread of the absorbed laser energy out of the interaction zone. Mass-limited targets, such as droplets, are shown here to enhance the achievable fast ion energy significantly due to an increase in the hot electron concentration. For given target dimensions, the existence is demonstrated for an optimum laser beam diameter when ion acceleration is efficient and geometrical energy losses are still acceptable. Ion energy also depends on the target geometrical form and rounded targets are found to enhance the energy of accelerated ions. The acceleration process is accompanied by generation of the dipole radiation in addition to the ordinary scattering of the electromagnetic wave.

Stellar explosions powered by the Blandford–Znajek mechanism

Abstract In this Letter, we briefly describe the first results of our numerical study on the possibility of magnetic origin of relativistic jets of long-duration gamma-ray bursters within the collapsar scenario. We track the collapse of massive rotating stars on to a rotating central black hole using axisymmetric general relativistic magnetohydrodynamic code that utilizes a realistic equation of state of stellar matter, and takes into account the cooling associated with emission of neutrinos and the energy losses due to dissociation of nuclei. The neutrino heating is not included. We describe the solution for one particular model where the progenitor star has magnetic field B= 3 × 1010 G. The solution exhibits strong explosion driven by the Poynting-dominated jets whose power exceeds 2 × 1051 erg s−1. The jets originate mainly from the black hole, and they are powered via the Blandford–Znajek mechanism.

Energy levels and radiative rates for transitions in B-like to F-like Kr ions (Kr XXXII–XXVIII)

Abstract Energy levels, radiative rates, oscillator strengths, line strengths, and lifetimes have been calculated for transitions in B-like to F-like Kr ions, Kr XXXIII–XXVIII. For the calculations, the fully relativistic GRASP code has been adopted, and results are reported for all electric dipole (E1), electric quadrupole (E2), magnetic dipole (M1), and magnetic quadrupole (M2) transitions among the lowest 125, 236, 272, 226, and 113 levels of Kr XXXII, Kr XXXI, Kr XXX, Kr XXIX, and Kr XXVIII, respectively, belonging to the n ⩽ 3 configurations. Comparisons are made with earlier available theoretical and experimental results, and some discrepancies have been noted and explained.

Separation of Accelerated Electrons and Positrons in the Relativistic Reconnection

We study the acceleration of electrons and positrons in a relativistic magnetic field reconnection using a 2.5 dimensional particle-in-cell electromagnetic relativistic code. We consider a model with two current sheets and periodic boundary conditions. The electrons and positrons are very effectively accelerated during the tearing and coalescence processes of the reconnection. We found that near the X-points of the reconnection the positions of electrons and positrons differ. This separation process is in agreement with those studied in previous papers analytically or by test particle simulations. We expect that in dependence on the magnetic field connectivity this local separation can lead to global spatial separation of the accelerated electrons and positrons. A similar simulation in an electron-proton plasma with the proton-electron mass ratio mi/me = 16 is made.

The maximum mass of differentially rotating neutron stars

Binary neutron star merger lead to the formation of a massive, rapidly and differentially rotating neutron star (or a strange star) or to the prompt collapse to a black hole. The maximum mass of a differentially rotating remnant is crucial for distinguishing between these two final objects. We study the effect of differential rotation on the maximum mass of neutron stars (strange stars). We numerically construct stellar models using a highly accurate relativistic code based on a multi-domain spectral method. We find much larger mass increases for strange stars than for neutron stars for the same degree of differential rotation.

Theoretical calculation of the transition spectra of highly charged tungsten ions in the EUV region

A relativistic atomic code, MCDF (Multi-Configuration Dirac–Fock), was used to calculate the transition probabilities of highly charged tungsten ions ranging from W 33+ to W 37+ . Transition probabilities at the wavelength range of 40–75 Å were obtained for the transitions from 4p 5 4d n +1 +4p 6 4d n −1 4f to 4p 6 4d n by using the electronic configuration of Mo ions.

Periodic standing-wave approximation: Post-Minkowski computations

The periodic standing-wave method studies circular orbits of compact objects coupled to helically symmetric standing-wave gravitational fields. From this solution an approximation is extracted for the strong field, slowly inspiralling motion of black holes and binary stars. Previous work on this model has dealt with nonlinear scalar models, and with linearized general relativity. Here we present the results of the method for the post-Minkowski (PM) approximation to general relativity, the first step beyond linearized gravity. We compute the PM approximation in two ways: first, via the standard approach of computing linearized gravitational fields and constructing from them quadratic driving sources for second-order fields, and second, by solving the second-order equations as an ``exact'' nonlinear system. The results of these computations have two distinct applications: (i) The computational infrastructure for the exact PM solution will be directly applicable to full general relativity. (ii) The results will allow us to begin supplying initial data to collaborators running general relativistic evolution codes.

Nonlinear dispersion of resonance extraordinary wave in a plasma with strong magnetic field

In this paper, the efficiency of electron acceleration by a short, powerful laser pulse propagating across an external magnetic field is investigated. Conditions for the decay of a laser pulse with frequency close to the upper hybrid resonance frequency are analyzed. It is also shown that a laser pulse propagating as an extraordinary wave in cold, magnetized, low-density plasma takes the form of a nonlinear wave with the modulated amplitude (envelope soliton). Finally, simulation results on the interaction of an electromagnetic pulse with a semi-infinite plasma, obtained with the help of an electromagnetic relativistic PIC code, are discussed and a comparison with the obtained theoretical results is presented.

General relativistic simulations of passive-magneto-rotational core collapse with microphysics

This paper presents results from axisymmetric simulations of magneto-rotational stellar core collapse to neutron stars in general relativity using the passive field approximation for the magnetic field. These simulations are performed using a new general relativistic numerical code specifically designed to study this astrophysical scenario. The code is an extension of an existing (and thoroughly tested) hydrodynamics code, which has been applied in the recent past to study relativistic rotational core collapse. It is based on the conformally-flat approximation of Einstein’s field equations and conservative formulations of the magneto-hydrodynamics equations. The code has been recently upgraded to incorporate a tabulated, microphysical equation of state and an approximate deleptonization scheme. This allows us to perform the most realistic simulations of magneto-rotational core collapse to date, which are compared with simulations employing a simplified (hybrid) equation of state, widely used in the relativistic core collapse community. Furthermore, state-of-the-art (unmagnetized) initial models from stellar evolution are used. In general, stellar evolution models predict weak magnetic fields in the progenitors, which justifies our simplification of performing the computations under the approach that we call the passive field approximation for the magnetic field. Our results show that for the core collapse models with microphysics the saturation of the magnetic field cannot be reached within dynamical time scales by winding up the poloidal magnetic field into a toroidal one. We estimate the effect of other amplification mechanisms including the magneto-rotational instability (MRI) and several types of dynamos. We conclude that for progenitors with astrophysically expected (i.e. weak) magnetic fields, the MRI is the only mechanism that could amplify the magnetic field on dynamical time scales. The uncertainties about the strength of the magnetic field at which the MRI saturates are discussed. All our microphysical models exhibit post-bounce convective overturn in regions surrounding the inner part of the proto-neutron star. Since this has a potential impact on enhancing the MRI, it deserves further investigation with more accurate neutrino treatment or alternative microphysical equations of state.