Hasil untuk "Astrophysics"

A. Mignone, G. Bodo, S. Massaglia et al.

We present a new numerical code, PLUTO, for the solution of hypersonic flows in 1, 2, and 3 spatial dimensions and different systems of coordinates. The code provides a multiphysics, multialgorithm modular environment particularly oriented toward the treatment of astrophysical flows in presence of discontinuities. Different hydrodynamic modules and algorithms may be independently selected to properly describe Newtonian, relativistic, MHD, or relativistic MHD fluids. The modular structure exploits a general framework for integrating a system of conservation laws, built on modern Godunov-type shock-capturing schemes. Although a plethora of numerical methods has been successfully developed over the past two decades, the vast majority shares a common discretization recipe, involving three general steps: a piecewise polynomial reconstruction followed by the solution of Riemann problems at zone interfaces and a final evolution stage. We have checked and validated the code against several benchmarks available in literature. Test problems in 1, 2, and 3 dimensions are discussed.

J. Pringle

D. Verner, G. Ferland, K. Korista et al.

We present a complete set of analytic fits to the nonrelativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections. {copyright} {ital 1996 The American Astronomical Society.}

R. Brito, V. Cardoso, P. Pani

Introduction.- Superradiance in flat spacetime.- Superradiance in black hole physics.- Black holes and superradiant instabilities.- Black hole superradiance in astrophysics.- Conclusions and Outlook.

S. Blyth, van der Thijs Hulst, M. Verheijen et al.

Bob Rehder

A desirable property of any theory of causal reasoning is to explain not only why people make causal reasoning errors but also <i>when</i> they make them. The <i>mutation sampler</i> is a rational process model of human causal reasoning that yields normatively correct inferences when sufficient cognitive resources are available but introduces systematic errors when they are not. The mutation sampler has been shown to account for a number of causal reasoning errors, including <i>Markov violations</i>, the phenomenon in which human reasoners treat causally related variables as statistically dependent when they are normatively independent. A Markov violation arises, for example, when an individual reasoning about a causal chain <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>X</mi><mo>→</mo><mi>Y</mi><mo>→</mo><mi>Z</mi></mrow></semantics></math></inline-formula> treats <i>X</i> as informative about the state of <i>Z</i> even when the state of <i>Y</i> is known. Recently, the mutation sampler was used to predict the existence of previously untested experimental conditions in which the <i>sign</i> of Markov violations would switch from positive to negative. Here, it was used to predict the existence of conditions in which Markov violations should <i>disappear</i> entirely. In fact, asking subjects to reason about a novel causal structure with nothing but <i>generative</i> causal relations (a cause makes its effect more likely) resulted in Markov violations in the usual positive direction. But simply describing one of four causal relations as <i>inhibitory</i> (the cause makes its effect less likely) resulted in the elimination of those violations. Theoretical model fitting confirmed how this novel result is predicted by the mutation sampler.

Isomiddin Nishonov, Javlon Rayimbaev, Saeed Ullah Khan et al.

Abstract Testing dark matter effects on gravity around black holes in the framework of gravity theories through observational data is an essential task of relativistic astrophysical studies. In this work, we first obtain a new spacetime solution for a black hole surrounded by perfect fluid dark matter (PFDM) in modified gravity (MOG). The MOG field is assumed to be a gravitational vector field. We investigate the vector fields with combined effects of PFDM on spacetime properties: event horizon radius, scalar invariants such as the Ricci scalar, the square of the Ricci tensor, and Kretchman scalars. We investigate the circular motion of test particles in the spacetime of the black hole, taking into account the MOG field interaction on the particle geodesics. The energy and angular momentum of the particles corresponding to circular orbits are studied. In addition, we explore how the PFDM and MOG fields change the position of innermost stable circular orbits (ISCOs) and their corresponding energy and angular momentum values. Moreover, we study the energy efficiency rate around the black hole in the Novikov and Thorns thin accretion disc model. We analyze collisional cases of the particles near the black hole and study the effects of the fields on the critical angular momentum in which particles can collide near the black hole and the center-of-mass energy of the colliding particles.

The CMS collaboration, A. Tumasyan, W. Adam et al.

Asalkhon Alimova, Farruh Atamurotov, Ahmadjon Abdujabbarov et al.

Abstract The motion of spinning particles around a quantum-corrected black hole is examined in this paper. We investigate the dynamics of spinning test particles by using the Mathisson–Papapetrou–Dixon equations, the Tulczyjew spin-supplementary condition, and restricting the motion to the equatorial plane. We determine the innermost stable circular orbit (ISCO), effective potential, and effective force and examine how these depend on the black hole’s $$\alpha $$ α parameter and the particle’s s spin. However, we also take into account a superluminal bound on the motion of the spinning particle since its kinematical four-velocity and dynamical four-momentum are not always parallel. We also show how the parameter $$\alpha $$ α affects the maximum value of the spin parameter s. We determine the critical angular momentum of the particle for which a collision is possible by investigating collisions of spinning particles close to the horizon of a black hole. Finally, we compute the particle’s center-of-mass energy $$\mathcal {E}_{cm}$$ E cm and analyze how the spin of the colliding particles affects it.

Marc Hon, Daniel Huber, Yaguang Li et al.

Asteroseismology of dwarf stars cooler than the Sun is very challenging owing to the low amplitudes and rapid timescales of oscillations. Here we present the asteroseismic detection of solar-like oscillations at 4-minute timescales ( ${\nu }_{\max }\sim 4300$ μ Hz) in the nearby K dwarf σ Draconis using extreme-precision Doppler velocity observations from the Keck Planet Finder and 20 s cadence photometry from NASA’s Transiting Exoplanet Survey Satellite. The star is the coolest dwarf star to date with both velocity and luminosity observations of solar-like oscillations, having amplitudes of 5.9 ± 0.8 cm s ^−1 and 0.8 ± 0.2 ppm, respectively. These measured values are in excellent agreement with established luminosity−velocity amplitude relations for oscillations and provide further evidence that mode amplitudes for stars with T _eff < 5500 K diminish in scale following an ( L / M ) ^1.5 relation. By modeling the star’s oscillation frequencies from photometric data, we measure an asteroseismic age of 4.5 ± 0.9 (ran) ± 1.2 (sys) Gyr. The observations demonstrate the capability of next-generation spectrographs and precise space-based photometry to extend observational asteroseismology to nearby cool dwarfs, which are benchmarks for stellar astrophysics and prime targets for directly imaging planets using future space-based telescopes.

Mukesh Kumar Singh, Shasvath J. Kapadia, Aditya Vijaykumar et al.

Templates modeling just the dominant mode of gravitational radiation are generally sufficient for the unbiased parameter inference of near-equal-mass compact binary mergers. However, neglecting the subdominant modes can bias the inference if the binary is significantly asymmetric, very massive, or has misaligned spins. In this work, we explore if neglecting these subdominant modes in the parameter estimation of nonspinning binary black hole mergers can bias the inference of their population-level properties such as mass and merger redshift distributions. Assuming the design sensitivity of the advanced LIGO-Virgo detector network, we find that neglecting subdominant modes will not cause a significant bias in the population inference, although including them will provide more precise estimates. This is primarily because asymmetric binaries are expected to be rarer in our detected sample, due to their intrinsic rareness and the observational selection effects. The increased precision in the measurement of the maximum black hole mass can help in better constraining the upper mass gap in the mass spectrum.

M. Maggiore

Ke Jia, Shiyong Zhou

Research on fault interaction and earthquake triggering, which is a hot issue in the field of source physics, can facilitate understanding of the underlying mechanisms of strong earthquakes and also has good application prospects in earthquake risk analysis and prediction research. Previous review articles provided detailed explanations from the perspectives of basic principles, methods, and applicability, as well as multiple earthquake case studies of stress triggering. However, the introduction to earthquake triggering from the perspective of seismicity analysis is not exhaustive, and the combination and complementarity of these two perspectives are not provided in detail. This paper summarizes the achievements and progress of research on fault interaction and earthquake triggering mechanism through the past few decades from the perspectives of physical and statistical models. The current challenges and possible future directions are reviewed and evaluated. From the perspective of the physical model, three important mechanisms of sources of fault interaction are analyzed: static stress triggering, dynamic stress triggering, and viscoelastic stress triggering, as well as the basic principles and methods of calculation. In the aspect of the statistical model, the basic principles and methods of seismicity analysis are introduced, and applications of the epidemic-type aftershock sequence (ETAS) model and b-value in fault interaction and earthquake triggering mechanism are analyzed. From the perspective of the combination of these two models, the unified connotation of mutual verification and the basic principle of the rate-and-state friction law are introduced. The analysis points out that the stress interaction between multiple faults or earthquakes can be comprehensively studied through the two different schools of Coulomb stress calculation and the ETAS model and that cross-validation can increase the reliability of the results. Retroactive application of rate-and-state friction law can provide a new perspective for understanding the earthquake triggering relationship and fault interaction.

Yijie Lv, Jiguang He, Weikai Xu et al.

In this article, a graph-theoretic method (taking advantage of constraints among sets associated with the corresponding parity-check matrices) is applied for the construction of a double low-density parity-check (D-LDPC) code (also known as LDPC code pair) in a joint source-channel coding (JSCC) system. Specifically, we pre-set the girth of the parity-check matrix for the LDPC code pair when jointly designing the two LDPC codes, which are constructed by following the set constraints. The constructed parity-check matrices for channel codes comprise an identity submatrix and an additional submatrix, whose column weights can be pre-set to be any positive integer numbers. Simulation results illustrate that the constructed D-LDPC codes exhibit significant performance improvement and enhanced flexible frame length (i.e., adaptability under various channel conditions) compared with the benchmark code pair.

A. Tumino, C. Bertulani, M. Cognata et al.

The Trojan Horse Method (THM) represents an indirect path to determine the bare nucleus astrophysical S-factor for reactions among charged particles at astrophysical energies. This is achieved by measuring the quasi-free cross section of a suitable three-body process. The method is also suited to study neutron-induced reactions, especially in the case of radioactive ion beams. A comprehensive review of the theoretical as well as experimental features behind the THM is presented here. An overview is given of some recent applications to demonstrate the method's practical use for reactions that have a great impact on selected astrophysical scenarios. Expected final online publication date for the Annual Review of Nuclear and Particle Science, Volume 71 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

R. Rando, A. De Angelis, M. Mallamaci

e-ASTROGAM is an observatory space mission dedicated to the study of the gamma radiation in the range from 0.3 MeV to 3 GeV. The detector is composed by a Silicon tracker, a calorimeter, and an anticoincidence system. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, eASTROGAM will open a new window on the non-thermal Universe. In particular it will determine the origin of key isotopes fundamental for the understanding of supernova explosions and the chemical evolution of our Galaxy. It will also shed light on the processes behind the acceleration of cosmic rays in our Galaxy.

S. P. Portegies Zwart

Computer use in astronomy continues to increase, and so also its impact on the environment. To minimize the effects, astronomers should avoid interpreted scripting languages such as Python, and favour the optimal use of energy-efficient workstations.

A. Tielens

Kaiwen Shi, Haiyan Su, Xinlong Feng

In this article, we mainly consider a first order penalty finite element method (PFEM) for the 2D/3D unsteady incompressible magnetohydrodynamic (MHD) equations. The penalty method applies a penalty term to relax the constraint “<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∇</mo><mo>·</mo><mi mathvariant="italic">u</mi><mo>=</mo><mn>0</mn></mrow></semantics></math></inline-formula>”, which allows us to transform the saddle point problem into two smaller problems to solve. The Euler semi-implicit scheme is based on a first order backward difference formula for time discretization and semi-implicit treatments for nonlinear terms. It is worth mentioning that the error estimates of the fully discrete PFEM are rigorously derived, which depend on the penalty parameter <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>ϵ</mi></semantics></math></inline-formula>, the time-step size <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>τ</mi></semantics></math></inline-formula>, and the mesh size <i>h</i>. Finally, two numerical tests show that our scheme is effective.

T. Zhang, Y. Ebihara

Abstract An increase in geomagnetically induced currents (GICs) is an inevitable result of geomagnetic field disturbances, and is harmful to the power grid, in particular, at high latitudes. At mid and low latitudes, the amplitude of the GICs is, in general, small, but large‐amplitude GICs are often observed during magnetic storms. It is of importance to understand major characteristics and extreme values of GICs at mid and low latitudes. For the geoelectric field disturbances ΔE observed at Kakioka (27.8° geomagnetic latitude) in Japan in 1996–2004, we performed superposed epoch analyses with respect to three types of geomagnetic disturbances: (a) storm sudden commencements (SSCs)/sudden impulses (SIs), (b) main phase of magnetic storms, and (c) bay disturbances. It is shown that the SSCs/SIs and the main phase of the magnetic storms are equally important for causing large‐amplitude disturbances of ΔE at Kakioka. GICs are thought to be amplified when the SIs and/or the bay disturbances occur during the magnetic storms. The maximum value of ΔE tends to be correlated with the maximum value of ΔH during the three types of events, where ΔH is the horizontal component of the geomagnetic field. Assuming that a quasi‐linear relationship between the maximum ΔE and the maximum ΔH is valid, we estimated GICs at three substations in Japan for an extreme SSCs/SIs, and the extreme magnetic storms. This scheme could be applicable to estimate roughly the GICs against extreme events, and to forecast the maximum GICs in a real‐time manner.