arXiv subjects

Bestin James

Publications and source records attributed to Bestin James.

5 recordsLinked to original sources

arXiv2026

Numerical modelling of recombination-driven stellar winds

Low-temperature stars have a significant population of neutral hydrogen increasing with height in their atmospheres. This suggests eventual recombination as we move away from their deep photospheric layers. Recombination can potentially drive stellar winds, particularly in cool and evolved stars. We aim to verify the possibility of stellar winds driven by recombination. We also aim to constitute an initial model for this type wind driving. We examine the possibility of recombination-driven outflows across the HR diagram by comparing the gravitational potential energy per atom to the hydrogen ionization energy. Then we start from a simple analytical model with the addition of an extra heating term to the gas in the Parker wind model. We model the heating term, analogous to the heat released by recombination, and examine how it can drive an outflow. We implement the model in the code CASTRO and check the hydrodynamic stability of the solution. The added heating drives a steady transonic outflow in our models, when tested with the parameters of an AGB star. The heating function affects the solution and the location of the sonic point depends on the location of the heating function itself. Even though the derived hydrodynamic equations are independent of the density, we determine the processes that set the mass-loss rate. By comparing the heating from collisional recombination and cooling due to radiative recombination, we show that relatively high densities are required to release the recombination energy as heat, implying mass-loss rates of the order of 0.1 solar mass per year. Our comparison of energies across the HR diagram suggests a possibility of recombination-driven winds in evolved red giants and AGB stars. The main deciding factor for wind launching is the radial distribution of heat released by recombination in a star's atmosphere. This can in turn affect the mass-loss rate.

astro-ph.SR↗

arXiv2024

Black hole outflows initiated by a large-scale magnetic field

Accreting black hole sources show variable outflows at different mass scales. For instance, in the case of galactic nuclei, our own galactic center Sgr A* exhibits flares and outbursts in the X-ray and infrared bands. Recent studies suggest that the inner magnetospheres of these sources have a pronounced effect on such emissions. Accreting plasma carries the frozen-in magnetic flux along with it down to the black hole horizon. During the in-fall, the magnetic field intensifies and it can lead to a magnetically arrested state. We investigate the competing effects of inflows at the black hole horizon and the outflows developed in the accreting plasma due to the action of magnetic field in the inner magnetosphere and their implications. We start with a spherically symmetric Bondi-type inflow and introduce the magnetic field. In order to understand the influence of the initial configuration, we start the computations with an aligned magnetic field with respect to the black hole rotation axis. Then we proceed to the case of magnetic fields inclined to the black hole rotation axis. We employ the 2D and 3D versions of HARM code for the aligned field models while using the 3D version for the inclined field and compare the results of computations against each other. We observe how the magnetic lines of force start accreting with the plasma while an equatorial intermittent outflow develops and goes on pushing some material away from the black hole partially along the equatorial plane, and partly ejecting it out of the plane in the vertical direction. In consequence, the accretion rate also fluctuates. The black hole spin direction prevails at later stages and it determines the flow geometry near the event horizon, whereas on larger scales the flow geometry stays influenced by the initial inclination of the field.

astro-ph.HE↗

arXiv2022

Modeling the GRB jet properties with 3D general relativistic simulations of magnetically arrested accretion flows

We investigate the dependence of the GRB jet structure and its evolution on the properties of the accreting torus in the central engine. Our models numerically evolve the accretion disk around a Kerr black hole using 3D general relativistic magnetohydrodynamic simulations. We use two different analytical hydrodynamical models of the accretion disk, based on the Fishbone-Moncrief and Chakrabarti solutions, as our initial states for the structure of the collapsar disk and the remnant after a binary neutron star merger, respectively. We impose poloidal magnetic fields of two different geometries upon the initial stable solutions. We study the formation and evolution of the magnetically arrested disk state and its effect on the properties of the emitted jet. The jets produced in our models are structured and have a relatively hollow core and reach higher Lorentz factors at an angle $\gtrsim 9{^\circ}$ from the axis. The jet in our short GRB model has an opening angle of up to $\sim 25^{\circ}$ while our long GRB engine produces a narrower jet, of up to $\sim 11^{\circ}$. We also study the time variability of the jets and provide an estimate of the minimum variability timescale in our models. The application of our models to the GRB jets in the binary neutron star post-merger system and to the ultra-relativistic jets launched from collapsing stars are briefly discussed.

astro-ph.HE↗

arXiv2021

Variability of magnetically-dominated jets from accreting black holes

Structured jets are recently invoked to explain the complex emission of gamma ray bursts, such as GW 170817. Based on the accretion simulations, the jets are expected to have a structure that is more complex than a simple top-hat. Also, the structure of launching regions of blazar jets should influence their large scale evolution. This is recently revealed by the interactions of jet components in TXS 0506+056, where the jet is observed at a viewing angle close to zero. Observational studies have also shown an anti-correlation between the jet variability, measured e.g. by its minimum variability time scale, and the Lorentz factor, that spans several orders of magnitude and covers both blazars and GRBs samples. Motivated by those observational properties of black hole sources, we investigate the accretion inflow and outflow properties, by means of numerical GR MHD simulations. We perform axisymmetric calculations of the structure and evolution of central engine, composed of magnetized torus around Kerr black hole that is launching a non-uniform jet. We probe the jet energetics at different points along the line of sight, and we measure the jet time variability as localized in these specific regions. We quantify our results by computing the minimum variability timescales and power density spectra. We reproduce the MTS-$Γ$ correlation and we attribute it to the black hole spin as the main driving parameter of the engine. We also find that the PDS slope is not strongly affected by the black hole spin, while it differs for various viewing angles.

astro-ph.HE↗

arXiv2020

Multi-messenger signals from short gamma ray bursts

We present the results of simulations done with the code HARM-COOL developed in the CTP PAS Warsaw research group over the years 2017-2019. It is based in the original GR MHD scheme proposed by Gammie et al. (2003) for the simulation of Active Galactic Nucleus, but now it has been suited for the engine of a short Gamma Ray Burst event. We compute time-dependent evolution of a black hole accretion disk, in two-dimensional, axisymmetric scheme. The code includes neutrino cooling and accounts for nuclear structure of dense, degenerate matter. Free protons, neutrons, and electron-positron pairs form a neutron-rich, magnetically driven outflow that provides site for subsequent r-process nucleosynthesis. Here the heavy elements up to the Uranium and Gold are synthesized and may contribute to the chemical enrichment of the circum-burst medium. Their radio-active decay will give signal in lower energies in a timescale of weeks-months after the GRB prompt phase. In addition, the magnetic fields are responsible for the launching of ultra-relativistic jets along the rotation axis of the central black hole, according to the well-known Blandford-Znajek mechanism. These jets are sites of variable high energy emission in gamma rays. We find that the magnetic field and the black hole spin account for the observed variability timescales and jet energetics.

astro-ph.HE↗