We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $\alpha$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $\gamma$-ray emission from X-ray binaries and active galactic nuclei.

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Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars

Preface 1. Historical overview 2. Observations of stellar winds 3. Basic concepts: isothermal winds 4. Basic concepts: non-isothermal winds 5. Coronal winds 6. Sound wave driven winds 7. Dust driven winds 8. Line driven winds 9. Magnetic rotator theory 10. Alfven wave driven winds 11. Outflowing disks from rotating stars 12. Winds colliding with the interstellar medium 13. The effects of mass loss on stellar evolution 14. Problems Appendices Bibliography Object index Index.

Introduction to Stellar Winds

In a single process, the merger of binary neutron star systems combines extreme gravity, the copious emission of gravitational waves, complex microphysics and electromagnetic processes, which can lead to astrophysical signatures observable at the largest redshifts. We review here the recent progress in understanding what could be considered Einstein's richest laboratory, highlighting in particular the numerous significant advances of the last decade. Although special attention is paid to the status of models, techniques and results for fully general-relativistic dynamical simulations, a review is also offered on the initial data and advanced simulations with approximate treatments of gravity. Finally, we review the considerable amount of work carried out on the post-merger phase, including black-hole formation, torus accretion onto the merged compact object, the connection with gamma-ray burst engines, ejected material, and its nucleosynthesis.

Binary Neutron Star Mergers: A Review of Einstein's Richest Laboratory

Some active galactic nuclei, microquasars, and gamma ray bursts may be powered by the electromagnetic braking of a rapidly rotating black hole. We investigate this possibility via axisymmetric numerical simulations of a black hole surrounded by a magnetized plasma. The plasma is described by the equations of general relativistic magnetohydrodynamics, and the effects of radiation are neglected. The evolution is followed for $2000 G M/c^3$, and the computational domain extends from inside the event horizon to typically $40 G M/c^2$. We compare our results to two analytic steady state models, including the force-free magnetosphere of Blandford & Znajek. Along the way we present a self-contained rederivation of the Blandford-Znajek model in Kerr-Schild (horizon penetrating) coordinates. We find that (1) low density polar regions of the numerical models agree well with the Blandford-Znajek model; (2) many of our models have an outward Poynting flux on the horizon in the Kerr-Schild frame; (3) none of our models have a net outward energy flux on the horizon; and (4) one of our models, in which the initial disk has net magnetic flux, shows a net outward angular momentum flux on the horizon. We conclude with a discussion of the limitations of our model, astrophysical implications, and problems to be addressed by future numerical experiments.

https://arxiv.org/pdf/astro-ph/0404512.pdf

A Measurement of the Electromagnetic Luminosity of a Kerr Black Hole

The afterglow emission from gamma-ray bursts (GRBs) is usually interpreted as synchrotron radiation from electrons accelerated at the GRB external shock that propagates with relativistic velocities into the magnetized interstellar medium. By means of multi-dimensional particle-in-cell simulations, we investigate the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0 ? 10?1. The pre-shock magnetic field is orthogonal to the flow, as generically expected for relativistic shocks. We find that relativistic perpendicular shocks propagating in electron-positron plasmas are efficient particle accelerators if the magnetization is ? 10?3. For electron-ion plasmas, the transition to efficient acceleration occurs for ? 3 ? 10?5. Here, the acceleration process proceeds similarly for the two species, since the electrons enter the shock nearly in equipartition with the ions, as a result of strong pre-heating in the self-generated upstream turbulence. In both electron-positron and electron-ion shocks, we find that the maximum energy of the accelerated particles scales in time as ?maxt 1/2. This scaling is shallower than the so-called (and commonly assumed) Bohm limit ?maxt, and it naturally results from the small-scale nature of the Weibel turbulence generated in the shock layer. In magnetized plasmas, the energy of the accelerated particles increases until it reaches a saturation value ?sat/?0 mic 2 ~ ??1/4, where ?0 mic 2 is the mean energy per particle in the upstream bulk flow. Further energization is prevented by the fact that the self-generated turbulence is confined within a finite region of thickness ??1/2 around the shock. Our results can provide physically grounded inputs for models of non-thermal emission from a variety of astrophysical sources, with particular relevance to GRB afterglows.

https://iopscience.iop.org/article/10.1088/0004-637X/771/1/54/pdf

The maximum energy of accelerated particles in relativistic collisionless shocks

In this paper, we investigate the evolution of the plasmoid- chain in a Poynting-dominated plasma. We model the relativistic current sheet with cold background plasma using the relativistic resistive magnetohydrodynamic approximation, and solve its temporal evolution numerically. We perform various calculations using different magnetization parameters of the background plasma and different Lundquist numbers. Numerical results show that the initially induced plasmoid triggers a secondary te aring instability, which gradually fills the current sheet with plasmoids, as has also been observed in the non-relativistic case. We find the plasmoid-chain greatly enhances the reconnection rate, which becomes independent of the Lundquist number, when this exceeds a critical value. In addition, we show the distribution of pla smoid size becomes a power law. Since magnetic reconnection is expected to play an important role in variou s high energy astrophysical phenomena, our results can be used for explaining the physical mechanism of them. Subject headings:magnetic fields, magnetohydrodynamics (MHD), relativisti c processes, plasmas

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Evolution of Relativistic Plasmoid Chains in a Poynting-dominated Plasma

We present a new numerical method of special relativistic resistive magnetohydrodynamics with scalar resistivity that can treat a range of phenomena, from non-relativistic to relativistic (shock, contact discontinuity, and Alfven wave). The present scheme calculates the numerical flux of fluid by using an approximate Riemann solver and electromagnetic field by using the method of characteristics. Since this scheme uses appropriate characteristic velocities, it is capable of accurately solving problems that cannot be approximated as ideal magnetohydrodynamics and whose characteristic velocity is much lower than the velocity of light. The numerical results show that our scheme can solve the above problems as well as nearly ideal MHD problems. Our new scheme is particularly well suited to systems with initially weak magnetic field and mixed phenomena of relativistic and non-relativistic velocity, for example magnetorotational instability in an accretion disk and super Alfvenic turbulence.

https://iopscience.iop.org/article/10.1088/0004-637X/735/2/113/pdf

A New Numerical Scheme for Resistive Relativistic Magnetohydrodynamics Using Method of Characteristics

In this paper, we investigate the evolution of the plasmoid-chain in a Poynting-dominated plasma. We model the relativistic current sheet with cold background plasma using the relativistic resistive magnetohydrodynamic approximation, and solve its temporal evolution numerically. We perform various calculations using different magnetization parameters of the background plasma and different Lundquist numbers. Numerical results show that the initially induced plasmoid triggers a secondary tearing instability, which gradually fills the current sheet with plasmoids, as has also been observed in the non-relativistic case. We find the plasmoid-chain greatly enhances the reconnection rate, which becomes independent of the Lundquist number, when this exceeds a critical value. In addition, we show the distribution of plasmoid size becomes a power law. Since magnetic reconnection is expected to play an important role in various high energy astrophysical phenomena, our results can be used for explaining the physical mechanism of them.

Evolution of the Relativistic Plasmoid-Chain in the Poynting-Dominated Plasma

We present a new numerical method of special relativistic resistive magnetohydrodynamics with scalar resistivity that can treat a range of phenomena, from nonrelativistic to relativistic (shock, contact discontinuity, and Alfv\'en wave). The present scheme calculates the numerical flux of fluid by using an approximate Riemann solver, and electromagnetic field by using the method of characteristics. Since this scheme uses appropriate characteristic velocities, it is capable of accurately solving problems that cannot be approximated as ideal magnetohydrodynamics and whose characteristic velocity is much lower than light velocity. The numerical results show that our scheme can solve the above problems as well as nearly ideal MHD problems. Our new scheme is particularly well suited to systems with initially weak magnetic field, and mixed phenomena of relativistic and non-relativistic velocity; for example, MRI in accretion disk, and super Alfv\'enic turbulence.

A New numerical scheme for resistive relativistic MHD using method of characteristics

We report turbulence effects on magnetic reconnection in relativistic plasmas using 3-dimensional relativistic resistive magnetohydrodynamics simulations. We found reconnection rate became independent of the plasma resistivity due to turbulence effects similarly to non-relativistic cases. We also found compressible turbulence effects modified the turbulent reconnection rate predicted in non-relativistic incompressible plasmas; The reconnection rate saturates and even decays as the injected velocity approaches to the Alfv\'en velocity. Our results indicate the compressibility cannot be neglected when compressible component becomes about half of incompressible mode occurring when the Alfv\'en Mach number reaches about $0.3$. The obtained maximum reconnection rate is around $0.05$ to $0.1$, which will be able to reach around $0.1$ to $0.2$ if injection scales are comparable to the sheet length.

https://iopscience.iop.org/article/10.1088/0004-637X/815/1/16/pdf

Makoto Takamoto

Papers

Evolution of Relativistic Plasmoid Chains in a Poynting-dominated Plasma

A New Numerical Scheme for Resistive Relativistic Magnetohydrodynamics Using Method of Characteristics

Evolution of the Relativistic Plasmoid-Chain in the Poynting-Dominated Plasma

A New numerical scheme for resistive relativistic MHD using method of characteristics

TURBULENT RECONNECTION in RELATIVISTIC PLASMAS and EFFECTS of COMPRESSIBILITY