arXiv subjects

Alexander Tchekhovskoy

Publications and source records attributed to Alexander Tchekhovskoy.

At least 91 records · Page 5Linked to original sources

arXiv2013

Time-Dependent 3D Magnetohydrodynamic Pulsar Magnetospheres: Oblique Rotators

The current state of the art in pulsar magnetosphere modeling assumes the force-free limit of magnetospheric plasma. This limit retains only partial information about plasma velocity and neglects plasma inertia and temperature. We carried out time-dependent 3D relativistic magnetohydrodynamic (MHD) simulations of oblique pulsar magnetospheres that improve upon force-free by retaining the full plasma velocity information and capturing plasma heating in strong current layers. We find rather low levels of magnetospheric dissipation, with less than 10% of pulsar spindown energy dissipated within a few light cylinder radii, and the MHD spindown that is consistent with that in force-free. While oblique magnetospheres are qualitatively similar to the rotating split-monopole force-free solution at large radii, we find substantial quantitative differences with the split-monopole, e.g., the luminosity of the pulsar wind is more equatorially concentrated than the split-monopole at high obliquities, and the flow velocity is modified by the emergence of reconnection flow directed into the current sheet.

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arXiv2013

Energy Extraction from Spinning Black Holes via Relativistic Jets

It has for long been an article of faith among astrophysicists that black hole spin energy is responsible for powering the relativistic jets seen in accreting black holes. Two recent advances have strengthened the case. First, numerical general relativistic magnetohydrodynamic simulations of accreting spinning black holes show that relativistic jets form spontaneously. In at least some cases, there is unambiguous evidence that much of the jet energy comes from the black hole, not the disk. Second, spin parameters of a number of accreting stellar-mass black holes have been measured. For ballistic jets from these systems, it is found that the radio luminosity of the jet correlates with the spin of the black hole. This suggests a causal relationship between black hole spin and jet power, presumably due to a generalized Penrose process.

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arXiv2013

Semi-implicit scheme for treating radiation under M1 closure in general relativistic conservative fluid dynamics codes

A numerical scheme is described for including radiation in multi-dimensional general-relativistic conservative fluid dynamics codes. In this method, a covariant form of the M1 closure scheme is used to close the radiation moments, and the radiative source terms are treated semi-implicitly in order to handle both optically thin and optically thick regimes. The scheme has been implemented in a conservative general relativistic radiation hydrodynamics code KORAL. The robustness of the code is demonstrated on a number of test problems, including radiative relativistic shock tubes, static radiation pressure supported atmosphere, shadows, beams of light in curved spacetime, and radiative Bondi accretion. The advantages of M1 closure relative to other approaches such as Eddington closure and flux-limited diffusion are discussed, and its limitations are also highlighted.

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arXiv2013

Swift J1644+57 gone MAD: the case for dynamically-important magnetic flux threading the black hole in a jetted tidal disruption event

The unusual transient Swift J1644+57 likely resulted from a collimated relativistic jet powered by accretion onto a massive black hole (BH) following the tidal disruption (TD) of a star. Several mysteries cloud the interpretation of this event: (1) extreme flaring and `plateau' shape of the X-ray/gamma-ray light curve during the first 10 days after the gamma-ray trigger; (2) unexpected rebrightening of the forward shock radio emission months after trigger; (3) no obvious evidence for jet precession, despite misalignment typically expected between the angular momentum of the accretion disk and BH; (4) recent abrupt shut-off in jet X-ray emission after 1.5 years. Here we show that all of these seemingly disparate mysteries are naturally resolved by one assumption: the presence of strong magnetic flux Phi threading the BH. Initially, Phi is weak relative to high fall-back mass accretion rate, Mdot, and the disk and jets precess about the BH axis = our line of sight. As Mdot drops, Phi becomes dynamically important and leads to a magnetically-arrested disk (MAD). MAD naturally aligns disk and jet axis along the BH spin axis, but only after a violent rearrangement phase (jet wobbling). This explains the erratic light curve at early times and the lack of precession at later times. We use our model for Swift J1644+57 to constrain BH and disrupted star properties, finding that a solar-mass main sequence star disrupted by a relatively low mass, M~10^5-10^6 Msun, BH is consistent with the data, while a WD disruption (though still possible) is disfavored. The magnetic flux required to power Swift J1644+57 is too large to be supplied by the star itself, but it could be collected from a quiescent `fossil' accretion disk present in the galactic nucleus prior to the TD. The presence (lack of) of such a fossil disk could be a deciding factor in what TD events are accompanied by powerful jets.[abridged]

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arXiv2012

Alignment of Magnetized Accretion Disks and Relativistic Jets with Spinning Black Holes

Accreting black holes (BHs) produce intense radiation and powerful relativistic jets, which are affected by the BH's spin magnitude and direction. While thin disks might align with the BH spin axis via the Bardeen-Petterson effect, this does not apply to jet systems with thick disks. We used fully three-dimensional general relativistic magnetohydrodynamical simulations to study accreting BHs with various BH spin vectors and disk thicknesses with magnetic flux reaching saturation. Our simulations reveal a "magneto-spin alignment" mechanism that causes magnetized disks and jets to align with the BH spin near BHs and further away to reorient with the outer disk. This mechanism has implications for the evolution of BH mass and spin, BH feedback on host galaxies, and resolved BH images for SgrA* and M87.

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arXiv2012

General Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes

Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height $H$ to cylindrical radius $R$ ratio of $|H/R|\sim 0.2--1$) accretion flows around BHs with various dimensionless spins ($a/M$, with BH mass $M$) and with initially toroidally-dominated ($ϕ$-directed) and poloidally-dominated ($R-z$ directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough $|a/M|$, coherent large-scale (i.e. $\gg H$) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. For sufficiently high $|a/M|$ or low $|H/R|$ the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric "magnetically choked accretion flow" (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with $\gtrsim 100$% efficiency for $|a/M|\gtrsim 0.9$. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk quasi-periodic oscillation (JD-QPO) mechanism. The high-frequency QPO has spherical harmonic $|m|=1$ mode period of $τ\sim 70GM/c^3$ for $a/M\sim 0.9$ with coherence quality factors $Q\gtrsim 10$. [abridged]

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arXiv2012

Prograde and Retrograde Black Holes: Whose Jet is More Powerful?

The outflow efficiency (eta) from black hole (BH) accretion disc systems is known to depend upon both the BH spin (a) and the amount of large-scale magnetic flux threading the BH and disc. Semi-analytical flux-trapping models suggest retrograde BHs should trap much more large-scale magnetic flux near the BH leading to much higher eta than for prograde BHs. We self-consistently determine the amount of large-scale magnetic flux trapped by rapidly spinning (a = -0.9 and 0.9) BHs using global 3D time-dependent non-radiative general relativistic magnetohydrodynamic simulations of thick (h/r ~ 0.3-0.6) discs. We find that BH-trapped flux builds up until it is strong enough to disrupt the inner accretion disc. Contrary to prior flux-trapping models, which do not include the back-reaction of magnetic flux on the disc, our simulations show prograde BHs trap more magnetic flux, leading to about 3 times higher eta than retrograde BHs for |a| = 0.9. Both spin orientations can produce highly efficient jets, eta ~ 100%, with increasing eta for increasing disc thickness. The similarity of eta for prograde and retrograde BHs makes it challenging to infer the sign of BH spin based on jet energetics alone.

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arXiv2012

General Relativistic Modeling of Magnetized Jets from Accreting Black Holes

Recent advances in general relativistic magnetohydrodynamic modeling of jets offer unprecedented insights into the inner workings of accreting black holes that power the jets in active galactic nuclei (AGN) and other accretion systems. I will present the results of recent studies that determine spin-dependence of jet power and discuss the implications for the AGN radio loud/quiet dichotomy and recent observations of high jet power in a number of AGN.

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arXiv2012

On the Spin-Down of Intermittent Pulsars

Magnetospheres of pulsars are thought to be filled with plasma, and variations in plasma supply can affect both pulsar emission properties and spin-down rates. A number of recently discovered "intermittent" pulsars switch between two distinct states: an "on", radio-loud state, and an "off", radio-quiet state. Spin-down rates in the two states differ by a large factor, $\sim 1.5-2.5$, which is not easily understood in the context of current models. In this Letter we present self-consistent numerical solutions of "on" and "off" states of intermittent pulsar magnetospheres. We model the "on" state as a nearly ideal force-free magnetosphere with abundant magnetospheric plasma supply. The lack of radio emission in the "off" state is associated with plasma supply disruption that results in lower plasma density on the open field lines. We model the "off" state using nearly vacuum conditions on the open field lines and nearly ideal force-free conditions on the closed field lines, where plasma can remain trapped even in the absence of pair production. The toroidal advection of plasma in the closed zone in the "off" state causes spin-downs that are a factor of $\sim 2$ higher than vacuum values, and we naturally obtain a range of spin-down ratios between the "on" and "off" states, $\sim 1.2-2.9$, which corresponds to a likely range of pulsar inclination angles of $30{-}90^\circ$. We consider the implications of our model to a number of poorly understood but possibly related pulsar phenomena, including nulling, timing noise, and rotating radio transients.

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arXiv2011

Resistive Solutions for Pulsar Magnetospheres

The current state of the art in the modeling of pulsar magnetospheres invokes either the vacuum or force-free limits for the magnetospheric plasma. Neither of these limits can simultaneously account for both the plasma currents and the accelerating electric fields that are needed to explain the morphology and spectra of high-energy emission from pulsars. To better understand the structure of such magnetospheres, we combine accelerating fields and force-free solutions by considering models of magnetospheres filled with resistive plasma. We formulate Ohm's Law in the minimal velocity fluid frame and construct a family of resistive solutions that smoothly bridges the gap between the vacuum and the force-free magnetosphere solutions. The spin-down luminosity, open field line potential drop, and the fraction of open field lines all transition between the vacuum and force-free values as the plasma conductivity varies from zero to infinity. For fixed inclination angle, we find that the spin-down luminosity depends linearly on the open field line potential drop. We consider the implications of our resistive solutions for the spin down of intermittent pulsars and sub-pulse drift phenomena in radio pulsars.

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arXiv2011

Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole

We describe global, 3D, time-dependent, non-radiative, general-relativistic, magnetohydrodynamic simulations of accreting black holes (BHs). The simulations are designed to transport a large amount of magnetic flux to the center, more than the accreting gas can force into the BH. The excess magnetic flux remains outside the BH, impedes accretion, and leads to a magnetically arrested disc. We find powerful outflows. For a BH with spin parameter a = 0.5, the efficiency with which the accretion system generates outflowing energy in jets and winds is eta ~ 30%. For a = 0.99, we find eta ~ 140%, which means that more energy flows out of the BH than flows in. The only way this can happen is by extracting spin energy from the BH. Thus the a = 0.99 simulation represents an unambiguous demonstration, within an astrophysically plausible scenario, of the extraction of net energy from a spinning BH via the Penrose-Blandford-Znajek mechanism. We suggest that magnetically arrested accretion might explain observations of active galactic nuclei with apparent eta ~ few x 100%.

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arXiv2011

Constraints on Cold Magnetized Shocks in Gamma-Ray Bursts

We consider a model in which the ultra-relativistic jet in a gamma-ray burst (GRB) is cold and magnetically accelerated. We assume that the energy flux in the outflowing material is partially thermalized via internal shocks or a reverse shock, and we estimate the maximum amount of radiation that could be produced in such magnetized shocks. We compare this estimate with the available observational data on prompt gamma-ray emission in GRBs. We find that, even with highly optimistic assumptions, the magnetized jet model is radiatively too inefficient to be consistent with observations. One way out is to assume that much of the magnetic energy in the post-shock, or even pre-shock, jet material is converted to particle thermal energy by some unspecified process, and then radiated. This can increase the radiative efficiency sufficiently to fit observations. Alternatively, jet acceleration may be driven by thermal pressure rather than magnetic fields. In this case, which corresponds to the traditional fireball model, sufficient prompt GRB emission could be produced either from shocks at a large radius or from the jet photosphere closer to the center.

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arXiv2010

Simulations of Magnetized Disks Around Black Holes: Effects of Black Hole Spin, Disk Thickness, and Magnetic Field Geometry

The standard general relativistic model of a razor-thin accretion disk around a black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at the radius of the innermost stable circular orbit (ISCO) and that, outside the ISCO, the shear stress is produced by an effective turbulent viscosity. However, astrophysical accretion disks are not razor-thin, it is uncertain whether the shear stress necessarily vanishes at the ISCO, and the magnetic field, which is thought to drive turbulence in disks, may contain large-scale structures that do not behave like a simple local scalar viscosity. We describe three-dimensional general relativistic magnetohydrodynamic simulations of accretion disks around black holes with a range of spin parameters, and we use the simulations to assess the validity of the NT model. Our fiducial initial magnetic field consists of multiple (alternating polarity) poloidal field loops whose shape is roughly isotropic in the disk in order to match the isotropic turbulence expected in the poloidal plane. For a thin disk with an aspect ratio |h/r| ~ 0.07 around a non-spinning black hole, we find a decrease in the accreted specific angular momentum of 2.9% relative to the NT model and an excess luminosity from inside the ISCO of 3.5%. The deviations in the case of spinning black holes are of the same order. In addition, the deviations decrease with decreasing |h/r|. We therefore conclude that magnetized thin accretion disks in x-ray binaries in the thermal/high-soft spectral state ought to be well-described by the NT model, especially at luminosities below 30% of Eddington where we expect a very small disk thickness |h/r| <~ 0.05. We also discuss how the stress and the luminosity inside the ISCO depend on the assumed initial magnetic field geometry. (abridged)

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arXiv2010

Magnetohydrodynamic Simulations of Gamma-Ray Burst Jets: Beyond the Progenitor Star

Achromatic breaks in afterglow light curves of gamma-ray bursts (GRBs) arise naturally if the product of the jet's Lorentz factor γand opening angle Θ_j satisfies (γΘ_j) >> 1 at the onset of the afterglow phase, i.e., soon after the conclusion of the prompt emission. Magnetohydrodynamic (MHD) simulations of collimated GRB jets generally give (γΘ_j) <~ 1, suggesting that MHD models may be inconsistent with jet breaks. We work within the collapsar paradigm and use axisymmetric relativistic MHD simulations to explore the effect of a finite stellar envelope on the structure of the jet. Our idealized models treat the jet-envelope interface as a collimating rigid wall, which opens up outside the star to mimic loss of collimation. We find that the onset of deconfinement causes a burst of acceleration accompanied by a slight increase in the opening angle. In our fiducial model with a stellar radius equal to 10^4.5 times that of the central compact object, the jet achieves an asymptotic Lorentz factor γ~ 500 far outside the star and an asymptotic opening angle Θ_j ~ 0.04 rad ~ 2 deg, giving (γΘ_j) ~ 20. These values are consistent with observations of typical long-duration GRBs, and explain the occurrence of jet breaks. We provide approximate analytic solutions that describe the numerical results well.

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arXiv2010

Black Hole Spin and the Radio Loud/Quiet Dichotomy of Active Galactic Nuclei

Radio loud active galactic nuclei (AGN) are on average 1000 times brighter in the radio band compared to radio quiet AGN. We investigate whether this radio loud/quiet dichotomy can be due to differences in the spin of the central black holes that power the radio-emitting jets. Using general relativistic magnetohydrodynamic simulations, we construct steady state axisymmetric numerical models for a wide range of black hole spins (dimensionless spin parameter 0.1 <= a <= 0.9999 and a variety of jet geometries. We assume that the total magnetic flux through the black hole horizon at radius r_H(a) is held constant. If the black hole is surrounded by a thin accretion disk, we find that the total black hole power output depends approximately quadratically on the angular frequency of the hole, P \propto Ω_H^2 \propto (a/r_H)^2. We conclude that, in this scenario, differences in the black hole spin can produce power variations of only a few tens at most. However, if the disk is thick such that the jet subtends a narrow solid angle around the polar axis, then the power dependence becomes much steeper, P \propto Ω_H^4 or even \propto Ω_H^6. Power variations of 1000 are then possible for realistic black hole spin distributions. We derive an analytic solution that accurately reproduces the steeper scaling of jet power with Ω_H, and we provide a numerical fitting formula that reproduces all our simulation results. We discuss other physical effects that might contribute to the observed radio loud/quiet dichotomy of AGN.

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arXiv2010

Numerical Studies of Relativistic Jets

The connection between collimation and acceleration of magnetized relativistic jets is discussed. The focus is on recent numerical simulations which shed light on some longstanding problems.

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arXiv2009

Efficiency of Magnetic to Kinetic Energy Conversion in a Monopole Magnetosphere

Unconfined relativistic outflows from rotating, magnetized compact objects are often well-modeled by assuming the field geometry is approximately a split-monopole at large radii. Earlier work has indicated that such an unconfined flow has an inefficient conversion of magnetic energy to kinetic energy. This has led to the conclusion that ideal magnetohydrodynamical (MHD) processes fail to explain observations of, e.g., the Crab pulsar wind at large radii where energy conversion appears efficient. In addition, as a model for astrophysical jets, the monopole field geometry has been abandoned in favor of externally confined jets since the latter appeared to be generically more efficient jet accelerators. We perform time-dependent axisymmetric relativistic MHD simulations in order to find steady state solutions for a wind from a compact object endowed with a monopole field geometry. Our simulations follow the outflow for 10 orders of magnitude in distance from the compact object, which is large enough to study both the initial "acceleration zone" of the magnetized wind as well as the asymptotic "coasting zone." We obtain the surprising result that acceleration is actually {\it efficient} in the polar region, which develops a jet despite not being confined by an external medium. Our models contain jets that have sufficient energy to account for moderately energetic long and short gamma-ray burst (GRB) events (~10^{51}--10^{52} erg), collimate into narrow opening angles (opening half-angle θ_j \approx 0.03 rad), become matter-dominated at large radii (electromagnetic energy flux per unit matter energy flux σ<1), and move at ultrarelativistic Lorentz factors (γ_j ~ 200 for our fiducial model). (abridged)

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arXiv2009

Stability of Relativistic Force-Free Jets

We consider a two-parameter family of cylindrical force-free equilibria, modeled to match numerical simulations of relativistic force-free jets. We study the linear stability of these equilibria, assuming a rigid impenetrable wall at the outer cylindrical radius R_j. We find that equilibria in which the Lorentz factor γ(R) increases monotonically with increasing radius R are stable. On the other hand, equilibria in which γ(R) reaches a maximum value at an intermediate radius and then declines to a smaller value γ_j at R_j are unstable. The most rapidly growing mode is an m=1 kink instability which has a growth rate ~ (0.4 / γ_j) (c/R_j). The e-folding length of the equivalent convected instability is ~2.5 γ_j R_j. For a typical jet with an opening angle θ_j ~ few / γ_j, the mode amplitude grows weakly with increasing distance from the base of the jet, much slower than one might expect from a naive application of the Kruskal-Shafranov stability criterion.

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