arXiv subjects

Eric G. Blackman

Publications and source records attributed to Eric G. Blackman.

At least 163 records · Page 9Linked to original sources

arXiv2003

A Compact X-ray Source and Possible X-ray Jets within the Planetary Nebula Menzel 3

We report the discovery, by the Chandra X-ray Observatory, of X-ray emission from the bipolar planetary nebula Menzel 3. In Chandra CCD imaging, Mz 3 displays hot (3-6x10^6 K) gas within its twin, coaxial bubbles of optical nebulosity, as well as a compact X-ray source at the position of its central star(s). The brightest diffuse X-ray emission lies along the polar axis of the optical nebula, suggesting a jet-like configuration. The observed combination of an X-ray-emitting point source and possible X-ray jet(s) is consistent with models in which accretion disks and, potentially, magnetic fields shape bipolar planetary nebulae via the generation of fast, collimated outflows.

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arXiv2002

Helical surface structures

Over the past few years there has been growing interest in helical magnetic field structures seen at the solar surface, in coronal mass ejections, as well as in the solar wind. Although there is a great deal of randomness in the data, on average the extended structures are mostly left-handed on the northern hemisphere and right-handed on the southern. Surface field structures are also classified as dextral (= right bearing) and sinistral (= left bearing) occurring preferentially in the northern and southern hemispheres respectively. Of particular interest here is a quantitative measurement of the associated emergence rates of helical structures, which translate to magnetic helicity fluxes. In this review, we give a brief survey of what has been found so far and what is expected based on models. Particular emphasis is put on the scale dependence of the associated fields and an attempt is made to estimate the helicity flux of the mean field vs. fluctuating field.

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arXiv2002

Doubly Helical Coronal Ejections from Dynamos and their Role in Sustaining the Solar Cycle

Two questions about the solar magnetic field might be answered together once their connection is identified. The first is important for large scale dynamo theory: what prevents the magnetic backreaction forces from shutting down the dynamo cycle? The second question is: what determines the handedness of twist and writhe in magnetized coronal ejecta? Magnetic helicity conservation is important for answering both questions. Conservation implies that dynamo generation of large scale writhed structures is accompanied by the oppositely signed twist along these structures. The latter is associated with the backreaction force. We suggest that coronal mass ejections (CME's) simultaneously liberate small scale twist and large scale writhe of opposite sign, helping to prevent the cycle from quenching and enabling a net magnetic flux change in each hemisphere. Observations and helicity spectrum measurements from a simulation of a rising flux ribbon support this idea. We show a new pictorial of dynamo flux generation that includes the backreaction and magnetic helicity conservation by characterizing the field as a 2-D ribbon rather than a 1-D line.

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arXiv2002

On Magnetic Field Amplification in Gamma-Ray Burst Sources

Magnetic fields play a dual role in gamma-ray bursts (GRBs). First, GRB and afterglow spectra (the latter interpreted as emission from external shocks) imply synchrotron radiation in a magnetic field that is a significant fraction of equipartition with the particle energy density. Second, magnetized rotators with $\sim 10^{15}$ Gauss field may power GRB by transporting Poynting flux to large distances where it dissipates and also drives an external shock. The field amplification at external shocks and in the engine involve separate processes. External shock fields are likely either seeded by a pre-GRB wind, or are amplified by two-stream plasma instabilities with MHD turbulence playing a subsequent role. In the engine, the large scale fields are likely produced by MHD helical dynamos, since flux accretion cannot easily compete with turbulent diffusion, and because structures must be large enough to rise to coronae before diffusing. Why helical dynamos are feasible, and their relation to the magnetorotational instability are among the points discussed.

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arXiv2002

Line Profiles from Different Accretion Engine Geometries

Line profiles represent an important probe of the geometry and dynamics of the central accretion engines in AGN. Although a flat disk illuminated from above can explain the relativistic Fe K alpha line profile in some cases, current data, plausible disk physics, and future comparison to the iron and other AGN lines invite some consideration of other geometries. General features of line profiles calculated fom concave discs, warped discs, thick clumpy (inflow/outflow) discs, and disks with spiral structure, are briefly summarized.

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arXiv2002

A New Dynamical Mean-Field Dynamo Theory and Closure Approach

We develop a new nonlinear mean field dynamo theory that couples field growth to the time evolution of the magnetic helicity and the turbulent electromotive force, $\emfb$. We show that the difference between kinetic and current helicities emerges naturally as the growth driver when the time derivative of $\emfb$ is coupled into the theory. The solutions predict significant field growth in a kinematic phase and a saturation rate/strength that is magnetic Reynolds number dependent/independent in agreement with numerical simulations. The amplitude of early time oscillations provides a diagnostic for the closure.

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arXiv2002

Dynamic nonlinearity in large scale dynamos with shear

We supplement the mean field dynamo growth equation with the total magnetic helicity evolution equation. This provides an explicitly time dependent model for alpha quenching in dynamo theory. For dynamos without shear, this approach accounts for the observed large scale field growth and saturation in numerical simulations. After a significant kinematic phase, the dynamo is resistively quenched, i.e. the saturation time depends on the microscopic resistivity. This is independent of whether or not the turbulent diffusivity is resistively quenched. We find that the approach is also successful for dynamos that include shear and exhibit migratory waves (cycles). In this case however, whether or not the cycle period remains of the order of the dynamical time scale at large magnetic Reynolds numbers does depend how on how the turbulent magnetic diffusivity quenches. Since this is unconstrained by magnetic helicity conservation, the diffusivity is presently an input parameter. Comparison to current numerical experiments suggests a turbulent diffusivity that depends only weakly on the magnetic Reynolds number, but higher resolution simulations are needed.

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arXiv2002

Recent Developments in Magnetic Dynamo Theory

Some recent results and open issues in magnetic dynamo theory are addressed. The distinction between small-scale and mean-field dynamo (MFD) action in forced turbulent flows is emphasized. Though useful, the MFD has been controversial. This is partly due to concerns about its need for helically forced turbulence, but mainly because simple "textbook" treatments are kinematic and linear. The non-linear backreaction of the growing magnetic field has been suspected to prematurely quench MFD action. To resolve the controversy, we must first understand those non-linear MFDs which can be numerically simulated. Recently, there has been progress on this front. For simple MFDs in closed systems, dynamical quenching models that incorporate a transfer of magnetic helicity between small and large scales agree reasonably well with fully 3-D numerical periodic box simulations. Unresolved issues such as the quenching of turbulent diffusion and the additional physics needed to apply these results to real systems are also discussed herein. The following summarizes conceptual progress in describing mean-field magnetic energy growth in the simplest MFDs: For a closed turbulent flow, the non-linear mean-field dynamo, is first fast and kinematic, then slow and dynamic, and magnetic helicity transfer makes it so.

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arXiv2002

Effect of Fractional Kinetic Helicity on Turbulent Magnetic Dynamo Spectra

Magnetic field amplification in astrophysics ultimately requires an understanding of magnetohydrodynamic turbulence. Kinetic helicity has long been known to be important for large scale field growth in forced MHD turbulence, and has been recently demonstrated numerically to be asymptotically consistent with slow mean field dynamo action in a periodic box. Here we show numerically that the magnetic spectrum at and below the forcing scale is also strongly influenced by kinetic helicity. We identify a critical value, $f_{h,crit}$ above which the magnetic spectrum develops maxima at wavenumber $= 1$ scale {\it and} at the forcing scale, For $f< f_{h,crit}$ the field peaks only at the resistive scale. Kinetic helicity may thus be important not only for generating a large scale field, but also for establishing observed peaks in magnetic spectra at the forcing scale. The turbulent Galactic disk provides an example where both large scale ($>$ supernova forcing scale) fields and small scale ($\le$ forcing scale, with peak at forcing scale) fields are observed. We discuss this, and the potential application to the protogalaxy, but also emphasize the limitations in applying our results to these systems.

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arXiv2002

Dynamical Quenching of the $α^2$ Dynamo

We present a two-scale approximation for the dynamics of a nonlinear $α^2$ dynamo. Solutions of the resulting nonlinear equations agree with the numerical simulations of Brandenburg (2001), and show that $α$ is quenched by the buildup of magnetic helicity at the forcing scale $1/k_2$ as the $α$ effect transfers it from the large scale $1/k_1$. For times $t > (k_1/k_2)R_{M,2}$ in eddy turnover units (where $R_{M,2}$ is the magnetic Reynolds number of the forcing scale), $α$ is resistively limited in the form predicted for the steady-state case. However, for $t << R_{M,2}$, $α$ takes on its kinematic value, independent of $R_{M,2}$, allowing the production of large-scale magnetic energy equal to $k_1/k_2$ times equipartition. Thus the dynamic theory of $α$ predicts substantial "fast" growth of large-scale field despite being "slow" at large times.

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arXiv2001

Iron line profiles from black hole accretion discs with spiral velocity structure

We calculate the iron line profiles from accretion discs with spiral velocity structures around Schwarzschild black holes. We find that quasi-periodic bumps appear in the the profiles, thereby providing a test for spiral wave patterns. This study is motivated by recent work showing that spiral density waves can result from MHD instabilities even in non-self-gravitating discs, and by improved spectral resolution of forthcoming X-ray missions.

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arXiv2001

Dynamos in Asymptotic-Giant-Branch Stars As the Origin of Magnetic Fields Shaping Planetary Nebulale

Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage$^{1}$. A shock forms near the boundary between the winds, which creates a relatively dense shell that provides the characteristic appearance of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape$^{2}$. While a magnetic field could launch and collimate a bipolar outflow, the origin of such a field has hitherto been unclear, as previous work suggested that a field could not be generated${^3}$. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, in a dynamo at the interface between a rapidly rotating core and the more slowly rotating envelope of the star. The field is strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly$^{4}$ slow rotation of most white dwarf stars.

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arXiv2000

How astrophysical mean field dynamos can circumvent existing quenching constraints

Mean field dynamo theory is a leading candidate to explain the observed large scale magnetic fields of galaxies and stars. However, controversy arises over the extent of premature quenching by the backreaction of the growing field. We distinguish between rapid mean field dynamo action, which is required by astrophysical systems, and resistively limited action. We show how the flow of magnetic helicity is important for rapid action. Existing numerical and analytic work suggesting that mean field dynamos are prematurely quenched and resistively limited include approximations or boundary conditions which suppress the magnetic helicity flow from the outset. Thus they do not unambiguously reveal whether real astrophysical mean field dynamos are dynamically suppressed when the helicity flow is allowed. An outflow of helicity also implies an outflow of magnetic energy and so active coronae or winds should accompany mean field dynamos. Open boundaries alone may not be sufficient for rapid dynamo action and the additional physics of buoyancy and outflows may be required. Possible simulation approaches to test some of the principles are briefly discussed. Some limitations of the ``Zeldovich relation'' are also addressed.

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arXiv2000

Implications of mean field accretion disc theory for vorticity and magnetic field growth

In addition to the scalar Shakura-Sunyaev $α_{ss}$ turbulent viscosity transport term used in simple analytic accretion disc modeling, a pseudoscalar transport term also arises. The essence of this term can be captured even in simple models for which vertical averaging is interpreted as integration over a half-thickness and one separately studies each hemisphere. The additional term highlights a complementarity between mean field magnetic dynamo theory and accretion disc theory treated as a mean field theory. Such pseudoscalar terms have been studied, and can lead to large scale magnetic field and vorticity growth. Here it is shown that vorticity can grow even in the simplest azimuthal and half-height integrated disc model, for which mean quantities depend only on radius. The simplest vorticity growth solutions seem to have scales and vortex survival times consistent those required for facilitating planet formation. Also it is shown that when the magnetic back-reaction is included to lowest order, the pseudoscalar driving the magnetic field growth and that driving the vorticity growth will behave differently with respect to shearing and non-shearing flows: the former can reverse sign in the two cases, while the latter will have the same sign.

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arXiv2000

Mean Field Dynamo Saturation: Toward Understanding Conflicting Results

Mean field dynamos may explain the origin of large scale magnetic fields of galaxies, but controversy arises over the extent of dynamo quenching by the growing field. Here we explain how apparently conflicting results may be mutually consistent, by showing the role of magnetic helicity conservation and boundary terms usually neglected. We estimate the associated magnetic energy flowing out of the Galaxy but emphasize that the mechanism of field escape needs to be addressed.

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arXiv2000

Reprocessed emission line profiles from dense clouds in geometrically thick accretion engines

The central engines of active galactic nuclei (AGN) contain cold, dense material as well as hot X-ray emitting gas. The standard paradigm for the engine geometry is a cold thin disc sandwiched between hot X-ray coronae. Strong support for this geometry in Seyferts comes from the study of fluorescent iron line profiles, although the evidence is not ubiquitously air tight. The thin disc model of line profiles in AGN and in X-ray binaries should be bench marked against other plausible possibilities. One proposed alternative is an engine consisting of dense clouds embedded in an optically thin, geometrically thick X-ray emitting engine. This model is further motivated by studies of geometrically thick engines such as advection dominated accretion flows (ADAFs). Here we compute the reprocessed iron line profiles from dense clouds embedded in geometrically thick, optically thin X-ray emitting discs near a Schwarzchild black hole. We consider a range of cloud distributions and disc solutions, including ADAFs, pure radial infall, and bipolar outflows. We find that such models can reproduce line profiles similar to those from geometrically thin, optically thick discs and might help alleviate some of the problems encountered from the latter.

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arXiv2000

Some Consequences of Magnetic Fields in High Energy Sources

Magnetic fields likely play a fundamental intermediary role between gravity and radiation in many astrophysical rotators. They can, among other things, 1) induce and be amplified by turbulence, 2) energize coronae, 3) launch and collimate outflows in ``spring'' or ``fling'' mechanisms. The first is widely recognized to be important for angular momentum transport, but can also produce intrinsic variability and possibly vorticity growth. The second leads to the production of high energy flares, and also facilitates a test of general relativity from AGN observations. The third can operate from rotators with a large scale fields, though the origin of the requisite large scale fields is somewhat unresolved. I discuss these three points in more detail below, emphasizing some open questions.

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arXiv2000

MHD Stellar and Disk Winds: Application to Planetary Nebulae

MHD winds can emanate from both stars and surrounding accretion disks. It is of interest to know how much wind power is available and which (if either) of the two rotators dominates that power. We investigate this in the context of multi-polar planetary nebulae (PNe) and proto-planetary nebulae (PPNe), for which recent observations have revealed the need for a wind power source in excess of that available from radiation driving, and a possible need for magnetic shaping. We calculate the MHD wind power from a coupled disk and star, where the former results from binary disruption. The resulting wind powers depend only on the accretion rate and stellar properties. We find that if the stellar envelope were initially slowly rotating, the disk wind would dominate throughout the evolution. If the envelope of the star were rapidly rotating, the stellar wind could initially be of comparable power to the disk wind until the stellar wind carries away the star's angular momentum. Since an initially rapidly rotating star can have its spin and magnetic axes misaligned to the disk, multi-polar outflows can result from this disk wind system. For times greater than a spin-down time, the post-AGB stellar wind is slaved to the disk for both slow and rapid initial spin cases and the disk wind luminosity dominates. We find a reasonably large parameter space where a hybrid star+disk MHD driven wind is plausible and where both or either can account for PPNe and PNe powers. We also speculate on the morphologies which may emerge from the coupled system. The coupled winds might help explain the shapes of a number of remarkable multi-shell or multi-polar nebulae. Magnetic activity such as X-ray flares may be associated with the both central star and the disk and would be a valuable diagnostic for the dynamical role of MHD processes in PNe.

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