59 research outputs found
Electromagnetic power of merging and collapsing compact objects
[Abridged] Electromagnetic emission can be produced as a precursor to the
merger, as a prompt emission during the collapse of a NS and at the spin-down
stage of the resulting BH. We demonstrate that the time evolution of the
axisymmetric force-free magnetic fields can be expressed in terms of the
hyperbolic Grad-Shafranov equation. We find exact non-linear time-dependent
split-monopole structure of magnetosphere driven by spinning and collapsing NS
in Schwarzschild geometry. Based on this solution, we argue that the collapse
of a NS into the BH happens smoothly, without natural formation of current
sheets or other dissipative structures on the open field lines and, thus, does
not allow the magnetic field to become disconnected from the star and escape to
infinity. Thus, as long as an isolated Kerr BH can produce plasma and currents,
it does not lose its open magnetic field lines, its magnetospheric structure
evolved towards a split monopole and the BH spins down electromagnetically. The
"no hair theorem", which assumes that the outside medium is a vacuum, is not
applicable in this case: highly conducting plasma introduces a topological
constraint forbidding the disconnection of the magnetic field lines from the
BH. Eventually, a single random large scale spontaneous reconnection event will
lead to magnetic field release, shutting down the electromagnetic BH engine
forever. We also discuss the nature of short Gamma Ray Bursts and suggest that
the similarity of the early afterglows properties of long and short GRBs can be
related to the fact that in both cases a spinning BH can retains magnetic field
for sufficiently long time to extract a large fraction of its rotation energy
and produce high energy emission via the internal dissipation in the wind
Simple waves in relativistic fluids
We consider the Riemann problem for relativistic flows of polytropic fluids
and find relations for the flow characteristics. Evolution of physical
quantities take especially simple form for the case of cold magnetized plasmas.
We find exact, explicit analytical solutions for one dimensional expansion of
magnetized plasma into vacuum, valid for arbitrary magnetization. We also
consider expansion into cold unmagnetized external medium both for stationary
initial conditions and for initially moving plasma, as well as reflection of
rarefaction wave from a wall. We also find self-similar structure of
three-dimensional magnetized outflows into vacuum, valid close to the
plasma-vacuum interface.
The key results of this work, the self-similar solutions, were incorporated
post-initial submission into appendices of the published version of Granot et
al. (2010)
Scattering and Diffraction in Magnetospheres of Fast Pulsars
We apply a theory of wave propagation through a turbulent medium to the
scattering of radio waves in pulsar magnetospheres. We find that under
conditions of strong density modulation the effects of magnetospheric
scintillations in diffractive and refractive regimes may be observable. The
most distinctive feature of the magnetospheric scintillations is their
independence on frequency.
Results based on diffractive scattering due to small scale inhomogeneities
give a scattering angle that may be as large as 0.1 radians, and a typical
decorrelation time of seconds.
Refractive scattering due to large scale inhomogeneities is also possible,
with a typical angle of radians and a correlation time of the order
of seconds. Temporal variation in the plasma density may also result
in a delay time of the order of seconds. The different scaling of the
above quantities with frequency may allow one to distinguish the effects of
propagation through a pulsar magnetosphere from the interstellar medium. In
particular, we expect that the magnetospheric scintillations are relatively
more important for nearby pulsars when observed at high frequencies.Comment: 19 pages, 1 Figur
Slowly balding black holes
The "no hair" theorem, a key result in General Relativity, states that an
isolated black hole is defined by only three parameters: mass, angular
momentum, and electric charge; this asymptotic state is reached on a
light-crossing time scale. We find that the "no hair" theorem is not formally
applicable for black holes formed from collapse of a rotating neutron star.
Rotating neutron stars can self-produce particles via vacuum breakdown forming
a highly conducting plasma magnetosphere such that magnetic field lines are
effectively "frozen-in" the star both before and during collapse. In the limit
of no resistivity, this introduces a topological constraint which prohibits the
magnetic field from sliding off the newly-formed event horizon. As a result,
during collapse of a neutron star into a black hole, the latter conserves the
number of magnetic flux tubes , where
is the initial
magnetic flux through the hemispheres of the progenitor and out to infinity. We
test this theoretical result via three-dimensional general relativistic plasma
simulations of rotating black holes that start with a neutron star dipole
magnetic field with no currents initially present outside the event horizon.
The black hole's magnetosphere subsequently relaxes to the split monopole
magnetic field geometry with self-generated currents outside the event horizon.
The dissipation of the resulting equatorial current sheet leads to a slow loss
of the anchored flux tubes, a process that balds the black hole on long
resistive time scales rather than the short light-crossing time scales expected
from the vacuum "no-hair" theorem.Comment: 14 pages, 2 figure
The electromagnetic model of Gamma Ray Bursts
I describe electromagnetic model of gamma ray bursts and contrast its main
properties and predictions with hydrodynamic fireball model and its
magnetohydrodynamical extension. The electromagnetic model assumes that
rotational energy of a relativistic, stellar-mass central source
(black-hole--accretion disk system or fast rotating neutron star) is converted
into magnetic energy through unipolar dynamo mechanism, propagated to large
distances in a form of relativistic, subsonic, Poynting flux-dominated wind and
is dissipated directly into emitting particles through current-driven
instabilities. Thus, there is no conversion back and forth between internal and
bulk energies as in the case of fireball model. Collimating effects of magnetic
hoop stresses lead to strongly non-spherical expansion and formation of jets.
Long and short GRBs may develop in a qualitatively similar way, except that in
case of long bursts ejecta expansion has a relatively short, non-relativistic,
strongly dissipative stage inside the star. Electromagnetic and fireball models
(as well as strongly and weakly magnetized fireballs) lead to different early
afterglow dynamics, before deceleration time. Finally, I discuss the models in
view of latest observational data in the Swift era.Comment: solicited contribution to Focus Issue of New Journal of Physics, 27
pages, 4 figure
On particle acceleration and trapping by Poynting flux dominated flows
Using particle-in-cell (PIC) simulations, we study the evolution of a
strongly magnetized plasma slab propagating into a finite density ambient
medium. Like previous work, we find that the slab breaks into discrete magnetic
pulses. The subsequent evolution is consistent with diamagnetic relativistic
pulse acceleration of \cite{liangetal2003}. Unlike previous work, we use the
actual electron to proton mass ratio and focus on understanding trapping vs.
transmission of the ambient plasma by the pulses and on the particle
acceleration spectra. We find that the accelerated electron distribution
internal to the slab develops a double-power law. We predict that emission from
reflected/trapped external electrons will peak after that of the internal
electrons. We also find that the thin discrete pulses trap ambient electrons
but allow protons to pass through, resulting in less drag on the pulse than in
the case of trapping of both species. Poynting flux dominated scenarios have
been proposed as the driver of relativistic outflows and particle acceleration
in the most powerful astrophysical jets.Comment: 25 pages, Accepted by Plasma Physics and Controlled Fusio
Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
(ABBREVIATED) We consider the structure of neutron star magnetospheres
threaded by large-scale electrical currents, and the effect of resonant Compton
scattering by the charge carriers (both electrons and ions) on the emergent
X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs,
these currents are maintained by magnetic stresses acting deep inside the star.
We construct self-similar, force-free equilibria of the current-carrying
magnetosphere with a power-law dependence of magnetic field on radius, B ~
r^(-2-p), and show that a large-scale twist softens the radial dependence to p
< 1. The spindown torque acting on the star is thereby increased in comparison
with a vacuum dipole. We comment on the strength of the surface magnetic field
in the SGR and AXP sources, and the implications of this model for the narrow
measured distribution of spin periods. A magnetosphere with a strong twist,
B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant
cyclotron scattering, independent of frequency (radius), surface magnetic field
strength, or charge/mass ratio of the scattering charge. When electrons and
ions supply the current, the stellar surface is also heated by the impacting
charges at a rate comparable to the observed X-ray output of the SGR and AXP
sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux
at the ion and electron cyclotron resonances will significantly modify the
emerging pulse profile and, through the Doppler effect, generate a non-thermal
tail to the X-ray spectrum. The sudden change in the pulse profile of SGR
1900+14 after the 27 August 1998 giant flare is related to an enhanced optical
depth to electron cyclotron scattering, resulting from a sudden twist imparted
to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.
Extreme Plasma Astrophysics
This is a science white paper submitted to the Astro-2020 and Plasma-2020
Decadal Surveys. The paper describes the present status and emerging
opportunities in Extreme Plasma Astrophysics -- a study of
astrophysically-relevant plasma processes taking place under extreme conditions
that necessitate taking into account relativistic, radiation, and QED effects.Comment: A science white paper submitted to the Astro-2020 and Plasma-2020
Decadal Surveys. 7 pages including cover page and references. Paper updated
in late March 2019 to include a several additional co-authors and references,
and a few small change
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