10,064 research outputs found
Nature of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
I summarize recent developments in the magnetar model of the Soft Gamma
Repeaters and Anomalous X-ray Pulsars, give a critical inventory of alternative
models for the AXPs, and outline the improved diagnostics expected from present
observational efforts.Comment: Invited review, Soft Gamma Repeaters: The Rome 2000 Mini-workshop,
eds. M. Feroci, S. Mereghetti, & L. Stell
Impulsive Electromagnetic Emission near a Black Hole
The electromagnetic signature of a point explosion near a Kerr black hole
(BH) is evaluated. The first repetitions produced by gravitational lensing are
not periodic in time; periodicity emerges only as the result of multiple
circuits of the prograde and retrograde light rings and is accompanied by
exponential dimming. Gravitational focusing creates a sequence of concentrated
caustic features and biases the detection of a repeating source toward
alignment of the BH spin with the plane of the sky. We consider the
polarization pattern in the case of emission by the Lorentz upboosting and
reflection of a magnetic field near the explosion site. Then the polarized
fraction of the detected pulse approaches unity, and rays propagating near the
equatorial plane maintain a consistent polarization direction. Near a slowly
accreting supermassive BH (SMBH), additional repetitions are caused by
reflection off annular fragments of an orbiting disk that has passed through an
ionization instability. These results are applied to the repeating fast radio
burst (FRB) source 121102, giving a concrete and predictive example of how FRB
detectability may be biased by lensing. A gravitational lensing delay of 10-30
s, and reflection delay up to s, are found for emission near the
innermost stable circular orbit of a SMBH; these
effects combine to produce interesting correlations between delay time and
burst fluence. A similar repetitive pulse envelope could be seen in the
gravitational wave signal produced by a collision between compact stars near a
SMBH.Comment: Published in the Astrophysical Journal, 32 pages, 17 figure
Tiny Electromagnetic Explosions
This paper considers electromagnetic transients of a modest total energy
( erg) and small initial size ( cm). They could be produced during collisions between relativistic
field structures (e.g. macroscopic magnetic dipoles) that formed around, or
before, cosmic electroweak symmetry breaking. The outflowing energy has a
dominant electromagnetic component; a subdominant thermal component
(temperature GeV) supplies inertia in the form of residual . A
thin shell forms that expands subluminally, attaining a Lorentz factor before decelerating. Drag is supplied by the reflection of an ambient
magnetic field, and by deflection of ambient free electrons. Emission of
low-frequency (GHz-THz) superluminal waves takes place through three channels:
i) reflection of the ambient magnetic field; ii) direct linear conversion of
the embedded magnetic field into a superluminal mode; and iii) excitation
outside the shell by corrugation of its surface. The escaping electromagnetic
pulse is very narrow (a few wavelengths) and so the width of the detected
transient is dominated by propagation effects. GHz radio transients are emitted
from i) the dark matter halos of galaxies and ii) the near-horizon regions of
supermassive black holes that formed by direct gas collapse and now accrete
slowly. Brighter and much narrower 0.01-1 THz pulses are predicted at a rate at
least comparable to fast radio bursts, experiencing weaker scattering and
absorption. The same explosions also accelerate protons up to eV
and heavier nuclei up to eV.Comment: 25 pages, 16 figures, Astrophysical Journal, in pres
Dissipation in Relativistic Outflows: A Multisource Overview
Relativistically expanding sources of X-rays and gamma-rays cover an enormous
range of (central) compactness and Lorentz factor. The underlying physics is
discussed, with an emphasis on how the dominant dissipative mode and the
emergent spectrum depend on these parameters. Photons advected outward from
high optical depth are a potentially important source of Compton seeds. Their
characteristic energy is bounded below by ~1 MeV in pair-loaded outflows of
relatively low compactness, and remains near ~1 MeV at very high compactness
and low matter loading. This is compared with the characteristic energy of O(1)
MeV observed in the rest frame spectra of many sources, including gamma-ray
bursts, OSSE jet sources, MeV Blazars, and the intense initial 0.1 s pulse of
the March 5 event. Additional topics discussed include the feedback of pair
creation on electron heating and the formation of non-thermal spectra, their
effectiveness at shielding the dissipative zone from ambient photons, direct
Compton damping of irregularities in the outflow, the relative importance of
various soft photon sources, and the softening of the emergent spectrum that
results from heavy matter loading. The implications of this work for X-ray and
optical afterglow from GRB's are briefly considered. Direct synchrotron
emission behind the forward shock is inhibited by the extremely low energy
density of the ambient magnetic field. Mildly relativistic ejecta off axis from
the main gamma-ray emitting cone become optically thin to scattering on a
timescale of ~1 day (E/10^{52} erg)^{1/2}, and can be a direct source of
afterglow radiation.Comment: 23 pages, August 1997, invited review, Cracow Conference on
Relativistic Jets in AGN, eds. M. Ostrowski, M. Sikora, G. Madejski, and M.
Begelman, Jagellonian University Press, p. 63 [cited in accompanying astro-ph
preprint, but not available on NASA ADS
Giant Primeval Magnetic Dipoles
Macroscopic magnetic dipoles are considered as cosmic dark matter. Permanent
magnetism in relativistic field structures can involve some form of
superconductivity, one example being current-carrying string loops (`springs')
with vanishing net tension. We derive the cross section for free classical
dipoles to collide, finding it depends weakly on orientation when mutual
precession is rapid. The collision rate of `spring' loops with tension in galactic halos approaches the measured rate of fast
radio bursts (FRBs) if the loops comprise most of the dark matter. A large
superconducting dipole (LSD) with mass g and size mm
will form a km magnetosphere moving through interstellar plasma.
Although hydromagnetic drag is generally weak, it is strong enough to capture
some LSDs into long-lived rings orbiting supermassive black holes (SMBHs) that
form by the direct collapse of massive gas clouds. Repeated collisions near
young SMBHs could dominate the global collision rate, thereby broadening the
dipole mass spectrum. Colliding LSDs produce tiny, hot electromagnetic
explosions. The accompanying paper shows that these explosions couple
effectively to propagating low-frequency electromagnetic modes, with output
peaking at 0.01-1 THz. We describe several constraints on, and predictions of,
LSDs as cosmic dark matter. The shock formed by an infalling LSD triggers
self-sustained thermonuclear burning in a C/O (ONeMg) white dwarf (WD) of mass
(). The spark is generally located well off
the center of the WD. The rate of LSD-induced explosions matches the observed
rate of Type Ia supernovae.Comment: 23 pages, 19 figures, Astrophysical Journal in pres
Spin and Magnetism of White Dwarfs
The magnetism and rotation of white dwarf (WD) stars are investigated in
relation to a hydromagnetic dynamo operating in the progenitor during shell
burning phases. The downward pumping of angular momentum in the convective
envelope, in combination with the absorption of a planet or tidal spin-up from
a binary companion, can trigger strong dynamo action near the core-envelope
boundary. Several arguments point to the outer core as the source for a
magnetic field in the WD remnant: the outer third of a WD
is processed during the shell burning phase(s) of the progenitor; the escape of
magnetic helicity through the envelope mediates the growth of (compensating)
helicity in the core, as is needed to maintain a stable magnetic field in the
remnant; and the intense radiation flux at the core boundary facilitates
magnetic buoyancy within a relatively thick tachocline layer. The helicity flux
into the growing core is driven by a dynamical imbalance with a
latitude-dependent rotational stress. The magnetic field deposited in an
isolated massive WD is concentrated in an outer shell of mass and can reach MG. A buried toroidal field experiences
moderate ohmic decay above an age Gyr, which may lead to growth or
decay of the external magnetic field. The final WD spin period is related to a
critical spin rate below which magnetic activity shuts off, and core and
envelope decouple; it generally sits in the range of hours to days. WD periods
ranging up to a year are possible if the envelope re-expands following a late
thermal pulse.Comment: 23 pages, 27 figures, accepted to the ApJ, typos and figures
correcte
Hot Electromagnetic Outflows. III. Displaced Fireball in a Strong Magnetic Field
The evolution of a dilute electron-positron fireball is calculated in the
regime of strong magnetization and very high compactness (l ~10^3-10^8).
Heating is applied at a low effective temperature (< 25 keV), and the fireball
is allowed to expand, so that the formation of a black-body spectral
distribution is inhibited by pair annihilation. The diffusion equation for
Compton scattering is coupled to a single-temperature pair gas and an exact
(trans-relativistic) cyclo-synchrotron photon source. We find that the photon
spectrum develops a quasi-thermal peak, with a power-law slope below it that is
characteristic of gamma-ray bursts. The formation of a thermal high-frequency
spectrum is checked using the full kinetic equations at l ~ 10^3. These results
have several implications for the central engine of GRBs, and the mechanism of
energy transport. 1. Baryon rest mass carries less than ~ 10^{-5} of the energy
flux at jet breakout inside ~ 10^{12} cm from the engine, with most carried by
the magnetic field. 2. This degree of baryon purity points to the presence of
an event horizon in the engine, and neutrons play a negligible role in the
prompt emission mechanism. 3. X-ray flashes are emitted by outflows carrying
enough baryons that the photosphere is pair-depleted, which we show results in
faster thermalization. 4. The relation between observed peak frequency and
burst luminosity is bounded below by the observed Amati et al. relation if jet
Lorentz factor ~ 1/(opening angle) at breakout. 5. Stellar models are used to
demonstrate an inconsistency between the highest observed GRB energies, and a
hydrodynamic nozzle: magnetic collimation is required. 6. The magnetized pair
gas is dilute enough that high-frequency Alfven waves may become charge
starved. Finally, we suggest that limitations on magnetic reconnection from
plasma collisionality have been overestimated.Comment: 29 pages, 34 figures, submitted to the Ap
Hot Electromagnetic Outflows I: Acceleration and Spectra
The theory of cold, relativistic, magnetohydrodynamic outflows is generalized
by the inclusion of an intense radiation source. In some contexts, such the
breakout of a gamma-ray burst jet from a star, the outflow is heated to a high
temperature at a large optical depth. Eventually it becomes transparent and is
pushed to a higher Lorentz factor by a combination of the Lorentz force and
radiation pressure. We obtain its profile, both inside and outside the fast
magnetosonic critical point, when the poloidal magnetic field is radial and
monopolar. Most of the energy flux is carried by the radiation field and the
toroidal magnetic field that is wound up close to the rapidly rotating engine.
Although the entrained matter carries little energy, it couples the radiation
field to the magnetic field. Then the fast critical point is pushed inward from
infinity and, above a critical radiation intensity, the outflow is accelerated
mainly by radiation pressure. We identify a distinct observational signature of
this hybrid outflow: a hardening of the radiation spectrum above the peak of
the seed photon distribution, driven by bulk Compton scattering. The
non-thermal spectrum -- obtained by a Monte Carlo method -- is most extended
when the Lorentz force dominates the acceleration, and the seed photon beam is
wider than the Lorentz cone of the MHD fluid. This effect is a generic feature
of hot, magnetized outflows interacting with slower relativistic material. It
may explain why some GRB spectra appear to peak at photon energies above the
original Amati et al. scaling. A companion paper addresses the case of jet
breakout, where diverging magnetic flux surfaces yield strong MHD acceleration
over a wider range of Lorentz factor.Comment: To be published in the Astrophysical Journa
Rotation and Magnetism of Massive Stellar Cores
The internal rotation and magnetism of evolved massive stars are considered
in response to i) the inward pumping of angular momentum through deep and
slowly rotating convective layers; and ii) the winding up of a helical magnetic
field in radiative layers. Field winding can transport angular momentum
effectively even when the toroidal field is limited by kinking. Magnetic
helicity is pumped into a growing radiative layer from an adjacent convective
envelope (or core). The receding convective envelope that forms during the
early accretion phase of a massive star is the dominant source of helicity in
its core, yielding a G polar magnetic field in a collapsed
neutron star (NS) remnant. Using MESA models of various masses, we find that
the NS rotation varies significantly, from s in a
13 model to ms in a model with an
extended core. Stronger inward pumping of angular momentum is found in more
massive stars, due to the growing thickness of the convective shells that form
during the later stages of thermonuclear burning. On the other hand, stars that
lose enough mass to form blue supergiants in isolation end up as very slow
rotators. The tidal spin-up of a 40 star by a massive binary
companion is found to dramatically increase the spin of the remnant black hole,
allowing a rotationally supported torus to form during the collapse. The
implications for post-collapse decay or amplification of the magnetic field are
also considered.Comment: 21 pages, 23 figures, minor revisions including expanded comparison
with previous work, Astrophysical Journal, in pres
Constrained Evolution of a Radially Magnetized Protoplanetary Disk: Implications for Planetary Migration
We consider the inner AU of a protoplanetary disk (PPD), at a stage
where angular momentum transport is driven by the mixing of a radial magnetic
field into the disk from a T-Tauri wind. Because the radial profile of the
imposed magnetic field is well constrained, a deterministic calculation of the
disk mass flow becomes possible. The vertical disk profiles obtained in Paper I
imply a stronger magnetization in the inner disk, faster accretion, and a
secular depletion of the disk material. Inward transport of solids allows the
disk to maintain a broad optical absorption layer even when the grain abundance
becomes too small to suppress its ionization. Thus a PPD may show a strong
middle-to-near infrared spectral excess even while its mass profile departs
radically from the minimum-mass solar nebula. The disk surface density is
buffered at g cm: below this, X-rays trigger strong enough
magnetorotational turbulence at the midplane to loft mm-cm sized particles high
in the disk, followed by catastrophic fragmentation. A sharp density gradient
bounds the inner depleted disk, and propagates outward to -2 AU over a
few Myr. Earth-mass planets migrate through the inner disk over a similar
timescale, whereas the migration of Jupiters is limited by the supply of gas.
Gas-mediated migration must stall outside 0.04 AU, where silicates are
sublimated and the disk shifts to a much lower column. A transition disk
emerges when the dust/gas ratio in the MRI-active layer falls below , where is the grain size.Comment: 22 pp, 18 figures, Astrophysical Journal, in pres
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