1,075 research outputs found
Magnetically-dominated jets inside collapsing stars as a model for gamma-ray bursts and supernova explosions
It has been suggested that magnetic fields play a dynamically-important role
in core-collapse explosions of massive stars. In particular, they may be
important in the collapsar scenario for gamma-ray bursts (GRB), where the
central engine is a hyper-accreting black hole or a millisecond magnetar. The
present paper is focussed on the magnetar scenario, with a specific emphasis on
the interaction of the magnetar magnetosphere with the infalling stellar
envelope. First, the ``Pulsar-in-a-Cavity'' problem is introduced as a paradigm
for a magnetar inside a collapsing star. The basic set-up of this fundamental
plasma-physics problem is described, outlining its main features, and simple
estimates are derived for the evolution of the magnetic field. In the context
of a collapsing star, it is proposed that, at first, the ram pressure of the
infalling plasma acts to confine the magnetosphere, enabling a gradual build-up
of the magnetic pressure. At some point, the growing magnetic pressure
overtakes the (decreasing) ram pressure of the gas, resulting in a
magnetically-driven explosion. The explosion should be highly anisotropic, as
the hoop-stress of the toroidal field, confined by the surrounding stellar
matter, collimates the magnetically-dominated outflow into two beamed
magnetic-tower jets. This creates a clean narrow channel for the escape of
energy from the central engine through the star, as required for GRBs. In
addition, the delayed onset of the collimated-explosion phase can explain the
production of large quantities of Nickel-56, as suggested by the GRB-Supernova
connection. Finally, the prospects for numerical simulations of this scenario
are discussed.Comment: Invited paper in the "Physics of Plasmas" (May 2007 special issue),
based on an invited talk at the 48th Annual Meeting of the APS Division of
Plasma Physics (Oct. 30 - Nov. 3, 2006, Philadelphia, PA); 24 pages, 7
figure
Stellar Explosions by Magnetic Towers
We propose a magnetic mechanism for the collimated explosion of a massive
star relevant for GRBs, XRFs and asymmetric supernovae. We apply Lynden-Bell's
magnetic tower scenario to the interior of a massive rotating star after the
core has collapsed to form a black hole with an accretion disk or a millisecond
magnetar acting as a central engine. We solve the force-free Grad-Shafranov
equation to calculate the magnetic structure and growth of a tower embedded in
a stellar environment. The pressure of the toroidal magnetic field,
continuously generated by differential rotation of the central engine, drives a
rapid expansion which becomes vertically collimated after lateral force balance
with the surrounding gas pressure is reached. The collimation naturally occurs
because hoop stress concentrates magnetic field toward the rotation axis and
inhibits lateral expansion. This leads to the growth of a self-collimated
magnetic tower. When embedded in a massive star, the supersonic expansion of
the tower drives a strong bow shock behind which an over-pressured cocoon
forms. The cocoon confines the tower by supplying collimating pressure and
provides stabilization against disruption due to MHD instabilities. Because the
tower consists of closed field lines starting and ending on the central engine,
mixing of baryons from the cocoon into the tower is suppressed. The channel
cleared by the growing tower is thus plausibly free of baryons and allows the
escape of magnetic energy from the central engine through the star. While
propagating down the stellar density gradient, the tower accelerates and
becomes relativistic. During the expansion, fast collisionless reconnection
becomes possible resulting in dissipation of magnetic energy which may be
responsible for GRB prompt emission.Comment: 19 pages, 8 figures, accepted to ApJ, updated references and
additional discussion adde
Relativistic Jets from Collapsars
We have studied the relativistic beamed outflow proposed to occur in the
collapsar model of gamma-ray bursts. A jet forms as a consequence of an assumed
energy deposition of erg/s within a cone
around the rotation axis of the progenitor star. The generated jet flow is
strongly beamed (\la few degrees) and reaches the surface of the stellar
progenitor (r cm) intact. At break-out the maximum Lorentz
factor of the jet flow is about 33. Simulations have been performed with the
GENESIS multi-dimensional relativistic hydrodynamic code.Comment: 6 pages, 2 figures, to appear in the proceedings of the conference
"Godunov methods: theory and applications", Oxford, October 199
The effect of alongcoast advection on pacific northwest shelf and slope water properties in relation to upwelling variability
The Northern California Current System experiences highly variable seasonal upwelling in addition to larger basin-scale variability, both of which can significantly affect its water chemistry. Salinity and temperature fields from a 7 year ROMS hindcast model of this region (43°N-50°N), along with extensive particle tracking, were used to study interannual variability in water properties over both the upper slope and the midshelf bottom. Variation in slope water properties was an order of magnitude smaller than on the shelf. Furthermore, the primary relationship between temperature and salinity anomalies in midshelf bottom water consisted of variation in density (cold/salty versus warm/fresh), nearly orthogonal to the anomalies along density levels (cold/fresh versus warm/salty) observed on the upper slope. These midshelf anomalies were well-explained (R2=0.6) by the combination of interannual variability in local and remote alongshore wind stress, and depth of the California Undercurrent (CUC) core. Lagrangian analysis of upper slope and midshelf bottom water shows that both are affected simultaneously by large-scale alongcoast advection of water through the northern and southern boundaries. The amplitude of anomalies in bottom oxygen and dissolved inorganic carbon (DIC) on the shelf associated with upwelling variability are larger than those associated with typical variation in alongcoast advection, and are comparable to observed anomalies in this region. However, a large northern intrusion event in 2004 illustrates that particular, large-scale alongcoast advection anomalies can be just as effective as upwelling variability in changing shelf water properties on the interannual scale
The origin of the negative torque density in disk-satellite interaction
Tidal interaction between a gaseous disk and a massive orbiting perturber is
known to result in angular momentum exchange between them. Understanding
astrophysical manifestations of this coupling such as gap opening by planets in
protoplanetary disks or clearing of gas by binary supermassive black holes
(SMBHs) embedded in accretion disks requires knowledge of the spatial
distribution of the torque exerted on the disk by a perturber. Recent
hydrodynamical simulations by Dong et al (2011) have shown evidence for the
tidal torque density produced in a uniform disk to change sign at the radial
separation of scale heights from the perturber's orbit, in clear
conflict with the previous studies. To clarify this issue we carry out a linear
calculation of the disk-satellite interaction putting special emphasis on
understanding the behavior of the perturbed fluid variables in physical space.
Using analytical as well as numerical methods we confirm the reality of the
negative torque density phenomenon and trace its origin to the overlap of
Lindblad resonances in the vicinity of the perturber's orbit - an effect not
accounted for in previous studies. These results suggest that calculations of
the gap and cavity opening in disks by planets and binary SMBHs should rely on
more realistic torque density prescriptions than the ones used at present.Comment: 18 pages, 6 figures, accepted to Ap
Nucleosynthesis of Nickel-56 from Gamma-Ray Burst Accretion Disks
We examine the prospects for producing Nickel-56 from black hole accretion
disks, by examining a range of steady state disk models. We focus on relatively
slowly accreting disks in the range of 0.05 - 1 solar masses per second, as are
thought to be appropriate for the central engines of long-duration gamma-ray
bursts. We find that significant amounts of Nickel-56 are produced over a wide
range of parameter space. We discuss the influence of entropy, outflow
timescale and initial disk position on mass fraction of Nickel-56 which is
produced. We keep careful track of the weak interactions to ensure reliable
calculations of the electron fraction, and discuss the role of the neutrinos.Comment: 10 pages, 9 figure
Prospects for obtaining an r-process from Gamma Ray Burst Disk Winds
We discuss the possibility that r-process nucleosynthesis may occur in the
winds from gamma ray burst accretion disks. This can happen if the temperature
of the disk is sufficiently high that electron antineutrinos are trapped as
well as neutrinos. This implies accretion disks with greater than a solar mass
per second accretion rate, although lower accretion rates with higher black
hole spin parameters may provide viable environments as well. Additionally, the
outflow from the disk must either have relatively low entropy, e.g. around s =
10, or the initial acceleration of the wind must be slow enough that it is
neutrino and antineutrino capture as opposed to electron and positron capture
that sets the electron fraction.Comment: 8 pages, submitted to Nucl. Phys. A as part of the Nuclei in Cosmos 8
proceeding
Disk-satellite interaction in disks with density gaps
Gravitational coupling between a gaseous disk and an orbiting perturber leads
to angular momentum exchange between them which can result in gap opening by
planets in protoplanetary disks and clearing of gas by binary supermassive
black holes (SMBHs) embedded in accretion disks. Understanding the co-evolution
of the disk and the orbit of the perturber in these circumstances requires
knowledge of the spatial distribution of the torque exerted by the latter on a
highly nonuniform disk. Here we explore disk-satellite interaction in disks
with gaps in linear approximation both in Fourier and in physical space,
explicitly incorporating the disk non-uniformity in the fluid equations.
Density gradients strongly displace the positions of Lindblad resonances in the
disk (which often occur at multiple locations), and the waveforms of modes
excited close to the gap edge get modified compared to the uniform disk case.
The spatial distribution of the excitation torque density is found to be quite
different from the existing prescriptions: most of the torque is exerted in a
rather narrow region near the gap edge where Lindblad resonances accumulate,
followed by an exponential fall-off with the distance from the perturber.
Despite these differences, for a given gap profile the full integrated torque
exerted on the disk agrees with the conventional uniform disk theory prediction
at the level of ~10%. The nonlinearity of the density wave excited by the
perturber is shown to decrease as the wave travels out of the gap, slowing down
its nonlinear evolution and damping. Our results suggest that gap opening in
protoplanetary disks and gas clearing around SMBH binaries can be more
efficient than the existing theories predict. They pave the way for
self-consistent calculations of the gap structure and the orbital evolution of
the perturber using accurate prescription for the torque density behavior.Comment: corrected typos in reference
Hydrodynamical Response of a Circumbinary Gas Disk to Black Hole Recoil and Mass Loss
Finding electromagnetic (EM) counterparts of future gravitational wave (GW)
sources would bring rich scientific benefits. A promising possibility, in the
case of the coalescence of a super-massive black hole binary (SMBHB), is that
prompt emission from merger-induced disturbances in a supersonic circumbinary
disk may be detectable. We follow the post-merger evolution of a thin,
zero-viscosity circumbinary gas disk with two-dimensional simulations, using
the hydrodynamic code FLASH. We analyze perturbations arising from the 530 km/s
recoil of a 10^6 M_sun binary, oriented in the plane of the disk, assuming
either an adiabatic or a pseudo-isothermal equation of state for the gas. We
find that a single-armed spiral shock wave forms and propagates outward,
sweeping up about 20% of the mass of the disk. The morphology and evolution of
the perturbations agrees well with those of caustics predicted to occur in a
collisionless disk. Assuming that the disk radiates nearly instantaneously to
maintain a constant temperature, we estimate the amount of dissipation and
corresponding post-merger light-curve. The luminosity rises steadily on the
time-scale of months, and reaches few times 10^{43} erg/s, corresponding to
about 10% of the Eddington luminosity of the central SMBHB. We also analyze the
case in which gravitational wave emission results in a 5% mass loss in the
merger remnant. The mass-loss reduces the shock overdensities and the overall
luminosity of the disk by 15-20%, without any other major effects on the spiral
shock pattern.Comment: 16 pages with 14 figures, submitted to MNRA
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