3,260 research outputs found
Relativistic Precessing Jets and Cosmological Gamma Ray Bursts
We discuss the possibility that gamma-ray bursts may result from cosmological
relativistic blob emitting neutron star jets that precess past the line of
sight. Beaming reduces the energy requirements, so that the jet emission can
last longer than the observed burst duration. One precession mode maintains a
short duration time scale, while a second keeps the beam from returning to the
line of sight, consistent with the paucity of repeaters. The long life of these
objects reduces the number required for production as compared to short lived
jets. Blobs can account for the time structure of the bursts. Here we focus
largely on kinematic and time scale considerations of beaming, precession, and
blobs--issues which are reasonably independent of the acceleration and jet
collimation mechanisms. We do suggest that large amplitude electro-magnetic
waves could be a source of blob acceleration.Comment: 15 pages, plain TeX, accepted to ApJ
Magnetic helicity fluxes in interface and flux transport dynamos
Dynamos in the Sun and other bodies tend to produce magnetic fields that
possess magnetic helicity of opposite sign at large and small scales,
respectively. The build-up of magnetic helicity at small scales provides an
important saturation mechanism. In order to understand the nature of the solar
dynamo we need to understand the details of the saturation mechanism in
spherical geometry. In particular, we want to understand the effects of
magnetic helicity fluxes from turbulence and meridional circulation. We
consider a model with just radial shear confined to a thin layer (tachocline)
at the bottom of the convection zone. The kinetic alpha owing to helical
turbulence is assumed to be localized in a region above the convection zone.
The dynamical quenching formalism is used to describe the build-up of mean
magnetic helicity in the model, which results in a magnetic alpha effect that
feeds back on the kinetic alpha effect. In some cases we compare with results
obtained using a simple algebraic alpha quenching formula. In agreement with
earlier findings, the magnetic alpha effect in the dynamical alpha quenching
formalism has the opposite sign compared with the kinetic alpha effect and
leads to a catastrophic decrease of the saturation field strength with
increasing magnetic Reynolds numbers. However, at high latitudes this quenching
effect can lead to secondary dynamo waves that propagate poleward due to the
opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and
from meridional circulation alleviate catastrophic quenching.Comment: 9 pages, 14 figures, submitted to A &
Planets Rapidly Create Holes in Young Circumstellar Discs
Recent spectral observations by the Spitzer Space Telescope (SST) reveal that
some discs around young ( yr old) stars have
remarkably sharp transitions to a low density inner region in which much of the
material has been cleared away. It has been recognized that the most plausible
mechanism for the sharp transition at a specific radius is the gravitational
influence of a massive planet. This raises the question of whether the planet
can also account for the hole extending all the way to the star. Using high
resolution numerical simulations, we show that Jupiter-mass planets drive
spiral waves which create holes on time scales times shorter than
viscous or planet migration times. We find that the theory of spiral-wave
driven accretion in viscous flows by Takeuchi et al. (1996) can be used to
provide a consistent interpretation of the simulations. In addition, although
the hole surface densities are low, they are finite, allowing mass accretion
toward the star. Our results therefore imply that massive planets can form
extended, sharply bounded spectral holes which can still accommodate
substantial mass accretion rates. The results also imply that holes are more
likely than gaps for Jupiter mass planets around solar mass stars.Comment: accepted by Ap
Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
Conversion of gravitational energy into radiation in accretion discs and the
origin of large scale magnetic fields in astrophysical rotators have often been
distinct topics of research. In semi-analytic work on both problems it has been
useful to presume large scale symmetries, necessarily resulting in mean field
theories. MHD turbulence makes the underlying systems locally asymmetric and
nonlinear. Synergy between theory and simulations should aim for the
development of practical mean field models that capture essential physics and
can be used for observational modeling. Mean field dynamo (MFD) theory and
alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century
MFD theory has more nonlinear predictive power compared to 20th century MFD
theory, whereas accretion theory is still in a 20th century state. In fact,
insights from MFD theory are applicable to accretion theory and the two are
artificially separated pieces of what should be a single theory. I discuss
pieces of progress that provide clues toward a unified theory. A key concept is
that large scale magnetic fields can be sustained via local or global magnetic
helicity fluxes or via relaxation of small scale magnetic fluctuations, without
the kinetic helicity driver of 20th century textbooks. These concepts may help
explain the formation of large scale fields that supply non-local angular
momentum transport via coronae and jets in a unified theory of accretion and
dynamos. In diagnosing the role of helicities and helicity fluxes in disk
simulations, each disk hemisphere should be studied separately to avoid being
misled by cancelation that occurs as a result of reflection asymmetry. The
fraction of helical field energy in disks is expected to be small compared to
the total field in each hemisphere as a result of shear, but can still be
essential for large scale dynamo action.Comment: For the Proceedings of the Third International Conference and
Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28
August 2011 at the Abdus Salam International Centre for Theoretical Physics,
Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica
Scripta (corrected small items to match version in print
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
An Explanation for the Bimodal Distribution of Gamma-Ray Bursts: Millisecond Pulsars from Accretion-Induced Collapse
Cosmological gamma-ray bursts (GRBs) could be driven by dissipation of pure
electromagnetic energy (Poynting flux) extracted from rapidly rotating compact
objects with strong magnetic fields. One such possibility is a young
millisecond pulsar (MSP) formed from accretion-induced collapse (AIC) of a
white dwarf. The combination of an efficient magnetic dynamo, likely operating
during the first seconds of the initially hot and turbulent MSP interior, and
the subsequent modest beaming of gamma-ray emitting outflows, would easily
account for energy constraints. But the remarkable feature of such models is
that they may naturally explain the hitherto unexplained bimodal distribution
in GRB time durations. The two burst classes could correspond to MSPs that form
spinning above and below a gravitationally unstable limit respectively. In the
former case, the spin-down time scale is due to gravitational radiation
emission () while the spin-down time scale of the latter is due to
electromagnetic dipole emission (). These two time scales account for
the short and long GRB durations, i.e. the observed bimodal GRB duration
distribution. A natural prediction is that the short duration GRBs would be
accompanied by strong gravitational radiation emission which is absent from the
longer class. Both would show millisecond variabilities.Comment: 10 pages, Ap
Radio Continuum Jet in NGC 7479
The barred galaxy NGC 7479 hosts a remarkable jet-like radio continuum
feature: bright, 12-kpc long in projection, and hosting an aligned magnetic
field. The degree of polarization is 6%-8% along the jet, and remarkably
constant, which is consistent with helical field models. The radio brightness
of the jet suggests strong interaction with the ISM and hence a location near
the disk plane. We observed NGC 7479 at four wavelengths with the VLA and
Effelsberg radio telescopes. The equipartition strength is 35-40 micro-G for
the total and >10 micro-G for the ordered magnetic field in the jet. The jet
acts as a bright, polarized background. Faraday rotation between 3.5 and 6 cm
and depolarization between 6 and 22 cm can be explained by magneto-ionic gas in
front of the jet, with thermal electron densities of ~0.06 cm**(-3) in the bar
and ~0.03 cm**(-3) outside the bar. The regular magnetic field along the bar
points toward the nucleus on both sides. The regular field in the disk reveals
multiple reversals, probably consisting of field loops stretched by a shearing
gas flow in the bar. The projection of the jet bending in the sky plane is in
the sense opposite to that of the underlying stellar and gaseous spiral
structure. The bending in 3-D is most easily explained as a precessing jet,
with an age less than 10**6 years. Our observations are consistent with very
recent triggering, possibly by a minor merger. NGC 7479 provides a unique
opportunity to study interaction-triggered 15-kpc scale radio jets within a
spiral galaxy.Comment: 18 pages, 21 figures, accepted for publication in the Astrophysical
Journa
The XMM/BeppoSAX observation of Mkn 841
Mkn 841 has been observed simultaneously by XMM and BeppoSAX in January 2001.
Due to operational contingency, the 30ks XMM observation was split into two
parts, separated by about 15 hours. We first report the presence of a narrow
iron line which appears to be rapidly variable between the two pointings,
requiring a non-standard interpretation. We then focus on the analysis of the
broad band (0.3-200 keV) continuum using the XMM/EPIC, RGS and SAX/PDS data.
The Mkn 841 spectrum is well fitted by a comptonization model in a geometry
more photon-fed than a simple slab geometry above a passive disk. It presents a
relatively large reflection (R>2) which does not agree with an apparently weak
iron line. It also show the presence of a strong soft excess wellfitted by a
comptonized spectrum in a cool plasma, suggesting the presence of a
multi-temperature corona.Comment: 4 pages, 5 figures. Proc. of the meeting: "The Restless High-Energy
Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't
Zand, and R.A.M.J. Wijers Ed
Recommended from our members
Evaluating a financial service opportunity via judgemental modelling
The financial services market is currently undergoing tremendous change, due to the twin influences of developments in the field of Electronic Data Interchange (EDI) and changes in the legislation determining how banks will be allowed to operate after 1992. Describes research whose objective was to evaluate the viability of introducing a new service to handle cross-border payments, undertaken for a large multinational bank, and focused on respondents from a variety of firms within the European Community. Uses a combination of a PC-based judgemental modelling package and in-depth interviews to develop a clear understanding of the needs of both the originators′ and the vendors′ requirements. Although based on a relatively small sample, the detailed findings permit development of an effective approach to segmenting the market for the particular service
Jet Deflection via Cross winds: Laboratory Astrophysical Studies
We present new data from High Energy Density (HED) laboratory experiments
designed to explore the interaction of a heavy hypersonic radiative jet with a
cross wind. The jets are generated with the MAGPIE pulsed power machine where
converging conical plasma flows are produced from a cylindrically symmetric
array of inclined wires. Radiative hypersonic jets emerge from the convergence
point. The cross wind is generated by ablation of a plastic foil via
soft-X-rays from the plasma convergence region. Our experiments show that the
jets are deflected by the action of the cross wind with the angle of deflection
dependent on the proximity of the foil. Shocks within the jet beam are apparent
in the data. Analysis of the data shows that the interaction of the jet and
cross wind is collisional and therefore in the hydro-dynamic regime. MHD plasma
code simulations of the experiments are able to recover the deflection
behaviour seen in the experiments. We consider the astrophysical relevance of
these experiments applying published models of jet deflection developed for AGN
and YSOs. Fitting the observed jet deflections to quadratic trajectories
predicted by these models allows us to recover a set of plasma parameters
consistent with the data. We also present results of 3-D numerical simulations
of jet deflection using a new astrophysical Adaptive Mesh Refinement code.
These simulations show highly structured shocks occurring within the beam
similar to what was observed in the experimentsComment: Submitted to ApJ. For a version with figures go to
http://web.pas.rochester.edu/~afrank/labastro/CW/Jet-Wind-Frank.pd
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