120 research outputs found
Public Broadcasting: A Medium in Search of Solutions
Simulation based methods used to estimate uncertainties in models have recently gained much interest. We study the bootstrap and how this method could be used in system identification related problems. The main contribution is a proposed algorithm to estimate the probability density function in case of undermodeling. We illustrate the performance of the bootstrap resampling method by simulation examples, which are in good agreement with Monte Carlo simulations
Crust-core interactions and the magnetic dipole orientation in neutron stars
We develop an effective model for a neutron star with a magnetosphere. It
takes into account the electromagnetic torques acting on the magnetic dipole,
the friction forces between the crust and the core, and the gravitational
corrections. Anomalous electromagnetic torques, usually neglected in a rigid
star model, play here a crucial role for the alignement of the magnetic dipole.
The crust-core coupling time implied by the model is consistent with the
observational data and other theoretical estimations. This model describes the
main features of the behavior of the magnetic dipole during the life of the
star, and in particular gives a natural explanation for the n<3 value of the
breaking index in a young neutron star.Comment: 23 pages, 1 figur
Communication Research
Contains reports on nine research projects.Bell Telephone Laboratories, Incorporate
Communication Research
Contains reports on eight research projects.Bell Telephone Laboratories, IncorporatedCarnegie FoundationRockefeller FoundationOffice of Naval Researc
The Galactic Center with Roman
We advocate for a Galactic center (GC) field to be added to the Galactic
Bulge Time Domain Survey (GBTDS). The new field would yield high-cadence
photometric and astrometric measurements of an unprecedented 3.3
million stars toward the GC. This would enable a wide range of science cases,
such as finding star-compact object binaries that may ultimately merge as
LISA-detectable gravitational wave sources, constraining the mass function of
stars and compact objects in different environments, detecting populations of
microlensing and transiting exoplanets, studying stellar flares and variability
in young and old stars, and monitoring accretion onto the central supermassive
black hole. In addition, high-precision proper motions and parallaxes would
open a new window into the large-scale dynamics of stellar populations at the
GC, yielding insights into the formation and evolution of galactic nuclei and
their co-evolution with the growth of the supermassive black hole. We discuss
the possible trade-offs between the notional GBTDS and the addition of a GC
field with either an optimal or minimal cadence. Ultimately, the addition of a
GC field to the GBTDS would dramatically increase the science return of Roman
and provide a legacy dataset to study the mid-plane and innermost regions of
our Galaxy.Comment: 19 pages, 3 figures. Submitted to the NASA Roman Core Community
Surveys White Paper Cal
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
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