69 research outputs found
Are the magnetic fields of millisecond pulsars ~ 10^8 G?
It is generally assumed that the magnetic fields of millisecond pulsars
(MSPs) are G. We argue that this may not be true and the fields
may be appreciably greater. We present six evidences for this: (1) The G field estimate is based on magnetic dipole emission losses which is
shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are
claimed to have G on the basis of a Rayleygh-Taylor instability
accretion argument. We show that the accretion argument is questionable and the
upper limit G may be much higher; (3) Low magnetic field neutron
stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting
indicating a much higher magnetic field; (5) The data that predict G for MSPs also predict ages on the order of, and greater than, ten
billion years, which is much greater than normal pulsars. If the predicted ages
are wrong, most likely the predicted G fields of MSPs are wrong;
(6) When magnetic fields are measured directly with cyclotron lines in X-ray
binaries, fields G are indicated. Other scenarios should be
investigated. One such scenario is the following. Over 85% of MSPs are
confirmed members of a binary. It is possible that all MSPs are in large
separation binaries having magnetic fields G with their magnetic
dipole emission being balanced by low level accretion from their companions.Comment: 16 pages, accept for publication in Astrophysics and Space Scienc
Kinetic modelling of dissolution dynamic nuclear polarisation 13C magnetic resonance spectroscopy data for analysis of pyruvate delivery and fate in tumours
Dissolution dynamic nuclear polarisation (dDNP) of 13C-labelled pyruvate in magnetic resonance spectroscopy/imaging (MRS/MRSI) has the potential for monitoring tumour progression and treatment response. Pyruvate delivery, its metabolism to lactate and efflux were investigated in rat P22 sarcomas following simultaneous intravenous administration of hyperpolarised 13C-labelled pyruvate (13C1-pyruvate) and urea (13C-urea), a nonmetabolised marker. A general mathematical model of pyruvate-lactate exchange, incorporating an arterial input function (AIF), enabled the losses of pyruvate and lactate from tumour to be estimated, in addition to the clearance rate of pyruvate signal from blood into tumour, Kip, and the forward and reverse fractional rate constants for pyruvate-lactate signal exchange, kpl and klp. An analogous model was developed for urea, enabling estimation of urea tumour losses and the blood clearance parameter, Kiu. A spectral fitting procedure to blood time-course data proved superior to assuming a gamma-variate form for the AIFs. Mean arterial blood pressure marginally correlated with clearance rates. Kiu equalled Kip, indicating equivalent permeability of the tumour vasculature to urea and pyruvate. Fractional loss rate constants due to effluxes of pyruvate, lactate and urea from tumour tissue into blood (kpo, klo and kuo, respectively) indicated that T1s and the average flip angle, θ, obtained from arterial blood were poor surrogates for these parameters in tumour tissue. A precursor-product model, using the tumour pyruvate signal time-course as the input for the corresponding lactate signal time-course, was modified to account for the observed delay between them. The corresponding fractional rate constant, kavail, most likely reflected heterogeneous tumour microcirculation. Loss parameters, estimated from this model with different TRs, provided a lower limit on the estimates of tumour T1 for lactate and urea. The results do not support use of hyperpolarised urea for providing information on the tumour microcirculation over and above what can be obtained from pyruvate alone. The results also highlight the need for rigorous processes controlling signal quantitation, if absolute estimations of biological parameters are required
Spin Down of Rotating Compact Magnetized Strange Stars in General Relativity
We find that in general relativity slow down of the pulsar rotation due to
the magnetodipolar radiation is more faster for the strange star with
comparison to that for the neutron star of the same mass. Comparison with
astrophysical observations on pulsars spindown data may provide an evidence for
the strange star existence and, thus, serve as a test for distinguishing it
from the neutron star.Comment: 6 pages; Accepted for publication in Astrophysics and Space Scienc
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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