438 research outputs found
A general maximum entropy principle for self-gravitating perfect fluid
We consider a self-gravitating system consisting of perfect fluid with
spherical symmetry. Using the general expression of entropy density, we
extremize the total entropy under the constraint that the total number of
particles is fixed. We show that extrema of coincides precisely with the
relativistic Tolman-Oppenheimer-Volkoff (TOV) equation of hydrostatic
equilibrium. Furthermore, we apply the maximum entropy principle to a charged
perfect fluid and derive the generalized TOV equation. Our work provides a
strong evidence for the fundamental relationship between general relativity and
ordinary thermodynamics.Comment: 13 pages, no figure. The arguments have been improved so that the
assumption p=p(\rho) is no longer neede
Effects of Electromagnetic Field on the Dynamical Instability of Cylindrical Collapse
The objective of this paper is to discuss the dynamical instability in the
context of Newtonian and post Newtonian regimes. For this purpose, we consider
non-viscous heat conducting charged isotropic fluid as a collapsing matter with
cylindrical symmetry. Darmois junction conditions are formulated. The
perturbation scheme is applied to investigate the influence of dissipation and
electromagnetic field on the dynamical instability. We conclude that the
adiabatic index has smaller value for such a fluid in cylindrically
symmetric than isotropic sphere
Measurements Of Recoil Ion Longitudinal Momentum Transfer In Multiply Ionizing Collisions Of Fast Heavy Ions With Multielectron Targets
The longitudinal momentum transfer to the recoil ion in collisions of 1 MeV/amu bare F ions with Ne are resolved for final charge states of both projectile and recoil ions. We observe the recoil to be thrown backwards in electron-capture events, reflecting the physical impact of the electron translation factor. The size of the momentum transfer is in agreement with classical trajectory Monte Carlo calculations for low charge state recoil ions but not for high charge state recoil ions. © 1993 The American Physical Society
Increased net muscle protein balance in response to simultaneous and separate ingestion of carbohydrate and essential amino acids following resistance exercise
Relative to essential amino acids (EAAs), carbohydrate (CHO) ingestion stimulates a delayed response of net muscle protein balance (NBAL). We investigated if staggered ingestion of CHO and EAA would superimpose the response of NBAL following resistance exercise, thus resulting in maximal anabolic stimulation. Eight recreationally trained subjects completed 2 trials: combined (COMB - drink 1, CHO+EAA; drink 2, placebo) and separated (SEP - drink 1, CHO; drink 2, EAA) post-exercise ingestion of CHO and EAA. Drink 1 was administered 1 h following an acute exercise bout and was followed 1 h later by drink 2. A primed, continuous infusion of l-[ring-13C6]-phenylalanine was combined with femoral arteriovenous sampling and muscle biopsies for the determination of muscle protein kinetics. Arterial amino acid concentrations increased following ingestion of EAA in both conditions. No difference between conditions was observed for phenylalanine delivery to the leg (COMB: 167 ± 23 μmol·min-1·(100 mL leg vol)-1 × 6 h; SEP: 167 ± 21 μmol·min-1·(100 mL leg vol)-1 × 6 h, P > 0.05). In the first hour following ingestion of the drink containing EAA, phenylalanine uptake was 50% greater for the SEP trial than the COMB trial. However, phenylalanine uptake was similar for COMB (110 ± 19 mg) and SEP (117 ± 24 mg) over the 6 h period. These data suggest that whereas separation of CHO and EAA ingestion following exercise may have a transient physiological impact on NBAL, this response is not reflected over a longer period. Thus, separation of CHO and EAA ingestion is unnecessary to optimize post-exercise muscle protein metabolism
Unpulsed UBV Optical Emission from the Crab Pulsar
Based on observations of the Crab pulsar using the TRIFFID high speed imaging
photometer in the UBV bands using the Special Astrophysical Observatory's 6m
telescope in the Russian Caucasus, we report the detection of pronounced
emission during the so-called `off' phase of emission. Following de-extinction,
this unpulsed component of emission is shown to be consistent with a power law
with an exponent of alpha = -0.60 +/- 0.37, the uncertainty being dominated by
the error associated with the independent CCD photometry used to reference the
TRIFFID data. This suggests a steeper power law form than that reported
elsewhere in the literature for the total integrated spectrum, which is
essentially flat with alpha ~ 0.1, although the difference in this case is only
significant at the ~ 2 sigma level. Deeper reference integrated and TRIFFID
phase-resolved photometry in these bands in conjunction with further
observations in the UV and R region would constrain this fit further.Comment: 26 pages, 2 figures, uses aasms4.sty, accepted for publication in the
Astrophysical Journa
Pre- and post-selected ensembles and time-symmetry in quantum mechanics
An expression is proposed for the quantum mechanical state of a pre- and
post-selected ensemble, which is an ensemble determined by the final as well as
the initial state of the quantum systems involved. It is shown that the
probabilities calculated from the proposed state agree with previous
expressions, for cases where they both apply. The same probabilities are found
when they are calculated in the forward- or reverse-time directions. This work
was prompted by several problems raised by Shimony recently in relation to the
state, and time symmetry, of pre- and post-selected ensembles.Comment: RevTex4, 17 pages, no fig
Relativistic parsec-scale jets: II. Synchrotron emission
We calculate the optically thin synchrotron emission of fast electrons and
positrons in a spiral stationary magnetic field and a radial electric field of
a rotating relativistic strongly magnetized force-free jet consisting of
electron-positron pair plasma. The magnetic field has a helical structure with
a uniform axial component and a toroidal component that is maximal inside the
jet and decreasing to zero towards the boundary of the jet. Doppler boosting
and swing of the polarization angle of synchrotron emission due to the
relativistic motion of the emitting volume are calculated. The distribution of
the plasma velocity in the jet is consistent with the electromagnetic field
structure. Two spatial distributions of fast particles are considered: uniform,
and concentrated in the vicinity of the Alfven resonance surface. The latter
distribution corresponds to the regular acceleration by an electromagnetic wave
in the vicinity of its Alfven resonance surface inside the jet. The
polarization properties of the radiation have been obtained and compared with
the existing VLBI polarization measurements of parsec-scale jets in BL Lac
sources and quasars. Our results give a natural explanation of the observed
bimodality in the alignment between the electric field vector of the polarized
radiation and the projection of the jet axis on the plane of the sky. We
interpret the motion of bright knots as a phase velocity of standing spiral
eigenmodes of electromagnetic perturbations in a cylindrical jet. The degree of
polarization and the velocity of the observed proper motion of bright knots
depend upon the angular rotational velocity of the jet. The observed
polarizations and velocities of knots indicate that the magnetic field lines
are bent in the direction opposite to the direction of the jet rotation.Comment: 14 pages, 5 figures, Astron. Astroph. in pres
Simultaneous Absolute Timing of the Crab Pulsar at Radio and Optical Wavelengths
The Crab pulsar emits across a large part of the electromagnetic spectrum.
Determining the time delay between the emission at different wavelengths will
allow to better constrain the site and mechanism of the emission. We have
simultaneously observed the Crab Pulsar in the optical with S-Cam, an
instrument based on Superconducting Tunneling Junctions (STJs) with s time
resolution and at 2 GHz using the Nan\c{c}ay radio telescope with an instrument
doing coherent dedispersion and able to record giant pulses data. We have
studied the delay between the radio and optical pulse using simultaneously
obtained data therefore reducing possible uncertainties present in previous
observations. We determined the arrival times of the (mean) optical and radio
pulse and compared them using the tempo2 software package. We present the most
accurate value for the optical-radio lag of 255 21 s and suggest the
likelihood of a spectral dependence to the excess optical emission asociated
with giant radio pulses.Comment: 8 pages; accepted for publication in Astronomy and Astrophysic
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