39,296 research outputs found
On the magnon interaction in Haematite. 2: Magnon energy of the acoustical mode and magnetic critical fields
Previous spin wave theories of the antiferromagnet hematite were extended. The behavior of thermodynamic quantities around the Morin transition temperature was studied, and the latent heat of the Morin transition was calculated. The temperature dependence of the antiferromagnetic resonance frequency and the parallel and perpendicular critical spin-flop magnetic fields were calculated. It was found that the theory agrees well with experiment
Euclidean action for vacuum decay in a de Sitter universe
The behavior of the action of the instantons describing vacuum decay in a de
Sitter is investigated. For a near-to-limit instanton (a Coleman-de Luccia
instanton close to some Hawking-Moss instanton) we find approximate formulas
for the Euclidean action by expanding the scalar field and the metric of the
instanton in the powers of the scalar field amplitude. The order of the
magnitude of the correction to the Hawking-Moss action depends on the order of
the instanton (the number of crossings of the barrier by the scalar field): for
instantons of odd and even orders the correction is of the fourth and third
order in the scalar field amplitude, respectively. If a near-to-limit instanton
of the first order exists in a potential with the curvature at the top of the
barrier greater than 4 (Hubble constant), which is the case if the
fourth derivative of the potential at the top of the barrier is greater than
some negative limit value, the action of the instanton is less than the
Hawking-Moss action and, consequently, the instanton determines the outcome of
the vacuum decay if no other Coleman-de Luccia instanton is admitted by the
potential. A numerical study shows that for the quartic potential the physical
mode of the vacuum decay is given by the Coleman-de Luccia instanton of the
first order also in the region of parameters in which the potential admits two
instantons of the second order.Comment: 16 pages, 3 figures, references adde
A Novel Approach in Constraining Electron Spectra in Blazar Jets: The Case of Markarian 421
We report results from the observations of the well studied TeV blazar Mrk
421 with the Swift and the Suzaku satellites in December 2008. During the
observation, Mrk 421 was found in a relatively low activity state, with the
corresponding 2-10 keV flux of erg/s/cm^2. For the purpose
of robust constraining the UV-to-X-ray emission continuum we selected only the
data corresponding to truly simultaneous time intervals between Swift and
Suzaku, allowing us to obtain a good-quality, broad-band spectrum despite a
modest length (0.6 ksec) exposure. We analyzed the spectrum with the parametric
forward-fitting SYNCHROTRON model implemented in XSPEC assuming two different
representations of the underlying electron energy distribution, both well
motivated by the current particle acceleration models: a power-law distribution
above the minimum energy with an exponential cutoff at the
maximum energy , and a modified ultra-relativistic Maxwellian
with an equilibrium energy . We found that the latter implies
unlikely physical conditions within the blazar zone of Mrk 421. On the other
hand, the exponentially moderated power-law electron distribution gives two
possible sets of the model parameters: (i) flat spectrum with low minimum electron energy , and
(ii) steep spectrum with high minimum electron energy
. We discuss different interpretations of
both possibilities in the context of a diffusive acceleration of electrons at
relativistic, sub- or superluminal shocks. We also comment on how exactly the
gamma-ray data can be used to discriminate between the proposed different
scenarios.Comment: 18 pages, 2 figures; accepted for publication in the Astrophysical
Journa
Very Rapid High-Amplitude Gamma-ray Variability in Luminous Blazar PKS 1510-089 Studied with Fermi-LAT
Here we report on the detailed analysis of the gamma-ray light curve of a
luminous blazar PKS1510-089 observed in the GeV range with the Large Area
Telescope (LAT) onboard the Fermi satellite during the period 2011 September --
December. By investigating the properties of the detected three major flares
with the shortest possible time binning allowed by the photon statistics, we
find a variety of temporal characteristics and variability patterns. This
includes a clearly asymmetric profile (with a faster flux rise and a slower
decay) of the flare resolved on sub-daily timescales, a superposition of many
short uncorrelated flaring events forming the apparently coherent
longer-duration outburst, and a huge single isolated outburst unresolved down
to the timescale of three-hours. In the latter case we estimate the
corresponding gamma-ray flux doubling timescale to be below one hour, which is
extreme and never previously reported for any active galaxy in the GeV range.
The other unique finding is that the total power released during the studied
rapid and high-amplitude flares constitute the bulk of the power radiatively
dissipated in the source, and a significant fraction of the total kinetic
luminosity of the underlying relativistic outflow. Our analysis allows us to
access directly the characteristic timescales involved in shaping the energy
dissipation processes in the source, and to provide constraints on the location
and the structure of the blazar emission zone in PKS1510-089.Comment: 15 pages, 4 figures, 1 table. Accepted for publication in the
Astrophysical Journal Letter
Unfolding the Sulcus
Sulci are localized furrows on the surface of soft materials that form by a
compression-induced instability. We unfold this instability by breaking its
natural scale and translation invariance, and compute a limiting bifurcation
diagram for sulcfication showing that it is a scale-free, sub-critical {\em
nonlinear} instability. In contrast with classical nucleation, sulcification is
{\em continuous}, occurs in purely elastic continua and is structurally stable
in the limit of vanishing surface energy. During loading, a sulcus nucleates at
a point with an upper critical strain and an essential singularity in the
linearized spectrum. On unloading, it quasi-statically shrinks to a point with
a lower critical strain, explained by breaking of scale symmetry. At
intermediate strains the system is linearly stable but nonlinearly unstable
with {\em no} energy barrier. Simple experiments confirm the existence of these
two critical strains.Comment: Main text with supporting appendix. Revised to agree with published
version. New result in the Supplementary Informatio
Lattice model theory of the equation of state covering the gas, liquid, and solid phases
The three stable states of matter and the corresponding phase transitions were obtained with a single model. Patterned after Lennard-Jones and Devonshires's theory, a simple cubic lattice model containing two fcc sublattices (alpha and beta) is adopted. The interatomic potential is taken to be the Lennard-Jones (6-12) potential. Employing the cluster variation method, the Weiss and the pair approximations on the lattice gas failed to give the correct phase diagrams. Hybrid approximations were devised to describe the lattice term in the free energy. A lattice vibration term corresponding to a free volume correction is included semi-phenomenologically. The combinations of the lattice part and the free volume part yield the three states and the proper phase diagrams. To determine the coexistence regions, the equalities of the pressure and Gibbs free energy per molecule of the coexisting phases were utilized. The ordered branch of the free energy gives rise to the solid phase while the disordered branch yields the gas and liquid phases. It is observed that the triple point and the critical point quantities, the phase diagrams and the coexistence regions plotted are in good agreement with the experimental values and graphs for argon
Laboratory Experiment of Checkerboard Pupil Mask Coronagraph
We present the results of the first laboratory experiment of checkerboard
shaped pupil binary mask coronagraphs using visible light, in the context of
the R&D activities for future mid-infrared space missions such as the 3.5 m
SPICA telescope. The primary aim of this work is to demonstrate the
coronagraphic performance of checkerboard masks down to a
peak-to-peak contrast, which is required to detect self-luminous extra-solar
planets in the mid-infrared region. Two masks, consisting of aluminum films on
a glass substrates, were manufactured using nano-fabrication techniques with
electron beam lithography: one mask was optimized for a pupil with a 30%
central obstruction and the other was for a pupil without obstruction. The
theoretical contrast for both masks was and no adaptive optics system
was employed. For both masks, the observed point spread functions were quite
consistent with the theoretical ones. The average contrast measured within the
dark regions was and . The
coronagraphic performance significantly outperformed the requirement
and almost reached the theoretical limit determined by the mask designs. We
discuss the potential application of checkerboard masks for mid-infrared
coronagraphy, and conclude that binary masks are promising for future
high-contrast space telescopes.Comment: 6 pages, 6 figure
Semi-relativistic approximation to gravitational radiation from encounters with nonspinning black holes
The capture of compact bodies by black holes in galactic nuclei is an
important prospective source for low frequency gravitational wave detectors,
such as the planned Laser Interferometer Space Antenna. This paper calculates,
using a semirelativistic approximation, the total energy and angular momentum
lost to gravitational radiation by compact bodies on very high eccentricity
orbits passing close to a supermassive, nonspinning black hole; these
quantities determine the characteristics of the orbital evolution necessary to
estimate the capture rate. The semirelativistic approximation improves upon
treatments which use orbits at Newtonian-order and quadrupolar radiation
emission, and matches well onto accurate Teukolsky simulations for low
eccentricity orbits. Formulae are presented for the semirelativistic energy and
angular momentum fluxes as a function of general orbital parameters.Comment: 27 pages, 12 figures; v2: revised manuscript includes small changes
to make paper consistent with published version; v3: a statement about how to
generalise our results to hyperbolic orbits was incorrect, new version
includes published erratum as an appendi
Space charge and charge trapping characteristics of cross-linked polyethylene subjected to ac electric stresses
This paper reports on the result of space charge evolution in cross-linked polyethylene (XLPE) planar samples of approximately 220 ?m thick. The space charge measurement technique used in this study is the PEA method. There are two phases to this experiment. In the first phase, the samples were subjected to dc 30 kVdc/mm and ac (sinusoidal) electric stress level of 30 kVpk/mm at frequencies of 1 Hz, 10 Hz and 50 Hz ac. In addition, ac space charge under 30 kVrms/mm and 60 kVpk/mm electric stress at 50 Hz was also investigated. The volts off results showed that the amount of charge trapped in XLPE sample under dc electric stress is significantly bigger than samples under ac stress even when the applied ac stresses are substantially higher. The second phase of the experiment involves studying the dc space charge evolution in samples that were tested under ac stress during the first phase of the experiment. Ac ageing causes positive charge to become more dominant over negative charge. It was also discovered that ac ageing creates deeper traps, particularly for negative charge. This paper also gave a brief overview of the data processing methods used to analyse space charge under ac electric stress
- …