33,322 research outputs found
Dynamical Evolution of a Cylindrical Shell with Rotational Pressure
We prepare a general framework for analyzing the dynamics of a cylindrical
shell in the spacetime with cylindrical symmetry. Based on the framework, we
investigate a particular model of a cylindrical shell-collapse with rotational
pressure, accompanying the radiation of gravitational waves and massless
particles. The model has been introduced previously but has been awaiting for
proper analysis. Here the analysis is put forward: It is proved that, as far as
the weak energy condition is satisfied outside the shell, the collapsing shell
bounces back at some point irrespective of the initial conditions, and escapes
from the singularity formation.
The behavior after the bounce depends on the sign of the shell pressure in
the z-direction. When the pressure is non-negative, the shell continues to
expand without re-contraction. On the other hand, when the pressure is negative
(i.e. it has a tension), the behavior after the bounce can be more complicated
depending on the details of the model. However, even in this case, the shell
never reaches the zero-radius configuration.Comment: To appear in Phys. Rev.
Born-Infeld magnetars: larger than classical toroidal magnetic fields and implications for gravitational-wave astronomy
Magnetars are neutron stars presenting bursts and outbursts of X- and
soft-gamma rays that can be understood with the presence of very large magnetic
fields. Thus, nonlinear electrodynamics should be taken into account for a more
accurate description of such compact systems. We study that in the context of
ideal magnetohydrodynamics and make a realization of our analysis to the case
of the well-known Born-Infeld (BI) electromagnetism in order to come up with
some of its astrophysical consequences. We focus here on toroidal magnetic
fields as motivated by already known magnetars with low dipolar magnetic fields
and their expected relevance in highly magnetized stars. We show that BI
electrodynamics leads to larger toroidal magnetic fields when compared to
Maxwell's electrodynamics. Hence, one should expect higher production of
gravitational waves (GWs) and even more energetic giant flares from nonlinear
stars. Given current constraints on BI's scale field, giant flare energetics
and magnetic fields in magnetars, we also find that the maximum magnitude of
magnetar ellipticities should be . Besides, BI electrodynamics
may lead to a maximum increase of order of the GW energy radiated
from a magnetar when compared to Maxwell's, while much larger percentages may
arise for other physically motivated scenarios. Thus, nonlinear theories of the
electromagnetism might also be probed in the near future with the improvement
of GW detectors.Comment: 8 pages, no figures, accepted for publication in The European
Physical Journal C (EPJC
All-sky Relative Opacity Mapping Using Night Time Panoramic Images
An all-sky cloud monitoring system that generates relative opacity maps over
many of the world's premier astronomical observatories is described.
Photometric measurements of numerous background stars are combined with
simultaneous sky brightness measurements to differentiate thin clouds from sky
glow sources such as air glow and zodiacal light. The system takes a continuous
pipeline of all-sky images, and compares them to canonical images taken on
other nights at the same sidereal time. Data interpolation then yields
transmission maps covering almost the entire sky. An implementation of this
system is currently operating through the Night Sky Live network of CONCAM3s
located at Cerro Pachon (Chile), Mauna Kea (Hawaii), Haleakala (Hawaii), SALT
(South Africa) and the Canary Islands (Northwestern Africa).Comment: Accepted for publication in PAS
Experimental and theoretical evidences for the ice regime in planar artificial spin ices
In this work, we explore a kind of geometrical effect in the thermodynamics
of artificial spin ices (ASI). In general, such artificial materials are
athermal. Here, We demonstrate that geometrically driven dynamics in ASI can
open up the panorama of exploring distinct ground states and thermally magnetic
monopole excitations. It is shown that a particular ASI lattice will provide a
richer thermodynamics with nanomagnet spins experiencing less restriction to
flip precisely in a kind of rhombic lattice. This can be observed by analysis
of only three types of rectangular artificial spin ices (RASI). Denoting the
horizontal and vertical lattice spacings by a and b, respectively, then, a RASI
material can be described by its aspect ratio =a/b. The rhombic lattice
emerges when =. So, by comparing the impact of thermal
effects on the spin flips in these three appropriate different RASI arrays, it
is possible to find a system very close to the ice regime
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