2,460 research outputs found
Stability of an Ultra-Relativistic Blast Wave in an External Medium with a Steep Power-Law Density Profile
We examine the stability of self-similar solutions for an accelerating
relativistic blast wave which is generated by a point explosion in an external
medium with a steep radial density profile of a power-law index > 4.134. These
accelerating solutions apply, for example, to the breakout of a gamma-ray burst
outflow from the boundary of a massive star, as assumed in the popular
collapsar model. We show that short wavelength perturbations may grow but only
by a modest factor <~ 10.Comment: 12 pages, 3 figures, submitted to Physical Review
The Non-Relativistic Evolution of GRBs 980703 and 970508: Beaming-Independent Calorimetry
We use the Sedov-Taylor self-similar solution to model the radio emission
from the gamma-ray bursts (GRBs) 980703 and 970508, when the blastwave has
decelerated to non-relativistic velocities. This approach allows us to infer
the energy independent of jet collimation. We find that for GRB 980703 the
kinetic energy at the time of the transition to non-relativistic evolution,
t_NR ~ 40 d, is E_ST ~ (1-6)e51 erg. For GRB 970508 we find E_ST ~ 3e51 erg at
t_NR ~ 100 d, nearly an order of magnitude higher than the energy derived in
Frail, Waxman and Kulkarni (2000). This is due primarily to revised
cosmological parameters and partly to the maximum likelihood fit we use here.
Taking into account radiative losses prior to t_NR, the inferred energies agree
well with those derived from the early, relativistic evolution of the
afterglow. Thus, the analysis presented here provides a robust,
geometry-independent confirmation that the energy scale of cosmological GRBs is
about 5e51 erg, and additionally shows that the central engine in these two
bursts did not produce a significant amount of energy in mildly relativistic
ejecta at late time. Furthermore, a comparison to the prompt energy release
reveals a wide dispersion in the gamma-ray efficiency, strengthening our
growing understanding that E_gamma is a not a reliable proxy for the total
energy.Comment: Submitted to ApJ; 13 pages, 6 figures, 1 table; high-resolution
figures can be found at: http://www.astro.caltech.edu/~ejb/NR
Navigation in Curved Space-Time
A covariant and invariant theory of navigation in curved space-time with
respect to electromagnetic beacons is written in terms of J. L. Synge's
two-point invariant world function. Explicit equations are given for navigation
in space-time in the vicinity of the Earth in Schwarzschild coordinates and in
rotating coordinates. The restricted problem of determining an observer's
coordinate time when their spatial position is known is also considered
Confinement of supernova explosions in a collapsing cloud
We analyze the confining effect of cloud collapse on an expanding supernova
shockfront. We solve the differential equation for the forces on the shockfront
due to ram pressure, supernova energy, and gravity. We find that the expansion
of the shockfront is slowed and in fact reversed by the collapsing cloud.
Including radiative losses and a potential time lag between supernova explosion
and cloud collapse shows that the expansion is reversed at smaller distances as
compared to the non-radiative case. We also consider the case of multiple
supernova explosions at the center of a collapsing cloud. For instance, if we
scale our self-similar solution to a single supernova of energy 10^51 ergs
occurring when a cloud of initial density 10^2 H/cm^3 has collapsed by 50%, we
find that the shockfront is confined to ~15 pc in ~1 Myrs. Our calculations are
pertinent to the observed unusually compact non-thermal radio emission in blue
compact dwarf galaxies (BCDs). More generally, we demonstrate the potential of
a collapsing cloud to confine supernovae, thereby explaining how dwarf galaxies
would exist beyond their first generation of star formation.Comment: 3 pages, 4 figure
Self-similar cosmologies in 5D: spatially flat anisotropic models
In the context of theories of Kaluza-Klein type, with a large extra
dimension, we study self-similar cosmological models in 5D that are
homogeneous, anisotropic and spatially flat. The "ladder" to go between the
physics in 5D and 4D is provided by Campbell-Maagard's embedding theorems. We
show that the 5-dimensional field equations determine the form of
the similarity variable. There are three different possibilities: homothetic,
conformal and "wave-like" solutions in 5D. We derive the most general
homothetic and conformal solutions to the 5D field equations. They require the
extra dimension to be spacelike, and are given in terms of one arbitrary
function of the similarity variable and three parameters. The Riemann tensor in
5D is not zero, except in the isotropic limit, which corresponds to the case
where the parameters are equal to each other. The solutions can be used as 5D
embeddings for a great variety of 4D homogeneous cosmological models, with and
without matter, including the Kasner universe. Since the extra dimension is
spacelike, the 5D solutions are invariant under the exchange of spatial
coordinates. Therefore they also embed a family of spatially {\it
inhomogeneous} models in 4D. We show that these models can be interpreted as
vacuum solutions in braneworld theory. Our work (I) generalizes the 5D
embeddings used for the FLRW models; (II) shows that anisotropic cosmologies
are, in general, curved in 5D, in contrast with FLRW models which can always be
embedded in a 5D Riemann-flat (Minkowski) manifold; (III) reveals that
anisotropic cosmologies can be curved and devoid of matter, both in 5D and 4D,
even when the metric in 5D explicitly depends on the extra coordinate, which is
quite different from the isotropic case.Comment: Typos corrected. Minor editorial changes and additions in the
Introduction and Summary section
Self-similar imploding relativistic shock waves
Self-similar solutions to the problem of a strong imploding relativistic
shock wave are calculated. These solutions represent the relativistic
generalisation of the Newtonian Gouderley-Landau-Stanyukovich problem of a
strong imploding spherical shock wave converging to a centre. The solutions are
found assuming that the pre-shocked flow has a uniform density, and are
accurate for sufficiently large times after the formation of the shock wave.Comment: 22 pages, 4 figures. Minor corrections and a discussion of the
singular C_ characteristic added. Accepted for publication in Physics of
Fluid
Egological study of the phenomenon of subjective space
The article presents a phenomenological analysis of the space of transcendental subjectivity. The relevance of this study is to form an idea of the internal spatial structures of the pure I. The purpose of this study is to identify the complex nature of self-consciousness. To achieve this goal, the egological analysis was based on a mereological approach, which prepares the consideration of the space of pure consciousness from the point of view of the whole, composed of parts. The application of spatial categories to the pure Self is based on the expansion of the proximity space.
The results obtained indicate the need to create an egology of space in order to study the nature of spatial vision. The proposed research focuses not on the variety of available spaces, but on the process in which their formation takes place in subjective spatial vision. The influence of the subjective factor on the understanding of space is demonstrated by the example of mathematics. The mathematical notion of “proximity space” is subjected to a phenomenological extension, which reveals the phenomenon of spatiality of the pure I, or the inner space of the subject, which reflects the hierarchy of preferences or evaluations.
The primary spatial structure of pure consciousness is revealed, which is the division of pure I into two parts. The splitting of the Self into two constituent parts and its representation in the form of a spatial model with its top and bottom is funded by a deeper phenomenon — subjective spatiality. This structure can serve as a foundation for the subsequent correlative study of objective and subjective factors influencing the formation of various egological structures. Based on the analysis, a conclusion is made about the essential relationship of the pure I and the inner space. The importance of this conclusion is supported by an analysis of Husserl’s works, in which he touches upon the issues of the spatiality of transcendental subjectivity. The scientific novelty of the proposed work is that the distinction between absolute and relative intimacy is introduced into egological research. Only within the framework of relative intimacy is it possible to observe and systematically investigate the pure I. In the absolute position it remains anonymous because of its extreme intimacy
Self-similar solutions for relativistic shocks emerging from stars with polytropic envelopes
We consider a strong ultrarelativistic shock moving through a star whose
envelope has a polytrope-like density profile. When the shock is close to the
star's outer boundary, its behavior follows the self-similar solution given by
Sari (2005) for implosions in planar geometry. Here we outline this solution
and find the asymptotic solution as the shock reaches the star's edge. We then
show that the motion after the shock breaks out of the star is described by a
self-similar solution remarkably like the solution for the motion inside the
star. In particular, the characteristic Lorentz factor, pressure, and density
vary with time according to the same power laws both before and after the shock
breaks out of the star. After emergence from the star, however, the
self-similar solution's characteristic position corresponds to a point behind
the leading edge of the flow rather than at the shock front, and the relevant
range of values for the similarity variable changes. Our numerical integrations
agree well with the analytic results both before and after the shock reaches
the star's edge.Comment: 18 pages, 5 figures, submitted to Ap
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