32,333 research outputs found
Resolving Gamma-Ray Burst 000301C with a Gravitational Microlens
The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic,
short time-scale variability that is difficult to reconcile with the standard
relativistic shock model. We interpret the observed light curves as a
microlensing event superimposed on power-law flux decays typical of afterglows.
In general, a relativistic GRB shock appears on the sky as a thin ring
expanding at a superluminal speed. Initially the ring is small relative to its
angular separation from the lens and so its flux is magnified by a constant
factor. As the ring grows and sweeps across the lens its magnification reaches
a maximum. Subsequently, the flux gradually recovers its unlensed value. This
behavior involves only three free parameters in its simplest formulation and
was predicted theoretically by Loeb & Perna (1998). Fitting the available
R-band photometric data of GRB 000301C to a simple model of the microlensing
event and a broken power-law for the afterglow, we find reasonable values for
all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99
for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8
days after the burst. The entire optical-IR data imply a width of the GRB ring
of order 10% of its radius, similar to theoretical expectations. The angular
resolution provided by microlensing is better than a micro-arcsecond. We infer
a mass of approximately 0.5 M_Sun for a lens located half way to the source at
z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens,
but current data provides only an upper-limit on its surface brightness at the
GRB position.Comment: to appear in the ApJ Letters, 13 pages, 3 figures (one additional
figure included); all data used for the fits available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB000301C/ and through WWW at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
Cylindrically symmetric wormholes
This paper discusses traversable wormholes that differ slightly but
significantly from those of the Morris-Thorne type under the assumption of
cylindrical symmetry. The throat is a piecewise smooth cylindrical surface
resulting in a shape function that is not differentiable at some value. It is
proposed that the regular derivative be replaced by a one-sided derivative at
this value. The resulting wormhole geometry satisfies the weak energy
condition.Comment: Supplied missing figures; 15 pages AMSTe
Maximum Mass-Radius Ratio for Compact General Relativistic Objects in Schwarzschild- de Sitter Geometry
Upper limits for the mass-radius ratio are derived for arbitrary general
relativistic matter distributions in the presence of a cosmological constant.
General restrictions for the red shift and total energy (including the
gravitational contribution) for compact objects in the Schwarzschild-de Sitter
geometry are also obtained in terms of the cosmological constant and of the
mean density of the star.Comment: 8 pages, no figure
A detailed analysis of structure growth in theories of gravity
We investigate the connection between dark energy and fourth order gravity by
analyzing the behavior of scalar perturbations around a
Friedmann-Robertson-Walker background. The evolution equations for scalar
perturbation are derived using the covariant and gauge invariant approach and
applied to two widely studied gravity models. The structure of the
general fourth order perturbation equations and the analysis of scalar
perturbations lead to the discovery of a characteristic signature of fourth
order gravity in the matter power spectrum, the details of which have not seen
before in other works in this area. This could provide a crucial test for
fourth order gravity on cosmological scales.Comment: 27 pages and 35 figure
Symplectic SUSY Gauge Theories with Antisymmetric Matter
We investigate the confining phase vacua of supersymmetric Sp(2\NC) gauge
theories that contain matter in both fundamental and antisymmetric
representations. The moduli spaces of such models with \NF=3 quark flavors
and \NA=1 antisymmetric field are analogous to that of SUSY QCD with
\NF=\NC+1 flavors. In particular, the forms of their quantum superpotentials
are fixed by classical constraints. When mass terms are coupled to
W_{(\NF=3,\NA=1)} and heavy fields are integrated out, complete towers of
dynamically generated superpotentials for low energy theories with fewer
numbers of matter fields can be derived. Following this approach, we deduce
exact superpotentials in and theories which cannot be
determined by symmetry considerations or integrating in techniques. Building
upon these simple symplectic group results, we also examine the ground state
structures of several and models. We
emphasize that the top-down approach may be used to methodically find dynamical
superpotentials in many other confining supersymmetric gauge theories.Comment: 21 pages, Revte
Statistics of work performed on a forced quantum oscillator
Various aspects of the statistics of work performed by an external classical
force on a quantum mechanical system are elucidated for a driven harmonic
oscillator. In this special case two parameters are introduced that are
sufficient to completely characterize the force protocol. Explicit results for
the characteristic function of work and the respective probability distribution
are provided and discussed for three different types of initial states of the
oscillator: microcanonical, canonical and coherent states. Depending on the
choice of the initial state the probability distributions of the performed work
may grossly differ. This result in particular holds also true for identical
force protocols. General fluctuation and work theorems holding for
microcanonical and canonical initial states are confirmed
Honey bee colony losses
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