3,662 research outputs found
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Optimal measurement precision of a nonlinear interferometer
We study the best attainable measurement precision when a double-well trap
with bosons inside acts as an interferometer to measure the energy difference
of the atoms on the two sides of the trap. We introduce time independent
perturbation theory as the main tool in both analytical arguments and numerical
computations. Nonlinearity from atom-atom interactions will not indirectly
allow the interferometer to beat the Heisenberg limit, but in many regimes of
the operation the Heisenberg limit scaling of measurement precision is
preserved in spite of added tunneling of the atoms and atom-atom interactions,
often even with the optimal prefactor.Comment: very close to published versio
Probing anisotropies of gravitational-wave backgroundswith a space-based interferometer II: Perturbative reconstruction of a low-frequency skymap
We present a perturbative reconstruction method to make a skymap of
gravitational-wave backgrounds (GWBs) observed via space-based interferometer.
In the presence of anisotropies in GWBs, the cross-correlated signals of
observed GWBs are inherently time-dependent due to the non-stationarity of the
gravitational-wave detector. Since the cross-correlated signal is obtained
through an all-sky integral of primary signals convolving with the antenna
pattern function of gravitational-wave detectors, the non-stationarity of
cross-correlated signals, together with full knowledge of antenna pattern
functions, can be used to reconstruct an intensity map of the GWBs. Here, we
give two simple methods to reconstruct a skymap of GWBs based on the
perturbative expansion in low-frequency regime. The first one is based on
harmonic-Fourier representation of data streams and the second is based on
"direct" time-series data. The latter method enables us to create a skymap in a
direct manner. The reconstruction technique is demonstrated in the case of the
Galactic gravitational wave background observed via planned space
interferometer, LISA. Although the angular resolution of low-frequency skymap
is rather restricted, the methodology presented here would be helpful in
discriminating the GWBs of galactic origins by those of the extragalactic
and/or cosmological origins.Comment: 23 pages, 12 figures, Phys.Rev.D (2005) in pres
Cylindrical gravitational waves in expanding universes: Models for waves from compact sources
New boundary conditions are imposed on the familiar cylindrical gravitational
wave vacuum spacetimes. The new spacetime family represents cylindrical waves
in a flat expanding (Kasner) universe. Space sections are flat and nonconical
where the waves have not reached and wave amplitudes fall off more rapidly than
they do in Einstein-Rosen solutions, permitting a more regular null inifinity.Comment: Minor corrections to references. A note added in proo
Pion-Nucleon Scattering Relations at Next-to-Leading Order in 1/N_c
We obtain relations between partial-wave amplitudes for pi-N-->pi-N and
pi-N-->pi-Delta directly from large N_c QCD. While linear relations among
certain amplitudes holding at leading order (LO) in 1/N_c were derived in the
context of chiral soliton models two decades ago, the present work employs a
fully model-independent framework based on consistency with the large N_c
expansion. At LO we reproduce the soliton model results; however, this method
allows for systematic corrections. At next-to-leading order (NLO), most
relations require additional unknown functions beyond those appearing at
leading order (LO) and thus have little additional predictive power. However,
three NLO relations for the pi-N-->pi-Delta reaction are independent of unknown
functions and make predictions accurate at this order. The amplitudes relevant
to two of these relations were previously extracted from experiment. These
relations describe experiment dramatically better than their LO counterparts.Comment: 8 pages, 2 figures; references adde
Reconstructing a Simple Polytope from its Graph
Blind and Mani (1987) proved that the entire combinatorial structure (the
vertex-facet incidences) of a simple convex polytope is determined by its
abstract graph. Their proof is not constructive. Kalai (1988) found a short,
elegant, and algorithmic proof of that result. However, his algorithm has
always exponential running time. We show that the problem to reconstruct the
vertex-facet incidences of a simple polytope P from its graph can be formulated
as a combinatorial optimization problem that is strongly dual to the problem of
finding an abstract objective function on P (i.e., a shelling order of the
facets of the dual polytope of P). Thereby, we derive polynomial certificates
for both the vertex-facet incidences as well as for the abstract objective
functions in terms of the graph of P. The paper is a variation on joint work
with Michael Joswig and Friederike Koerner (2001).Comment: 14 page
Virtual Resonant States in Two-Photon Decay Processes: Lower-Order Terms, Subtractions, and Physical Interpretations
We investigate the two-photon decay rate of a highly excited atomic state
which can decay to bound states of lower energy via cascade processes. We show
that a naive treatment of the process, based on the introduction of
phenomenological decay rates for the intermediate, resonant states, leads to
lower-order terms which need to be subtracted in order to obtain the coherent
two-photon correction to the decay rate. The sum of the lower-order terms is
exactly equal to the one-photon decay rate of the initial state, provided the
naive two-photon decay rates are summed over all available two-photon channels.
A quantum electrodynamics (QED) treatment of the problem leads to an
"automatic" subtraction of the lower-order terms.Comment: 8 pages, RevTe
Composite Fermions in Negative Effective Magnetic Field: A Monte-Carlo Study
The method of Jain and Kamilla [PRB {\bf 55}, R4895 (1997)] allows numerical
generation of composite fermion trial wavefunctions for large numbers of
electrons in high magnetic fields at filling fractions of the form nu=p/(2mp+1)
with m and p positive integers. In the current paper we generalize this method
to the case where the composite fermions are in an effective (mean) field with
opposite sign from the actual physical field, i.e. when p is negative. We
examine both the ground state energies and the low energy neutral excitation
spectra of these states. Using particle-hole symmetry we can confirm the
correctness of our method by comparing results for the series m=1 with p>0
(previously calculated by others) to our results for the conjugate series m=1
with p <0. Finally, we present similar results for ground state energies and
low energy neutral excitations for the states with m=2 and p <0 which were not
previously addressable, comparing our results to the m=1 case and the p > 0,
m=2 cases.Comment: 11 page
Casimir Forces between Compact Objects: I. The Scalar Case
We have developed an exact, general method to compute Casimir interactions
between a finite number of compact objects of arbitrary shape and separation.
Here, we present details of the method for a scalar field to illustrate our
approach in its most simple form; the generalization to electromagnetic fields
is outlined in Ref. [1]. The interaction between the objects is attributed to
quantum fluctuations of source distributions on their surfaces, which we
decompose in terms of multipoles. A functional integral over the effective
action of multipoles gives the resulting interaction. Each object's shape and
boundary conditions enter the effective action only through its scattering
matrix. Their relative positions enter through universal translation matrices
that depend only on field type and spatial dimension. The distinction of our
method from the pairwise summation of two-body potentials is elucidated in
terms of the scattering processes between three objects. To illustrate the
power of the technique, we consider Robin boundary conditions , which interpolate between Dirichlet and Neumann cases as
is varied. We obtain the interaction between two such spheres
analytically in a large separation expansion, and numerically for all
separations. The cases of unequal radii and unequal are studied. We
find sign changes in the force as a function of separation in certain ranges of
and see deviations from the proximity force approximation even at
short separations, most notably for Neumann boundary conditions.Comment: 27 pages, 9 figure
Universal topological phase of 2D stabilizer codes
Two topological phases are equivalent if they are connected by a local
unitary transformation. In this sense, classifying topological phases amounts
to classifying long-range entanglement patterns. We show that all 2D
topological stabilizer codes are equivalent to several copies of one universal
phase: Kitaev's topological code. Error correction benefits from the
corresponding local mappings.Comment: 4 pages, 3 figure
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