25,450 research outputs found
A Simultaneous Optical and X-ray Variability Study of the Orion Nebula Cluster. II. A Common Origin in Magnetic Activity
We present a statistical analysis of simultaneous optical and X-ray light
curves, spanning 600 ks, for 814 pre-main-sequence (PMS) stars in the Orion
Nebula Cluster. The aim of this study is to establish the relationship, if any,
between the sites of optical and X-ray variability, and thereby to elucidate
the origins of X-ray production in PMS stars. In a previous paper we showed
that optical and X-ray variability in PMS stars are very rarely
time-correlated. Here, using time-averaged variability indicators to examine
the joint occurrences of optical and X-ray variability, we confirm that the two
forms of variability are not directly causally related. However, a strong and
highly statistically significant correlation is found between optical
variability and X-ray luminosity. As this correlation is found to be
independent of accretion activity, we argue that X-ray production in PMS stars
must instead be intimately connected with the presence and strength of
optically variable, magnetically active surface regions (i.e. spots) on these
stars. Moreover, because X-ray variability and optical variability are rarely
time-correlated, we conclude that the sites of X-ray production are not
exclusively co-spatial with these regions. We argue that solar-analog coronae,
heated by topologically complex fields, can explain these findings.Comment: To appear in the Astrophysical Journal. 33 pages, 3 figure
Evolution of the Fermi surface in phase fluctuating d-wave superconductors
One of the most puzzling aspects of the high superconductors is the
appearance of Fermi arcs in the normal state of the underdoped cuprate
materials. These are loci of low energy excitations covering part of the fermi
surface, that suddenly appear above instead of the nodal quasiparticles.
Based on a semiclassical theory, we argue that partial Fermi surfaces arise
naturally in a d-wave superconductor that is destroyed by thermal phase
fluctuations. Specifically, we show that the electron spectral function
develops a square root singularity at low frequencies for wave-vectors
positioned on the bare Fermi surface. We predict a temperature dependence of
the arc length that can partially account for results of recent angle resolved
photo emission (ARPES) experiments.Comment: Journal ref. adde
Dust Storms in Space?
Primarily from the Pioneer 8 and 9 results, it is concluded that the flux of picogram sized dust particles near the earth's orbit has been constant to within the observational limits over three years of observation. In particular, since dust streams are not observed, they cannot explain microphone detected events. However, the possibility of rare events due to dust blown directly off a cometary nucleus (such as that reported for Comet Bennett) cannot be completely ruled out
Monte Carlo simulation and global optimization without parameters
We propose a new ensemble for Monte Carlo simulations, in which each state is
assigned a statistical weight , where is the number of states with
smaller or equal energy. This ensemble has robust ergodicity properties and
gives significant weight to the ground state, making it effective for hard
optimization problems. It can be used to find free energies at all temperatures
and picks up aspects of critical behaviour (if present) without any parameter
tuning. We test it on the travelling salesperson problem, the Edwards-Anderson
spin glass and the triangular antiferromagnet.Comment: 10 pages with 3 Postscript figures, to appear in Phys. Rev. Lett
Rugged Metropolis Sampling with Simultaneous Updating of Two Dynamical Variables
The Rugged Metropolis (RM) algorithm is a biased updating scheme, which aims
at directly hitting the most likely configurations in a rugged free energy
landscape. Details of the one-variable (RM) implementation of this
algorithm are presented. This is followed by an extension to simultaneous
updating of two dynamical variables (RM). In a test with Met-Enkephalin in
vacuum RM improves conventional Metropolis simulations by a factor of about
four. Correlations between three or more dihedral angles appear to prevent
larger improvements at low temperatures. We also investigate a multi-hit
Metropolis scheme, which spends more CPU time on variables with large
autocorrelation times.Comment: 8 pages, 5 figures. Revisions after referee reports. Additional
simulations for temperatures down to 220
Simulated Tempering: A New Monte Carlo Scheme
We propose a new global optimization method ({\em Simulated Tempering}) for
simulating effectively a system with a rough free energy landscape (i.e. many
coexisting states) at finite non-zero temperature. This method is related to
simulated annealing, but here the temperature becomes a dynamic variable, and
the system is always kept at equilibrium. We analyze the method on the Random
Field Ising Model, and we find a dramatic improvement over conventional
Metropolis and cluster methods. We analyze and discuss the conditions under
which the method has optimal performances.Comment: 12 pages, very simple LaTeX file, figures are not included, sorr
Multicanonical Study of the 3D Ising Spin Glass
We simulated the Edwards-Anderson Ising spin glass model in three dimensions
via the recently proposed multicanonical ensemble. Physical quantities such as
energy density, specific heat and entropy are evaluated at all temperatures. We
studied their finite size scaling, as well as the zero temperature limit to
explore the ground state properties.Comment: FSU-SCRI-92-121; 7 pages; sorry, no figures include
A single-photon sampling architecture for solid-state imaging
Advances in solid-state technology have enabled the development of silicon
photomultiplier sensor arrays capable of sensing individual photons. Combined
with high-frequency time-to-digital converters (TDCs), this technology opens up
the prospect of sensors capable of recording with high accuracy both the time
and location of each detected photon. Such a capability could lead to
significant improvements in imaging accuracy, especially for applications
operating with low photon fluxes such as LiDAR and positron emission
tomography.
The demands placed on on-chip readout circuitry imposes stringent trade-offs
between fill factor and spatio-temporal resolution, causing many contemporary
designs to severely underutilize the technology's full potential. Concentrating
on the low photon flux setting, this paper leverages results from group testing
and proposes an architecture for a highly efficient readout of pixels using
only a small number of TDCs, thereby also reducing both cost and power
consumption. The design relies on a multiplexing technique based on binary
interconnection matrices. We provide optimized instances of these matrices for
various sensor parameters and give explicit upper and lower bounds on the
number of TDCs required to uniquely decode a given maximum number of
simultaneous photon arrivals.
To illustrate the strength of the proposed architecture, we note a typical
digitization result of a 120x120 photodiode sensor on a 30um x 30um pitch with
a 40ps time resolution and an estimated fill factor of approximately 70%, using
only 161 TDCs. The design guarantees registration and unique recovery of up to
4 simultaneous photon arrivals using a fast decoding algorithm. In a series of
realistic simulations of scintillation events in clinical positron emission
tomography the design was able to recover the spatio-temporal location of 98.6%
of all photons that caused pixel firings.Comment: 24 pages, 3 figures, 5 table
Mass Predictions for Pseudoscalar Charmonium and Bottomonium Hybrids in QCD Sum-Rules
Masses of the pseudoscalar charmonium and bottomonium
hybrids are determined using QCD Laplace sum-rules. The effects of the
dimension-six gluon condensate are included in our analysis and result in a
stable sum-rule analysis, whereas previous studies of these states were unable
to optimize mass predictions. The pseudoscalar charmonium hybrid is predicted
to have a mass of approximately 3.8 GeV and the corresponding bottomonium
prediction is 10.6 GeV. Calculating the full correlation function, rather than
only the imaginary part, is shown to be necessary for accurate formulation of
the sum-rules. The charmonium hybrid mass prediction is discussed within the
context of the X Y Z resonances.Comment: 10 pages, 7 embedded figures. Analysis extended and refined in v
Non equilibrium statistical physics with fictitious time
Problems in non equilibrium statistical physics are characterized by the
absence of a fluctuation dissipation theorem. The usual analytic route for
treating these vast class of problems is to use response fields in addition to
the real fields that are pertinent to a given problem. This line of argument
was introduced by Martin, Siggia and Rose. We show that instead of using the
response field, one can, following the stochastic quantization of Parisi and
Wu, introduce a fictitious time. In this extra dimension a fluctuation
dissipation theorem is built in and provides a different outlook to problems in
non equilibrium statistical physics.Comment: 4 page
- …