6,976 research outputs found
The X-ray modulation of PSR J2032+4127/MT91 213 during the Periastron Passage in 2017
We present the Neil Gehrels Swift Observatory (Swift), Fermi Large Area
Telescope (Fermi-LAT), and Karl G. Jansky Very Large Array (VLA) observations
of the gamma-ray binary PSR J2032+4127/MT91 213, of which the periastron
passage has just occurred in November 2017. In the Swift X-ray light curve, the
flux was steadily increasing before mid-October 2017, however, a sharp X-ray
dip on a weekly time-scale is seen during the periastron passage, followed by a
post-periastron X-ray flare lasting for ~20 days. We suggest that the X-ray dip
is caused by (i) an increase of the magnetization parameter at the shock, and
(ii) the suppression due to the Doppler boosting effect. The 20-day
post-periastron flare could be a consequence of the Be stellar disk passage by
the pulsar. An orbital GeV modulation is also expected in our model, however,
no significant variability is seen in the Fermi-LAT light curve. We suspect
that the GeV emission resulted from the interaction between the binary's
members is hidden behind the bright magnetospheric emission of the pulsar.
Pulsar gating technique would be useful to remove the magnetospheric emission
and recover the predicted GeV modulation, if an accurate radio timing solution
over the periastron passage is provided in the future.Comment: 6 pages, including 2 figures. Accepted for publication in Ap
Large-q asymptotics of the random bond Potts model
We numerically examine the large-q asymptotics of the q-state random bond
Potts model. Special attention is paid to the parametrisation of the critical
line, which is determined by combining the loop representation of the transfer
matrix with Zamolodchikov's c-theorem. Asymptotically the central charge seems
to behave like c(q) = 1/2 log_2(q) + O(1). Very accurate values of the bulk
magnetic exponent x_1 are then extracted by performing Monte Carlo simulations
directly at the critical point. As q -> infinity, these seem to tend to a
non-trivial limit, x_1 -> 0.192 +- 0.002.Comment: 9 pages, no figure
Weak Randomness for large q-State Potts models in Two Dimensions
We have studied the effect of weak randomness on q-state Potts models for q >
4 by measuring the central charges of these models using transfer matrix
methods. We obtain a set of new values for the central charges and then show
that some of these values are related to one another by a factorization law.Comment: 8 pages, Latex, no figure
Cluster Monte Carlo study of multi-component fluids of the Stillinger-Helfand and Widom-Rowlinson type
Phase transitions of fluid mixtures of the type introduced by Stillinger and
Helfand are studied using a continuum version of the invaded cluster algorithm.
Particles of the same species do not interact, but particles of different types
interact with each other via a repulsive potential. Examples of interactions
include the Gaussian molecule potential and a repulsive step potential.
Accurate values of the critical density, fugacity and magnetic exponent are
found in two and three dimensions for the two-species model. The effect of
varying the number of species and of introducing quenched impurities is also
investigated. In all the cases studied, mixtures of -species are found to
have properties similar to -state Potts models.Comment: 25 pages, 5 figure
Oak forest carbon and water simulations:Model intercomparisons and evaluations against independent data
Models represent our primary method for integration of small-scale, process-level phenomena into a comprehensive description of forest-stand or ecosystem function. They also represent a key method for testing hypotheses about the response of forest ecosystems to multiple changing environmental conditions. This paper describes the evaluation of 13 stand-level models varying in their spatial, mechanistic, and temporal complexity for their ability to capture intra- and interannual components of the water and carbon cycle for an upland, oak-dominated forest of eastern Tennessee. Comparisons between model simulations and observations were conducted for hourly, daily, and annual time steps. Data for the comparisons were obtained from a wide range of methods including: eddy covariance, sapflow, chamber-based soil respiration, biometric estimates of stand-level net primary production and growth, and soil water content by time or frequency domain reflectometry. Response surfaces of carbon and water flux as a function of environmental drivers, and a variety of goodness-of-fit statistics (bias, absolute bias, and model efficiency) were used to judge model performance.
A single model did not consistently perform the best at all time steps or for all variables considered. Intermodel comparisons showed good agreement for water cycle fluxes, but considerable disagreement among models for predicted carbon fluxes. The mean of all model outputs, however, was nearly always the best fit to the observations. Not surprisingly, models missing key forest components or processes, such as roots or modeled soil water content, were unable to provide accurate predictions of ecosystem responses to short-term drought phenomenon. Nevertheless, an inability to correctly capture short-term physiological processes under drought was not necessarily an indicator of poor annual water and carbon budget simulations. This is possible because droughts in the subject ecosystem were of short duration and therefore had a small cumulative impact. Models using hourly time steps and detailed mechanistic processes, and having a realistic spatial representation of the forest ecosystem provided the best predictions of observed data. Predictive ability of all models deteriorated under drought conditions, suggesting that further work is needed to evaluate and improve ecosystem model performance under unusual conditions, such as drought, that are a common focus of environmental change discussions
Finite-Size Scaling Study of the Surface and Bulk Critical Behavior in the Random-Bond 8-state Potts Model
The self-dual random-bond eight-state Potts model is studied numerically
through large-scale Monte Carlo simulations using the Swendsen-Wang cluster
flipping algorithm. We compute bulk and surface order parameters and
susceptibilities and deduce the corresponding critical exponents at the random
fixed point using standard finite-size scaling techniques. The scaling laws are
suitably satisfied. We find that a belonging of the model to the 2D Ising model
universality class can be conclusively ruled out, and the dimensions of the
relevant bulk and surface scaling fields are found to take the values
, , , to be compared to their Ising values:
15/8, 1, and 1/2.Comment: LaTeX file with Revtex, 4 pages, 4 eps figures, to appear in Phys.
Rev. Let
Critical Properties of Random Quantum Potts and Clock Models
We study zero temperature phase transitions in two classes of random quantum
systems -the -state quantum Potts and clock models. For models with purely
ferromagnetic interactions in one dimension, we show that for strong randomness
there is a second order transition with critical properties that can be
determined exactly by use of an RG procedure. Somewhat surprisingly, the
critical behaviour is completely independent of (for ).
For the clock model, we suggest the existence of a novel multicritical
point at intermediate randomness. We also consider the transition from
a paramagnet to a spin glass in an infinite range model. Assuming that the
transition is second order, we solve for the critical behaviour and find
independent exponents.Comment: 12 pages, REVTEX 3.0, 1 EPS figur
Quantum rotor theory of spinor condensates in tight traps
In this work, we theoretically construct exact mappings of many-particle
bosonic systems onto quantum rotor models. In particular, we analyze the rotor
representation of spinor Bose-Einstein condensates. In a previous work it was
shown that there is an exact mapping of a spin-one condensate of fixed particle
number with quadratic Zeeman interaction onto a quantum rotor model. Since the
rotor model has an unbounded spectrum from above, it has many more eigenstates
than the original bosonic model. Here we show that for each subset of states
with fixed spin F_z, the physical rotor eigenstates are always those with
lowest energy. We classify three distinct physical limits of the rotor model:
the Rabi, Josephson, and Fock regimes. The last regime corresponds to a
fragmented condensate and is thus not captured by the Bogoliubov theory. We
next consider the semiclassical limit of the rotor problem and make connections
with the quantum wave functions through use of the Husimi distribution
function. Finally, we describe how to extend the analysis to higher-spin
systems and derive a rotor model for the spin-two condensate. Theoretical
details of the rotor mapping are also provided here.Comment: 10 pages, 2 figure
- …