609 research outputs found
Universal scaling functions of critical Casimir forces obtained by Monte Carlo simulations
Effective Casimir forces induced by thermal fluctuations in the vicinity of
bulk critical points are studied by means of Monte Carlo simulations in
three-dimensional systems for film geometries and within the experimentally
relevant Ising and XY universality classes. Several surface universality
classes of the confining surfaces are considered, some of which are relevant
for recent experiments. A novel approach introduced previously EPL 80, 60009
(2007), based inter alia on an integration scheme of free energy differences,
is utilized to compute the universal scaling functions of the critical Casimir
forces in the critical range of temperatures above and below the bulk critical
temperature. The resulting predictions are compared with corresponding
experimental data for wetting films of fluids and with available theoretical
results.Comment: 21 pages, 17 figure
Measurement of the temperature of an ultracold ion source using time-dependent electric fields
We report on a measurement of the characteristic temperature of an ultracold
rubidium ion source, in which a cloud of laser-cooled atoms is converted to
ions by photo-ionization. Extracted ion pulses are focused on a detector with a
pulsed-field technique. The resulting experimental spot sizes are compared to
particle-tracking simulations, from which a source temperature
mK and the corresponding transversal reduced emittance m rad are determined. We find that this result is
likely limited by space charge forces even though the average number of ions
per bunch is 0.022.Comment: 8 pages, 11 figure
Cosmological Constraints from calibrated Yonetoku and Amati relation implies Fundamental plane of Gamma-ray bursts
We consider two empirical relations using data only from the prompt emission
of Gamma-Ray Bursts (GRBs), peak energy () - peak luminosity ()
relation (so called Yonetoku relation) and -isotropic energy () relation (so called Amati relation). We first suggest the independence
of the two relations although they have been considered similar and dependent.
From this viewpoint, we compare constraints on cosmological parameters,
and , from the Yonetoku and Amati relations
calibrated by low-redshift GRBs with . We found that they are
different in 1- level, although they are still consistent in 2-
level. This and the fact that both Amati and Yonetoku relations have systematic
errors larger than statistical errors suggest the existence of a hidden
parameter of GRBs. We introduce the luminosity time defined by as a hidden parameter to obtain a generalized Yonetoku
relation as . The new relation has much smaller systematic
error, 30%, and can be regarded as "Fundamental plane" of GRBs. We show a
possible radiation model for this new relation. Finally we apply the new
relation for high-redshift GRBs with to obtain
, which is consistent with the
concordance cosmological model within 2- level.Comment: 5 pages, 6 figures, published in JCA
Atomic-scale surface demixing in a eutectic liquid BiSn alloy
Resonant x-ray reflectivity of the surface of the liquid phase of the
BiSn eutectic alloy reveals atomic-scale demixing extending over
three near-surface atomic layers. Due to the absence of underlying atomic
lattice which typically defines adsorption in crystalline alloys, studies of
adsorption in liquid alloys provide unique insight on interatomic interactions
at the surface. The observed composition modulation could be accounted for
quantitatively by the Defay-Prigogine and Strohl-King multilayer extensions of
the single-layer Gibbs model, revealing a near-surface domination of the
attractive Bi-Sn interaction over the entropy.Comment: 4 pages (two-column), 3 figures, 1 table; Added a figure, updated
references, discussion; accepted at Phys. Rev. Let
Effects of the stellar wind on X-ray spectra of Cygnus X-3
We study X-ray spectra of Cyg X-3 from BeppoSAX, taking into account
absorption and emission in the strong stellar wind of its companion. We find
the intrinsic X-ray spectra are well modelled by disc blackbody emission, its
upscattering by hot electrons with a hybrid distribution, and by Compton
reflection. These spectra are strongly modified by absorption and reprocessing
in the stellar wind, which we model using the photoionization code cloudy. The
form of the observed spectra implies the wind is composed of two phases. A hot
tenuous plasma containing most of the wind mass is required to account for the
observed features of very strongly ionized Fe. Small dense cool clumps filling
<0.01 of the volume are required to absorb the soft X-ray excess, which is
emitted by the hot phase but not present in the data. The total mass-loss rate
is found to be (0.6--1.6) x 10^-5 solar masses per year. We also discuss the
feasibility of the continuum model dominated by Compton reflection, which we
find to best describe our data. The intrinsic luminosities of our models
suggest that the compact object is a black hole.Comment: MNRAS, in pres
Diffusive hidden Markov model characterization of DNA looping dynamics in tethered particle experiments
In many biochemical processes, proteins bound to DNA at distant sites are
brought into close proximity by loops in the underlying DNA. For example, the
function of some gene-regulatory proteins depends on such DNA looping
interactions. We present a new technique for characterizing the kinetics of
loop formation in vitro, as observed using the tethered particle method, and
apply it to experimental data on looping induced by lambda repressor. Our
method uses a modified (diffusive) hidden Markov analysis that directly
incorporates the Brownian motion of the observed tethered bead. We compare
looping lifetimes found with our method (which we find are consistent over a
range of sampling frequencies) to those obtained via the traditional
threshold-crossing analysis (which can vary depending on how the raw data are
filtered in the time domain). Our method does not involve any time filtering
and can detect sudden changes in looping behavior. For example, we show how our
method can identify transitions between long-lived, kinetically distinct states
that would otherwise be difficult to discern
XMM observations of the high-redshift quasar RXJ1028.6-0844 at z=4.276: soft X-ray spectral flattening
We present results from a new XMM-Newton observation of the high-redshift
quasar RXJ1028.6-0844 at a redshift of 4.276. The soft X-ray spectral
flattening, as reported by a study with ASCA previously (Yuan et al. 2000, ApJ
545, 625), is confirmed to be present, however, with reduced column density
when modelled by absorption. The inferred column density for absorption
intrinsic to the quasar is 2.1(+0.4-0.3)x10^22 (cm^-2) for cold matter, and
higher for ionised gas. The spectral flattening shows remarkable similarity
with those of two similar objects, GB1428+4217 (Worsley et al. 2004, MNRAS 350,
L67) and PMNJ0525-3343 (Worsley et al. 2004, MNRAS 350, 207). The results
improve upon those obtained from a previous short-exposure observation for
RXJ1028.6-0844 with XMM-Newton (Grupe et al. 2004, AJ 127, 1). A comparative
study of the two XMM-Newton observations reveals a change in the power-law
photon index from Gamma ~1.3 to 1.5 on timescales of about one year. A
tentative excess emission feature in the rest-frame 5-10keV band is suggested,
which is similar to that marginally suggested for GB1428+4217.Comment: 9 pages, accepted for publication in MNRAS; minor changes (added
footnote commenting on the use of the F-test, added references
Robust vetoes for gravitational-wave burst triggers using known instrumental couplings
The search for signatures of transient, unmodelled gravitational-wave (GW)
bursts in the data of ground-based interferometric detectors typically uses
`excess-power' search methods. One of the most challenging problems in the
burst-data-analysis is to distinguish between actual GW bursts and spurious
noise transients that trigger the detection algorithms. In this paper, we
present a unique and robust strategy to `veto' the instrumental glitches. This
method makes use of the phenomenological understanding of the coupling of
different detector sub-systems to the main detector output. The main idea
behind this method is that the noise at the detector output (channel H) can be
projected into two orthogonal directions in the Fourier space -- along, and
orthogonal to, the direction in which the noise in an instrumental channel X
would couple into H. If a noise transient in the detector output originates
from channel X, it leaves the statistics of the noise-component of H orthogonal
to X unchanged, which can be verified by a statistical hypothesis testing. This
strategy is demonstrated by doing software injections in simulated Gaussian
noise. We also formulate a less-rigorous, but computationally inexpensive
alternative to the above method. Here, the parameters of the triggers in
channel X are compared to the parameters of the triggers in channel H to see
whether a trigger in channel H can be `explained' by a trigger in channel X and
the measured transfer function.Comment: 14 Pages, 8 Figures, To appear in Class. Quantum Gra
New constraints on Planck-scale Lorentz Violation in QED from the Crab Nebula
We set constraints on O(E/M) Lorentz Violation in QED in an effective field
theory framework. A major consequence of such assumptions is the modification
of the dispersion relations for electrons/positrons and photons, which in turn
can affect the electromagnetic output of astrophysical objects. We compare the
information provided by multiwavelength observations with a full and
self-consistent computation of the broad-band spectrum of the Crab Nebula. We
cast constraints of order 10^{-5} at 95% confidence level on the lepton Lorentz
Violation parameters.Comment: 23 pages, 9 figures. v2: added comments and references, matches
version accepted by JCA
Model-Independent Comparisons of Pulsar Timings to Scalar-Tensor Gravity
Observations of pulsar timing provide strong constraints on scalar-tensor
theories of gravity, but these constraints are traditionally quoted as limits
on the microscopic parameters (like the Brans-Dicke coupling, for example) that
govern the strength of scalar-matter couplings at the particle level in
particular models. Here we present fits to timing data for several pulsars
directly in terms of the phenomenological couplings (masses, scalar charges,
moment of inertia sensitivities and so on) of the stars involved, rather than
to the more microscopic parameters of a specific model. For instance, for the
double pulsar PSR J0737-3039A/B we find at the 68% confidence level that the
masses are bounded by 1.28 < m_A/m_sun < 1.34 and 1.19 < m_B/m_sun < 1.25,
while the scalar-charge to mass ratios satisfy |a_A| < 0.21, |a_B| < 0.21 and
|a_B - a_A| < 0.002$. These constraints are independent of the details of the
scalar tensor model involved, and of assumptions about the stellar equations of
state. Our fits can be used to constrain a broad class of scalar tensor
theories by computing the fit quantities as functions of the microscopic
parameters in any particular model. For the Brans-Dicke and quasi-Brans-Dicke
models, the constraints obtained in this manner are consistent with those
quoted in the literature.Comment: 19 pages, 7 figure
- …