2,329 research outputs found
Polarization alignments of radio quasars in JVAS/CLASS surveys
We test the hypothesis that the polarization vectors of flat-spectrum radio
sources (FSRS) in the JVAS/CLASS 8.4-GHz surveys are randomly oriented on the
sky. The sample with robust polarization measurements is made of objects
and redshift information is known for of them. We performed two
statistical analyses: one in two dimensions and the other in three dimensions
when distance is available. We find significant large-scale alignments of
polarization vectors for samples containing only quasars (QSO) among the
varieties of FSRS's. While these correlations prove difficult to explain either
by a physical effect or by biases in the dataset, the fact that the QSO's which
have significantly aligned polarization vectors are found in regions of the sky
where optical polarization alignments were previously found is striking.Comment: 13 pages, 9 figures, submitted to MNRA
Percolation without FKG
We prove a Russo-Seymour-Welsh theorem for large and natural perturbative
families of discrete percolation models that do not necessarily satisfy the
Fortuin-Kasteleyn-Ginibre condition of positive association. In particular, we
prove the box-crossing property for the antiferromagnetic Ising model with
small parameter, and for certain discrete Gaussian fields with oscillating
correlation function
How to Discount Deep Reinforcement Learning: Towards New Dynamic Strategies
Using deep neural nets as function approximator for reinforcement learning
tasks have recently been shown to be very powerful for solving problems
approaching real-world complexity. Using these results as a benchmark, we
discuss the role that the discount factor may play in the quality of the
learning process of a deep Q-network (DQN). When the discount factor
progressively increases up to its final value, we empirically show that it is
possible to significantly reduce the number of learning steps. When used in
conjunction with a varying learning rate, we empirically show that it
outperforms original DQN on several experiments. We relate this phenomenon with
the instabilities of neural networks when they are used in an approximate
Dynamic Programming setting. We also describe the possibility to fall within a
local optimum during the learning process, thus connecting our discussion with
the exploration/exploitation dilemma.Comment: NIPS 2015 Deep Reinforcement Learning Worksho
Alignment of quasar polarizations with large-scale structures
We have measured the optical linear polarization of quasars belonging to
Gpc-scale quasar groups at redshift z ~ 1.3. Out of 93 quasars observed, 19 are
significantly polarized. We found that quasar polarization vectors are either
parallel or perpendicular to the directions of the large-scale structures to
which they belong. Statistical tests indicate that the probability that this
effect can be attributed to randomly oriented polarization vectors is of the
order of 1%. We also found that quasars with polarization perpendicular to the
host structure preferentially have large emission line widths while objects
with polarization parallel to the host structure preferentially have small
emission line widths. Considering that quasar polarization is usually either
parallel or perpendicular to the accretion disk axis depending on the
inclination with respect to the line of sight, and that broader emission lines
originate from quasars seen at higher inclinations, we conclude that quasar
spin axes are likely parallel to their host large-scale structures.Comment: Accepted for publication in Astronomy and Astrophysic
Investigating the impact of optical selection effects on observed rest frame prompt GRB properties
Measuring gamma-ray burst (GRB) properties in their rest-frame is crucial to
understand the physics at work in gamma-ray bursts. This can only be done for
GRBs with known redshift. Since redshifts are usually measured from the optical
spectrum of the afterglow, correlations between prompt and afterglow emissions
may introduce biases in the distribution of rest-frame properties of the prompt
emission. Our analysis is based on a sample of 90 GRBs with good optical
follow-up and well measured prompt emission. 76 of them have a measure of
redshift and 14 have no redshift. We estimate their optical brightness with
their R magnitude measured two hours after the trigger and compare the rest
frame prompt properties of different classes of GRB afterglow brightness. We
find that the optical brightness of GRBs in our sample is mainly driven by
their intrinsic afterglow luminosity. We show that GRBs with low and high
afterglow optical fluxes have similar Epi , Eiso , Liso , indicating that the
rest-frame distributions computed from GRBs with a redshift are not
significantly distorted by optical selection effects. However we found that the
rest frame T90 distribution is not immune to optical selection effect, which
favor the selection of GRBs with longer durations. Finally, we note that GRBs
in the upper part of the Epi-Eiso plane have fainter optical afterglows and we
show that optical selection effects strongly favor the detection of GRBs with
bright afterglows located close or below the best-fit Epi-Eiso relation, whose
redshift is easily measurable.Comment: 41 pages, 10 figures, 7 tables. arXiv admin note: substantial text
overlap with arXiv:1503.0276
Fast Selection of Spectral Variables with B-Spline Compression
The large number of spectral variables in most data sets encountered in
spectral chemometrics often renders the prediction of a dependent variable
uneasy. The number of variables hopefully can be reduced, by using either
projection techniques or selection methods; the latter allow for the
interpretation of the selected variables. Since the optimal approach of testing
all possible subsets of variables with the prediction model is intractable, an
incremental selection approach using a nonparametric statistics is a good
option, as it avoids the computationally intensive use of the model itself. It
has two drawbacks however: the number of groups of variables to test is still
huge, and colinearities can make the results unstable. To overcome these
limitations, this paper presents a method to select groups of spectral
variables. It consists in a forward-backward procedure applied to the
coefficients of a B-Spline representation of the spectra. The criterion used in
the forward-backward procedure is the mutual information, allowing to find
nonlinear dependencies between variables, on the contrary of the generally used
correlation. The spline representation is used to get interpretability of the
results, as groups of consecutive spectral variables will be selected. The
experiments conducted on NIR spectra from fescue grass and diesel fuels show
that the method provides clearly identified groups of selected variables,
making interpretation easy, while keeping a low computational load. The
prediction performances obtained using the selected coefficients are higher
than those obtained by the same method applied directly to the original
variables and similar to those obtained using traditional models, although
using significantly less spectral variables
Recurrent Neural Networks with more flexible memory: better predictions than rough volatility
We extend recurrent neural networks to include several flexible timescales
for each dimension of their output, which mechanically improves their abilities
to account for processes with long memory or with highly disparate time scales.
We compare the ability of vanilla and extended long short term memory networks
(LSTMs) to predict asset price volatility, known to have a long memory.
Generally, the number of epochs needed to train extended LSTMs is divided by
two, while the variation of validation and test losses among models with the
same hyperparameters is much smaller. We also show that the model with the
smallest validation loss systemically outperforms rough volatility predictions
by about 20% when trained and tested on a dataset with multiple time series.Comment: 9 page
On overfitting and asymptotic bias in batch reinforcement learning with partial observability
This paper provides an analysis of the tradeoff between asymptotic bias
(suboptimality with unlimited data) and overfitting (additional suboptimality
due to limited data) in the context of reinforcement learning with partial
observability. Our theoretical analysis formally characterizes that while
potentially increasing the asymptotic bias, a smaller state representation
decreases the risk of overfitting. This analysis relies on expressing the
quality of a state representation by bounding L1 error terms of the associated
belief states. Theoretical results are empirically illustrated when the state
representation is a truncated history of observations, both on synthetic POMDPs
and on a large-scale POMDP in the context of smartgrids, with real-world data.
Finally, similarly to known results in the fully observable setting, we also
briefly discuss and empirically illustrate how using function approximators and
adapting the discount factor may enhance the tradeoff between asymptotic bias
and overfitting in the partially observable context.Comment: Accepted at the Journal of Artificial Intelligence Research (JAIR) -
31 page
Electric-field control of the magnetic anisotropy in an ultrathin (Ga,Mn)As/(Ga,Mn)(As,P) bilayer
We report on the electric control of the magnetic anisotropy in an ultrathin
ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with competing in-plane and
out-of-plane anisotropies. The carrier distribution and therefore the strength
of the effective anisotropy is controlled by the gate voltage of a field effect
device. Anomalous Hall Effect measurements confirm that a depletion of carriers
in the upper (Ga,Mn)As layer results in the decrease of the in-plane
anisotropy. The uniaxial anisotropy field is found to decrease by a factor ~ 4
over the explored gate-voltage range, so that the transition to an out-of-plane
easy-axis configuration is almost reached
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