20,502 research outputs found
Robust Estimation for Linear Panel Data Models
In different fields of applications including, but not limited to,
behavioral, environmental, medical sciences and econometrics, the use of panel
data regression models has become increasingly popular as a general framework
for making meaningful statistical inferences. However, when the ordinary least
squares (OLS) method is used to estimate the model parameters, presence of
outliers may significantly alter the adequacy of such models by producing
biased and inefficient estimates. In this work we propose a new, weighted
likelihood based robust estimation procedure for linear panel data models with
fixed and random effects. The finite sample performances of the proposed
estimators have been illustrated through an extensive simulation study as well
as with an application to blood pressure data set. Our thorough study
demonstrates that the proposed estimators show significantly better
performances over the traditional methods in the presence of outliers and
produce competitive results to the OLS based estimates when no outliers are
present in the data set
Identification of baryon resonances in central heavy-ion collisions at energies between 1 and 2 AGeV
The mass distributions of baryon resonances populated in near-central
collisions of Au on Au and Ni on Ni are deduced by defolding the spectra
of charged pions by a method which does not depend on a specific resonance
shape. In addition the mass distributions of resonances are obtained from the
invariant masses of pairs. With both methods the deduced mass
distributions are shifted by an average value of -60 MeV/c relative to the
mass distribution of the free resonance, the distributions
descent almost exponentially towards mass values of 2000 MeV/c^2. The observed
differences between and pairs indicate a contribution
of isospin resonances. The attempt to consistently describe the
deduced mass distributions and the reconstructed kinetic energy spectra of the
resonances leads to new insights about the freeze out conditions, i.e. to
rather low temperatures and large expansion velocities.Comment: 30 pages, 13 figures, Latex using documentstyle[12pt,a4,epsfig], to
appear in Eur. Phys. J.
The AMIGA sample of isolated galaxies. XI. Optical characterisation of nuclear activity
Context.- This paper is part of a series involving the AMIGA project
(Analysis of the Interstellar Medium of Isolated GAlaxies), which identifies
and studies a statistically-significant sample of the most isolated galaxies in
the northern sky. Aims.- We present a catalogue of nuclear activity, traced by
optical emission lines, in a well-defined sample of the most isolated galaxies
in the local Universe, which will be used as a basis for studying the effect of
the environment on nuclear activity. Methods.- We obtained spectral data from
the 6th Data Release of the Sloan Digital Sky Survey, which were inspected in a
semi-automatic way. We subtracted the underlying stellar populations from the
spectra (using the software Starlight) and modelled the nuclear emission
features. Standard emission-line diagnostics diagrams were applied, using a new
classification scheme that takes into account censored data, to classify the
type of nuclear emission. Results.- We provide a final catalogue of
spectroscopic data, stellar populations, emission lines and classification of
optical nuclear activity for AMIGA galaxies. The prevalence of optical active
galactic nuclei (AGN) in AMIGA galaxies is 20.4%, or 36.7% including transition
objects. The fraction of AGN increases steeply towards earlier morphological
types and higher luminosities. We compare these results with a matched analysis
of galaxies in isolated denser environments (Hickson Compact Groups). After
correcting for the effects of the morphology and luminosity, we find that there
is no evidence for a difference in the prevalence of AGN between isolated and
compact group galaxies, and we discuss the implications of this result.
Conclusions.- We find that a major interaction is not a necessary condition for
the triggering of optical AGN.Comment: 16 pages, 11 figures, 12 tables, published in Astronomy and
Astrophysics. Figure 5 corrected: [OI] diagram adde
Radio loud AGN and the L_X - \sigma relation of galaxy groups and clusters
We use the ROSAT All-Sky Survey to study the X-ray properties of a sample of
625 groups and clusters of galaxies selected from the Sloan Digital Sky Survey.
We stack clusters with similar velocity dispersions and investigate whether
their average X-ray luminosities and surface brightness profiles vary with the
radio activity level of their central galaxies. We find that at a given value
of , clusters with a central radio AGN have more concentrated X-ray
surface brightness profiles, larger central galaxy masses, and higher X-ray
luminosities than clusters with radio-quiet central galaxies. The enhancement
in X-ray luminosity is more than a factor of two, is detected with better than
6 significance, and cannot be explained by X-ray emission from the
radio AGN itself. This difference is largely due to a subpopulation of
radio-quiet, high velocity dispersion clusters with low mass central galaxies.
These clusters are underluminous at X-ray wavelengths when compared to
otherwise similar clusters where the central galaxy is radio-loud, more
massive, or both.Comment: Section 5.2 is updated, more discussion on the dependence of L_X -
\sigma relation on the stellar mass of BCG
Radio Emission in the Cosmic Web
We explore the possibility of detecting radio emission in the \emph{cosmic
web} by analyzing shock waves in the MareNostrum cosmological simulation. This
requires a careful calibration of shock finding algorithms in Smoothed-Particle
Hydrodynamics simulations, which we present here. Moreover, we identify the
elements of the cosmic web, namely voids, walls, filaments and clusters with
the use of the SpineWeb technique, a procedure that classifies the structure in
terms of its topology. Thus, we are able to study the Mach number distribution
as a function of its environment. We find that the median Mach number, for
clusters is , for filaments is
, for walls is
, and for voids is
. We then estimate the radio emission
in the cosmic web using the formalism derived in Hoeft & Br\"{u}ggen (2007). We
also find that in order to match our simulations with observational data (e.g.,
NVSS radio relic luminosity function), a fraction of energy dissipated at the
shock of is needed, in contrast with the
proposed by Hoeft et al. (2008). We find that 41% of
clusters with host diffuse radio emission in the form
of radio relics. Moreover, we predict that the radio flux from filaments should
be Jy at a frequency of 150 MHz.Comment: 19 pages, 17 figures, accepted for publication in MNRAS. Minor
changes to tex fil
CXCL16/CXCR6 axis drives microglia/macrophages phenotype in physiological conditions and plays a crucial role in glioma
Microglia are patrolling cells that sense changes in the brain microenvironment and respond acquiring distinct phenotypes that can be either beneficial or detrimental for brain homeostasis. Anti-inflammatory microglia release soluble factors that might promote brain repair; however, in glioma, anti-inflammatory microglia dampen immune response and promote a brain microenvironment that foster tumor growth and invasion. The chemokine CXCL16 is expressed in the brain, where it is neuroprotective against brain ischemia, and it has been found to be over-expressed in glioblastoma (GBM). Considering that CXCL16 specific receptor CXCR6 is diffusely expressed in the brain including in microglia cells, we wanted to investigate the role of CXCL16 in the modulation of microglia cell activity and phenotype, and in the progression of glioma. Here we report that CXCL16 drives microglia polarization toward an anti-inflammatory phenotype, also restraining microglia polarization toward an inflammatory phenotype upon LPS and IFN? stimulation. In the context of glioma, we demonstrate that CXCL16 released by tumor cells is determinant in promoting glioma associated microglia/macrophages (GAMs) modulation toward an anti-inflammatory/pro-tumor phenotype, and that cxcr6ko mice, orthotopically implanted into the brain with GL261 glioma cells,survive longer compared to wild-type mice. We also describe that CXCL16/CXCR6 signaling acts directly on mouse glioma cells, as well as human primary GBM cells, promoting tumor cell growth, migration and invasion. All together these data suggest that CXCL16 signaling could represent a good target to modulate microglia phenotype in order to restrain inflammation or to limit glioma progression
Do we understand the interaction from the near threshold photoproduction on the deuteron?
The effects of final state interaction in incoherent photoproduction
on deuteron are studied within a three-body approach including a realistic
potential. The results are compared with available data, and differences with
other theoretical predictions are analyzed. The role of the
interaction and the possibility of extracting the scattering
parameters from this reaction are discussed.Comment: 9 pages revtex including 6 ps-figure
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