6,372 research outputs found
The generalized parton distributions of the nucleon in the NJL model based on the Faddeev approach
We study the generalized parton distributions, including the helicity-flip
ones, using Nambu-Jona-Lasinio model based on a relativistic Faddeev approach
with `static approximation'. Sum rules relating the generalized parton
distributions to nucleon electromagnetic form factors are satisfied. Moreover,
quark-antiquark contributions in the region are non-vanishing. Our
results are qualitatively similar to those calculated with Radyushkin's double
distribution ansatz using forward parton distribution functions calculated in
the NJL model as inputs.Comment: 35 pages, 15 figure
Using R via PHP for Teaching Purposes: R-php
This paper deals with the R-php statistical software, that is an environment for statistical analysis, freely accessible and attainable through the World Wide Web, based on R. Indeed, this software uses, as "engine" for statistical analyses, R via PHP and its design has been inspired by a paper of de Leeuw (1997). R-php is based on two modules: a base module and a point-and-click module. R-php base allows the simple editing of R code in a form. R-php point-and-click allows some statistical analyses by means of a graphical user interface (GUI): then, to use this module it is not necessary for the user to know the R environment, but all the allowed analyses can be performed by using the computer mouse. We think that this tool could be particularly useful for teaching purposes: one possible use could be in a University computer laboratory to permit a smooth approach of students to R.
A Software Tool for the Exponential Power Distribution: The normalp Package
In this paper we present the normalp package, a package for the statistical environment R that has a set of tools for dealing with the exponential power distribution. In this package there are functions to compute the density function, the distribution function and the quantiles from an exponential power distribution and to generate pseudo-random numbers from the same distribution. Moreover, methods concerning the estimation of the distribution parameters are described and implemented. It is also possible to estimate linear regression models when we assume the random errors distributed according to an exponential power distribution. A set of functions is designed to perform simulation studies to see the suitability of the estimators used. Some examples of use of this package are provided.
X-ray emission from star-forming galaxies - I. High-mass X-ray binaries
Based on a homogeneous set of X-ray, infrared and ultraviolet observations
from Chandra, Spitzer, GALEX and 2MASS archives, we study populations of
high-mass X-ray binaries (HMXBs) in a sample of 29 nearby star-forming galaxies
and their relation with the star formation rate (SFR). In agreement with
previous results, we find that HMXBs are a good tracer of the recent star
formation activity in the host galaxy and their collective luminosity and
number scale with the SFR, in particular, Lx~2.6 10^{39} SFR. However, the
scaling relations still bear a rather large dispersion of ~0.4 dex, which we
believe is of a physical origin. We present the catalog of 1057 X-ray sources
detected within the ellipse for galaxies of our sample and construct the
average X-ray luminosity function (XLF) of HMXBs with substantially improved
statistical accuracy and better control of systematic effects than achieved in
previous studies. The XLF follows a power law with slope of 1.6 in the
logLx~35-40 luminosity range with a moderately significant evidence for a break
or cut-off at Lx~10^{40} erg/s. As before, we did not find any features at the
Eddington limit for a neutron star or a stellar mass black hole. We discuss
implications of our results for the theory of binary evolution. In particular
we estimate the fraction of compact objects that once upon their lifetime
experienced an X-ray active phase powered by accretion from a high mass
companion and obtain a rather large number, fx~0.2 (0.1 Myr/tau_x) (tau_x is
the life time of the X-ray active phase). This is ~4 orders of magnitude more
frequent than in LMXBs. We also derive constrains on the mass distribution of
the secondary star in HMXBs.Comment: 23 pages, 14 figures, 5 tables, MNRAS - Accepted 2011 September 2
The X-ray emission of the Crab-like pulsar PSR J0537-6910
In this paper we present some preliminary result on the spectral and timing
analysis of the X-ray pulsed emission from the 16 ms pulsar PSR J0537-6910 in
the energy range 0.1--30 keV, based on archival BeppoSAX and RossiXTE
observations. This pulsar, discovered by Marshall et al.(1998) in the LMC field
with RXTE, is the fastest spinning pulsar associated with a supernova remnant.
It is characterized by strong glitch activity with the highest rate of all
known Crab-like system.Comment: 4 pages, 2 figures. Contribution to the proceedings of "The Restless
High-Energy Universe", Amsterdam, May 5-8, 2003. Editors: E.P.J. van den
Heuvel, J.J.M. in't Zand, R.A.M.J. Wijer
Some observations on bigeye tuna (Thunnus obesus) caught by the surface and longline fisheries for tunas in the Eastern Pacific Ocean
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