12,727 research outputs found
A note on the equivalence of a barotropic perfect fluid with a K-essence scalar field
In this short note, we obtain the necessary and sufficient condition for a
class of non-canonical single scalar field models to be exactly equivalent to
barotropic perfect fluids, under the assumption of an irrotational fluid flow.
An immediate consequence of this result is that the non-adiabatic pressure
perturbation in this class of scalar field systems vanishes exactly at all
orders in perturbation theory and on all scales. The Lagrangian for this
general class of scalar field models depends on both the kinetic term and the
value of the field. However, after a field redefinition, it can be effectively
cast in the form of a purely kinetic K-essence model.Comment: 4 pages, 1 figure; v2: References and footnotes 3 and 4 added.
Replaced to match published versio
Population of the Galactic X-ray binaries and eRosita
The population of the Galactic X-ray binaries has been mostly probed with
moderately sensitive hard X-ray surveys so far. The eRosita mission will
provide, for the first time a sensitive all-sky X-ray survey in the 2-10 keV
energy range, where the X-ray binaries emit most of the flux and discover the
still unobserved low-luminosity population of these objects. In this paper, we
briefly review the current constraints for the X-ray luminosity functions of
high- and low-mass X-ray binaries and present our own analysis based the
INTEGRAL 9-year Galactic survey, which yields improved constraints. Based on
these results, we estimate the number of new XRBs to be detected in the eRosita
all-sky surveyComment: accepted for publication in A&
On the Integrability of Classical Ruijsenaars-Schneider Model of Type
The problem of finding most general form of the classical integrable
relativistic models of many-body interaction of the type is
considered. In the simplest nontrivial case of ,the extra integral of
motion is presented in explicit form within the ansatz similar to the
nonrelativistic Calogero-Moser models. The resulting Hamiltonian has been found
by solving the set of two functional equations.Comment: 10 pages, LaTeX2e, no figure
Virtual photon structure functions and positivity constraints
We study the three positivity constraints among the eight virtual photon
structure functions, derived from the Cauchy-Schwarz inequality and which are
hence model-independent. The photon structure functions obtained from the
simple parton model show quite different behaviors in a massive quark or a
massless quark case, but they satisfy, in both cases, the three positivity
constraints. We then discuss an inequality which holds among the unpolarized
and polarized photon structure functions , and
, in the kinematic region , where is the mass squared of the probe (target) photon, and we examine
whether this inequality is satisfied by the perturbative QCD results.Comment: 24 pages, 13 eps figure
Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
Over half of GeV gamma-ray sources observed by the EGRET experiment have not
yet been identified as known astronomical objects. There is an isotropic
component of such unidentified sources, whose number is about 60 in the whole
sky. Here we calculate the expected number of dynamically forming clusters of
galaxies emitting gamma-rays by high energy electrons accelerated in the shock
wave when they form, in the framework of the standard theory of structure
formation. We find that a few tens of such forming clusters should be
detectable by EGRET and hence a considerable fraction of the isotropic
unidentified sources can be accounted for, if about 5% of the shock energy is
going into electron acceleration. We argue that these clusters are very
difficult to detect in x-ray or optical surveys compared with the conventional
clusters, because of their extended angular size of about 1 degree. Hence they
define a new population of ``gamma-ray clusters''. If this hypothesis is true,
the next generation gamma-ray telescopes such as GLAST will detect more than a
few thousands of gamma-ray clusters. It would provide a new tracer of
dynamically evolving structures in the universe, in contrast to the x-ray
clusters as a tracer of hydrodynamically stabilized systems. We also derive the
strength of magnetic field required for the extragalactic gamma-ray background
by structure formation to extend up to 100 GeV as observed, that is about
10^{-5} of the shock-heated baryon energy density.Comment: Accepted by ApJ after minor revisions. Received May 9, Accepted
August 3. 8 pages including 2 figure
Non-perturbative dynamics and charge fluctuations in effective chiral models
We discuss the properties of fluctuations of the electric charge in the
vicinity of the chiral crossover transition within effective chiral models at
finite temperature and vanishing net baryon density. The calculation includes
non-perturbative dynamics implemented within the functional renormalization
group approach. We study the temperature dependence of the electric charge
susceptibilities in the linear sigma model and explore the role of quantum
statistics. Within the Polyakov loop extended quark-meson model, we study the
influence of the coupling of quarks to mesons and to an effective gluon field
on charge fluctuations. We find a clear signal for the chiral crossover
transition in the fluctuations of the electric charge. Accordingly, we stress
the role of higher order cumulants as probes of criticality related to the
restoration of chiral symmetry and deconfinement.Comment: 12 pages, 3 figure
Optical and near-infrared photometric monitoring of the transient X-ray binary A0538-66 with REM
The transient Be/X-ray binary A0538-66 shows peculiar X-ray and optical
variability. Despite numerous studies, the intrinsic properties underlying its
anomalous behaviour remain poorly understood. Since 2014 September we are
conducting the first quasi-simultaneous optical and near-infrared photometric
monitoring of A0538-66 in seven filters with the Rapid Eye Mount (REM)
telescope, aiming to understand the properties of this binary system. We found
that the REM lightcurves show fast flares lasting one or two days that repeat
almost regularly every ~16.6 days, the orbital period of the neutron star. If
the optical flares are powered by X-ray outbursts through photon reprocessing,
the REM lightcurves indicate that A0538-66 is still active in X-rays: bright
X-ray flares (L_x > 1E37 erg/s) could be observable during the periastron
passages. The REM lightcurves show a long-term variability that is especially
pronounced in the g band and decreases with increasing wavelength, until it no
longer appears in the near-infrared lightcurves. In addition, A0538-66 is
fainter with respect to previous optical observations most likely due to the
higher absorption of the stellar radiation of a denser circumstellar disc. On
the basis of the current models, we interpret these observational results with
a circumstellar disc around the Be star observed nearly edge-on during a
partial depletion phase. The REM lightcurves also show short-term variability
on timescales of ~1 day possibly indicative of perturbations in the density
distribution of the circumstellar disc caused by the tidal interaction with the
neutron star.Comment: Accepted for publication in Astronomy & Astrophysic
Chiral thermodynamics of dense hadronic matter
We discuss phases of hot and dense hadronic matter using chiral Lagrangians.
A two-flavored parity doublet model constrained by the nuclear matter ground
state predicts chiral symmetry restoration. The model thermodynamics is shown
within the mean field approximation. A field-theoretical constraint on possible
phases from the anomaly matching is also discussed.Comment: 8 pages, 2 figures, to appear in the proceedings of 6th International
Workshop on Critical Point and Onset of Deconfinement (CPOD), 23-29 August
2010 at Joint Institute for Nuclear Research, Dubna, Russi
- …