7,445 research outputs found
Relation between the properties of the kilohertz quasi-periodic oscillations and spectral parameters in 4U 163653
We investigate the relation between the parameters of the energy spectrum and
the frequency and amplitude of the kilohertz quasi-periodic oscillations (kHz
QPOs) in the low-mass X-ray binary 4U 163653. We fit the -keV
spectrum of this source with a model that includes a thermal Comptonisation
component. We show that the frequencies of both kHz QPOs follow the same
relation as a function of the parameters of this spectral component, except for
a systematic frequency shift, whereas the rms fractional amplitude of each QPO
follows a different relation with respect to those same parameters. This
implies that, while the dynamical mechanism that sets the frequencies of the
QPO can be the same for both kHz QPOs, the radiative mechanisms that set the
amplitudes of the lower and the upper kHz QPO are likely different. We discuss
the implications of these results to the modelling of the kHz QPOs and the
possibility that the lower kHz QPO reflects a resonance between the
Comptonising medium and the photons from the accretion disc and/or the neutron
star surface.Comment: 7 pages, 4 figures, 2 tables. Accepted for publication in MNRA
Coupling between 4f and itinerant electrons in SmFeAsO1-xFx (0.15 < x < 0.2) superconductors: an NMR study
F NMR measurements in SmFeAsOF, for ,
are presented. The nuclear spin-lattice relaxation rate increases upon
cooling with a trend analogous to the one already observed in
CeCuAu, a quasi two-dimensional heavy-fermion intermetallic
compound with an antiferromagnetic ground-state. In particular, the behaviour
of the relaxation rate either in SmFeAsOF or in
CeCuAu can be described in the framework of the self-consistent
renormalization theory for weakly itinerant electron systems. Remarkably, no
effect of the superconducting transition on F is detected, a
phenomenon which can hardly be explained within a single band model.Comment: 4 figure
The reflection spectrum of the low-mass X-ray binary 4U 1636-53
We present 3-79 keV NuSTAR observations of the neutron star low-mass X-ray
binary 4U 1636-53 in the soft, transitional and hard state. The spectra display
a broad emission line at 5-10 keV. We applied several models to fit this line:
A GAUSSIAN line, a relativistically broadened emission line model, KYRLINE, and
two models including relativistically smeared and ionized reflection off the
accretion disc with different coronal heights, RELXILL and RELXILLLP. All
models fit the spectra well, however, the KYRLINE and RELXILL models yield an
inclination of the accretion disc of with respect to the line
of sight, which is at odds with the fact that this source shows no dips or
eclipses. The RELXILLLP model, on the other hand, gives a reasonable
inclination of . We discuss our results for these models in this
source and the possible primary source of the hard X-rays.Comment: 9 pages, 8 figure
Relation between spectral changes and the presence of the lower kHz QPO in the neutron-star low-mass X-ray binary 4U 1636-53
We fitted the -keV spectrum of all the observations of the
neutron-star low-mass X-ray binary 4U 163653 taken with the {\it Rossi X-ray
Timing Explorer} using a model that includes a thermal Comptonisation
component. We found that in the low-hard state the power-law index of this
component, , gradually increases as the source moves in the
colour-colour diagram. When the source undergoes a transition from the hard to
the soft state drops abruptly; once the source is in the soft state
increases again and then decreases gradually as the source spectrum
softens further. The changes in , together with changes of the electron
temperature, reflect changes of the optical depth in the corona. The lower
kilohertz quasi-periodic oscillation (kHz QPO) in this source appears only in
observations during the transition from the hard to the soft state, when the
optical depth of the corona is high and changes depends strongly upon the
position of the source in the colour-colour diagram. Our results are consistent
with a scenario in which the lower kHz QPO reflects a global mode in the system
that results from the resonance between, the disc and/or the neutron-star
surface, and the Comptonising corona.Comment: 9 pages, 6 figures. Accepted for publication in MNRA
Mitochondrial DNA lineages of Italian Giara and Sarcidano horses
Giara and Sarcidano are 2 of the 15 extant native Italian horse breeds with limited dispersal capability that originated from a larger number of individuals. The 2 breeds live in two distinct isolated locations on the island of Sardinia. To determine the genetic structure and evolutionary history of these 2 Sardinian breeds, the first hypervariable segment of the mitochondrial DNA (mtDNA) was sequenced and analyzed in 40 Giara and Sarcidano horses and compared with publicly available mtDNA data from 43 Old World breeds. Four different analyses, including genetic distance, analysis of molecular variance, haplotype sharing, and clustering methods, were used to study the genetic relationships between the Sardinian and other horse breeds. The analyses yielded similar results, and the FST values indicated that a high percentage of the total genetic variation was explained by between-breed differences. Consistent with their distinct phenotypes and geographic isolation, the two Sardinian breeds were shown to consist of 2 distinct gene pools that had no gene flow between them. Giara horses were clearly separated from the other breeds examined and showed traces of ancient separation from horses of other breeds that share the same mitochondrial lineage. On the other hand, the data from the Sarcidano horses fit well with variation among breeds from the Iberian Peninsula and North-West Europe: genetic relationships among Sarcidano and the other breeds are consistent with the documented history of this breed
Superconducting phase fluctuations in SmFeAsOF from diamagnetism at low magnetic field above
Superconducting fluctuations (SF) in SmFeAsOF (characterized
by superconducting transition temperature K) are
investigated by means of isothermal high-resolution dc magnetization
measurements. The diamagnetic response to magnetic fields up to 1 T above
is similar to what previously reported for underdoped cuprate
superconductors and it can be justified in terms of metastable superconducting
islands at non-zero order parameter lacking of long-range coherence because of
strong phase fluctuations. In the high-field regime ( T) scaling
arguments predicted on the basis of the Ginzburg-Landau theory of conventional
SF are found to be applicable, at variance with what observed in the low-field
regime. This fact enlightens that two different phenomena are simultaneously
present in the fluctuating diamagnetism, namely the phase SF of novel character
and the conventional SF. High magnetic fields (1.5 T )
are found to suppress the former while leaving unaltered the latter one.Comment: 7 pages, 5 figure
Direct evaluation of the isotope effect within the framework of density functional theory for superconductors
Within recent developments of density functional theory, its numerical implementation and of the superconducting density functional theory is nowadays possible to predict the superconducting critical temperature, Tc, with sufficient accuracy to anticipate the experimental verification. In this paper we present an analytical derivation of the isotope coefficient within the superconducting density functional theory. We calculate the partial derivative of Tc with respect to atomic masses. We verified the final expression by means of numerical calculations of isotope coefficient in monatomic superconductors (Pb) as well as polyatomic superconductors (CaC6). The results confirm the validity of the analytical derivation with respect to the finite difference methods, with considerable improvement in terms of computational time and calculation accuracy. Once the critical temperature is calculated (at the reference mass(es)), various isotope exponents can be simply obtained in the same run. In addition, we provide the expression of interesting quantities like partial derivatives of the deformation potential, phonon frequencies and eigenvectors with respect to atomic masses, which can be useful for other derivations and applications
INTEGRATION OF GEOMATIC TECHNIQUES FOR THE URBAN CAVITY SURVEY
Cagliari, county seat of Sardinia Region (Italy), situated in the southern part of the island, is characterized by a subsoil full of cavities. The excavations in fact, which lasted more than 4000 years, had a great development due also to the special geological characteristics of the city subsoil. The underground voids, which the city is rich in, belong to different classes such as hydraulic structures (aqueducts, cisterns, wells, etc.), settlement works (tunnels, bomb shelters, tombs etc.) and various works (quarries, natural caves, etc.). This paper describes the phases of the survey of a large cavity below a high-traffic square near the Faculty of Engineering in the city of Cagliari, where the research team works.. The cave, which is part of a larger complex, is important because it was used in the thirteenth century (known as the Pisan age) as a stone quarry. There are traces of this activity that have to be protected. Moreover, during the last forty years the continuous crossover of vehicles cracked the roof of the cave compromising the stability of the entire area. Consequently a plan was developed to make the whole cavity safe and usable for visits. The study of the safety of the cave has involved different professionals among which geologists, engineers, constructors. The goal of the University of Cagliari geomatic team was to solve two problems: to obtain geometrical information about the void and correctly place the cave in the context of existing maps. The survey and the products, useful for the investigation of the technicians involved, had to comply with tolerances of 3 cm in the horizontal and 5 cm in the vertical component. The approach chosen for this purpose was to integrate different geomatic techniques. The cave was surveyed using a laser scanner (Faro Photon 80) in order to obtain a 3D model of the cave from which all the geometrical information was derived, while both classic topography and GPS techniques were used to include the cave in the city map
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