16,768 research outputs found
On the compactness of the isolated neutron star RX J0720.4-3125
The data from all observations of RX J0720.4-3125 conducted by XMM-Newton
EPIC-pn with the same instrumental setup in 2000-2012 were reprocessed to form
a homogenous data set of solar barycenter corrected photon arrival times
registered from RX J0720.4-3125. A Bayesian method for the search, detection,
and estimation of the parameters of an unknown-shaped periodic signal was
employed as developed by Gregory & Loredo (1992).
A number of complex models (single and double peaked) of light curves from
pulsating neutron stars were statistically analyzed. The distribution of phases
for the registered photons was calculated by folding the arrival times with the
derived spin-period and the resulting distribution of phases approximated with
a mixed von Mises distribution, and its parameters were estimated by using the
Expected Maximization method. Spin phase-resolved spectra were extracted, and a
number of highly magnetized atmosphere models of an INS were used to fit
simultaneously, the results were verified via an MCMC approach. The
phase-folded light curves in different energy bands with high S/N ratio show a
high complexity and variations depending on time and energy.
They can be parameterized with a mixed von Mises distribution, i.e. with
double-peaked light curve profile showing a dependence of the estimated
parameters (mean directions, concentrations, and proportion) upon the energy
band, indicating that radiation emerges from at least two emitting areas.
The genuine spin-period of the isolated neutron star RX J0720-3125 derived as
more likely is twice of that reported in the literature (16.78s instead of
8.39s).
The gravitational redshift of RX J0720.4-3125 was determined to
and the compactness was estimated to
.Comment: Comments: 19 pages, 15 figures and 5 tables, Astronomy and
Astrophysics accepted. arXiv admin note: text overlap with arXiv:1108.389
Crossover between different regimes of inhomogeneous superconductivity in planar superconductor-ferromagnet hybrids
We studied experimentally the effect of a stripe-like domain structure in a
ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a
superconducting Pb microbridge. The system was designed in such a way that the
bridge is oriented perpendicular to the domain walls. It is demonstrated that
depending on the ratio between the amplitude of the nonuniform magnetic field
B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the
superconducting material, the regions of the reverse-domain superconductivity
in the H-T plane can be isolated or can overlap (H is the external magnetic
field, T is temperature). The latter case corresponds to the condition
B_0/H_{c2}<1 and results in the formation of superconductivity above the
magnetic domains of both polarities. We discovered the regime of edge-assisted
reverse-domain superconductivity, corresponding to localized superconductivity
near the edges of the bridge above the compensated magnetic domains. Direct
verification of the formation of inhomogeneous superconducting states and
external-field-controlled switching between normal state and inhomogeneous
superconductivity were obtained by low-temperature scanning laser microscopy.Comment: 11 pages, 12 figure
Transport, magnetic, and structural properties of LaCeMnO thin films. Evidence for hole-doping
Cerium-doped manganite thin films were grown epitaxially by pulsed laser
deposition at C and oxygen pressure Pa and were
subjected to different annealing steps. According to x-ray diffraction (XRD)
data, the formation of CeO as a secondary phase could be avoided for
Pa. However, transmission electron microscopy shows the presence
of CeO nanoclusters, even in those films which appear to be single phase in
XRD. With O annealing, the metal-to-insulator transition temperature
increases, while the saturation magnetization decreases and stays well below
the theoretical value for electron-doped LaCeMnO with mixed
Mn/Mn valences. The same trend is observed with decreasing film
thickness from 100 to 20 nm, indicating a higher oxygen content for thinner
films. Hall measurements on a film which shows a metal-to-insulator transition
clearly reveal holes as dominating charge carriers. Combining data from x-ray
photoemission spectroscopy, for determination of the oxygen content, and x-ray
absorption spectroscopy (XAS), for determination of the hole concentration and
cation valences, we find that with increasing oxygen content the hole
concentration increases and Mn valences are shifted from 2+ to 4+. The
dominating Mn valences in the films are Mn and Mn, and only a
small amount of Mn ions can be observed by XAS. Mn and Ce
XAS signals obtained in surface-sensitive total electron yield mode are
strongly reduced in the bulk-sensitive fluorescence mode, which indicates
hole-doping in the bulk for those films which do show a metal-to-insulator
transition.Comment: 8 pages, 10 figure
Ultra-High Energy Cosmic Rays: Some General Features, and Recent Developments Concerning Air Shower Computations
We present an introductory lecture on general features of cosmic rays, for
non-experts, and some recent developments concerning cascade equations for air
shower developments.Comment: invited talk, presented at the Hadron-RANP2004 worksho
Absorption Features in Spectra of Magnetized Neutron Stars
The X-ray spectra of some magnetized isolated neutron stars (NSs) show
absorption features with equivalent widths (EWs) of 50 - 200 eV, whose nature
is not yet well known. To explain the prominent absorption features in the soft
X-ray spectra of the highly magnetized (B ~ 10^{14} G) X-ray dim isolated NSs
(XDINSs), we theoretically investigate different NS local surface models,
including naked condensed iron surfaces and partially ionized hydrogen model
atmospheres, with semi-infinite and thin atmospheres above the condensed
surface. We also developed a code for computing light curves and integral
emergent spectra of magnetized neutron stars with various temperature and
magnetic field distributions over the NS surface. We compare the general
properties of the computed and observed light curves and integral spectra for
XDINS RBS\,1223 and conclude that the observations can be explained by a thin
hydrogen atmosphere above the condensed iron surface, while the presence of a
strong toroidal magnetic field component on the XDINS surface is unlikely. We
suggest that the harmonically spaced absorption features in the soft X-ray
spectrum of the central compact object (CCO) 1E 1207.4-5209 (hereafter 1E 1207)
correspond to peaks in the energy dependence of the free-free opacity in a
quantizing magnetic field, known as quantum oscillations. To explore observable
properties of these quantum oscillations, we calculate models of hydrogen NS
atmospheres with B ~ 10^{10} - 10^{11} G (i.e., electron cyclotron energy
E_{c,e} ~ 0.1 - 1 keV) and T_eff = 1 - 3 MK. Such conditions are thought to be
typical for 1E 1207. We show that observable features at the electron cyclotron
harmonics with EWs \approx 100 - 200 eV can arise due to these quantum
oscillations.Comment: 4 pages, 3 figures, conference "Astrophysics of Neutron Stars - 2010"
in honor of M. Ali Alpar, Izmir, Turke
Equilibrium states and invariant measures for random dynamical systems
Random dynamical systems with countably many maps which admit countable
Markov partitions on complete metric spaces such that the resulting Markov
systems are uniformly continuous and contractive are considered. A
non-degeneracy and a consistency conditions for such systems, which admit some
proper Markov partitions of connected spaces, are introduced, and further
sufficient conditions for them are provided. It is shown that every uniformly
continuous Markov system associated with a continuous random dynamical system
is consistent if it has a dominating Markov chain. A necessary and sufficient
condition for the existence of an invariant Borel probability measure for such
a non-degenerate system with a dominating Markov chain and a finite (16) is
given. The condition is also sufficient if the non-degeneracy is weakened with
the consistency condition. A further sufficient condition for the existence of
an invariant measure for such a consistent system which involves only the
properties of the dominating Markov chain is provided. In particular, it
implies that every such a consistent system with a finite Markov partition and
a finite (16) has an invariant Borel probability measure. A bijective map
between these measures and equilibrium states associated with such a system is
established in the non-degenerate case. Some properties of the map and the
measures are given.Comment: The article is published in DCDS-A, but without the 3rd paragraph on
page 4 (the complete removal of the paragraph became the condition for the
publication in the DCDS-A after the reviewer ran out of the citation
suggestions collected in the paragraph
Band-aid for information loss from black holes
We summarize, simplify and extend recent work showing that small deviations
from exact thermality in Hawking radiation, first uncovered by Kraus and
Wilczek, have the capacity to carry off the maximum information content of a
black hole. This goes a considerable way toward resolving a long-standing
"information-loss paradox"
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