6,693 research outputs found
Russo's formula for random interlacements
In this paper we obtain a couple of explicit expressions for the derivative
of the probability of an increasing event in the random interlacements model.
The event is supported in a finite subset of the lattice, and the derivative is
with respect to the intensity parameter of the model.Comment: 14 pages, 3 figures, added section, accepted for publication in the
Journal of Statistical Physic
On uniform closeness of local times of Markov chains and i.i.d. sequences
In this paper we consider the field of local times of a discrete-time Markov
chain on a general state space, and obtain uniform (in time) upper bounds on
the total variation distance between this field and the one of a sequence of
i.i.d. random variables with law given by the invariant measure of that
Markov chain. The proof of this result uses a refinement of the soft local time
method of [11].Comment: 42 pages, 2 figures, coupling construction of Section 4 corrected,
results unchange
IGR J14257-6117, a magnetic accreting white dwarf with a very strong X-ray orbital modulation
IGR J14257-6117 is an unclassified source in the hard X-ray catalogues.
Optical follow-ups suggest it could be a Cataclysmic Variable of the magnetic
type. We present the first high S/N X-ray observation performed by \XMM\ at
0.3--10 keV, complemented with 10--80 keV coverage by \Swift/BAT, aimed at
revealing the source nature. We detected for the first time a fast periodic
variability at 509.5\,s and a longer periodic variability at 4.05\,h, ascribed
to the white dwarf (WD) spin and binary orbital periods, respectively. These
unambiguously identify IGR J14257-6117 as a magnetic CV of the Intermediate
Polar (IP) type. The energy resolved light curves at both periods reveal
amplitudes decreasing with increasing energy, with the orbital modulation
reaching in the softest band. The energy spectrum shows optically
thin thermal emission with an excess at the iron complex, absorbed by two dense
media (), partially covering the X-ray
source. These are likely localised in the magnetically confined accretion flow
above the WD surface and at the disc rim, producing the energy dependent spin
and orbital variabilities, respectively. IGR J14257-6117, joins the group of
strongest orbitally modulated IPs now counting four systems. Drawing
similarities with low-mass X-ray binaries displaying orbital dips, these IPs
should be seen at large orbital inclinations allowing azimuthally extended
absorbing material fixed in the binary frame to intercept the line of sight.
For IGR J14257-6117, we estimate (). Whether
also the mass accretion rate plays a role in the large orbital modulations in
IPs cannot be established with the present data.Comment: Accepted for publication on MNRAS. 9 pages, 6 table, 5 figure
Unambiguous Detection of Reflection in Magnetic Cataclysmic Variables: Joint NuSTAR-XMM-Newton Observations of Three Intermediate Polars
In magnetic cataclysmic variables (CVs), X-ray emission regions are located
close to the white dwarf surface, which is expected to reflect a significant
fraction of intrinsic X-rays above 10 keV, producing a Compton reflection hump.
However, up to now, a secure detection of this effect in magnetic CVs has
largely proved elusive because of the limited sensitivity of non-imaging X-ray
detectors. Here we report our analysis of joint NuSTAR/XMM-Newton observations
of three magnetic CVs, V709 Cas, NY Lup, and V1223 Sgr. The improved hard X-ray
sensitivity of the imaging NuSTAR data has resulted in the first robust
detection of Compton hump in all three objects, with amplitudes of ~1 or
greater in NY Lup, and likely <1.0 in the other two. We also confirm earlier
report of a strong spin modulation above 10 keV in V709 Cas, and report the
first detection of small spin amplitudes in the others. We interpret this as
due to different height of the X-ray emitting region among these objects. A
height of ~0.2 white dwarf radii provides a plausible explanation for the low
reflection amplitude of V709 Cas. Since emission regions above both poles are
visible at certain spin phases, this can also explain the strong hard X-ray
spin modulation. A shock height of ~0.05 white dwarf radii can explain our
results on V1223 Sgr, while the shock height in NY Lup appears negligible.Comment: 16 pages including 3 figures and 2 tables; accepted for publication
in Astrophysical Journal Letter
A equação de Ramanujan-Nagell e algumas de suas generalizações
Dissertação (mestrado)—Universidade de BrasÃlia, Departamento de Matemática, 2013.O objetivo deste trabalho é mostrar algumas técnicas para resolução de equações
diofantinas. Métodos algébricos são ferramentas de grande utilidade para a resolução da equação equation x2 + 7 = yn, em que y = 2 ou Y é Ãmpar. O uso do método hipergeométrico traz um resultado recente (de 2008) no estudo da equação x2 + 7 =2n. m e técnicas algébricas garantem uma condição necessária para que essa última equação tenha solução. _______________________________________________________________________________________ ABSTRACTThe objective of this work is to show some techniques for solving Diophantine equations. Algebraic methods are useful tools for solving the equation x2 + 7 = yn, where y = 2 or y is odd. The use of the hypergeometric method brings a recent result (from 2008) in the study of the equation x2 + 7 = 2n.m and algebraic techniques ensure a necessary condition for the last equation to have a solution
Theoretical evidence for efficient p-type doping of GaN using beryllium
Ab initio calculations predict that Be is a shallow acceptor in GaN. Its
thermal ionization energy is 0.06 eV in wurtzite GaN; the level is valence
resonant in the zincblende phase. Be incorporation is severely limited by the
formation of Be_3N_2. We show however that co-incorporation with reactive
species can enhance the solubility. H-assisted incorporation should lead to
high doping levels in MOCVD growth after post-growth annealing at about 850 K.
Be-O co-incorporation produces high Be and O concentrations at MBE growth
temperatures.Comment: revised Feb 24 199
Macroscopic polarization and band offsets at nitride heterojunctions
Ab initio electronic structure studies of prototypical polar interfaces of
wurtzite III-V nitrides show that large uniform electric fields exist in
epitaxial nitride overlayers, due to the discontinuity across the interface of
the macroscopic polarization of the constituent materials. Polarization fields
forbid a standard evaluation of band offsets and formation energies: using new
techniques, we find a large forward-backward asymmetry of the offset (0.2 eV
for AlN/GaN (0001), 0.85 eV for GaN/AlN (0001)), and tiny interface formation
energies.Comment: RevTeX 4 pages, 2 figure
Effect of ELF e.m. fields on metalloprotein redox-active sites
The peculiarity of the distribution and geometry of metallic ions in enzymes
pushed us to set the hypothesis that metallic ions in active-site act like tiny
antennas able to pick up very feeble e.m. signals. Enzymatic activity of Cu2+,
Zn2+ Superoxide Dismutase (SOD1) and Fe2+ Xanthine Oxidase (XO) has been
studied, following in vitro generation and removal of free radicals. We
observed that Superoxide radicals generation by XO is increased by a weak field
having the Larmor frequency fL of Fe2+ while the SOD1 kinetics is sensibly
reduced by exposure to a weak field having the frequency fL of Cu2+ ion.Comment: 18 pages, 4 figure
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