340 research outputs found
The effects of magnetic field, age, and intrinsic luminosity on Crab-like pulsar wind nebulae
We investigate the time-dependent behavior of Crab-like pulsar wind nebulae
(PWNe) generating a set of models using 4 different initial spin-down
luminosities (), 8 values
of magnetic fraction ( 0.001, 0.01, 0.03, 0.1, 0.5, 0.9, 0.99, and
0.999, i.e., from fully particle dominated to fully magnetically dominated
nebulae), and 3 distinctive ages: 940, 3000, and 9000 years. We find that the
self-synchrotron Compton (SSC) contribution is irrelevant for =0.1, 1,
and 10% of the Crab power, disregarding the age and the magnetic fraction. SSC
only becomes relevant for highly energetic ( of the Crab), particle
dominated nebulae at low ages (of less than a few kyr), located in a FIR
background with relatively low energy density. Since no pulsar other than Crab
is known to have these features, these results clarify why the Crab Nebula, and
only it, is SSC dominated. No young PWN would be detectable at TeV energies if
the pulsar's spin-down power is 0.1% Crab or lower. For 1% of the Crab
spin-down, only particle dominated nebulae can be detected by H.E.S.S.-like
telescopes when young enough (with details depending on the precise injection
and environmental parameters). Above 10% of the Crab's power, all PWNe are
detectable by H.E.S.S.-like telescopes if they are particle dominated, no
matter the age. The impact of the magnetic fraction on the final SED is varied
and important, generating order of magnitude variations in the luminosity
output for systems that are otherwise the same (equal , , injection,
and environment).Comment: Accepted for publication in MNRA
Is there room for highly magnetized pulsar wind nebulae among those non-detected at TeV?
We make a time-dependent characterization of pulsar wind nebulae (PWNe)
surrounding some of the highest spin-down pulsars that have not yet been
detected at TeV. Our aim is assessing their possible level of magnetization. We
analyze the nebulae driven by J2022+3842 in G76.9+1.0, J0540-6919 in N158A (the
Crab twin), J1400--6325 in G310.6--1.6, and J1124--5916 in G292.0+0.18, none of
which have been found at TeV energies. For comparison we refer to published
models of G54.1+0.3, the Crab nebula, and develop a model for N157B in the
Large Magellanic Cloud (LMC). We conclude that further observations of N158A
could lead to its detection at VHE. According to our model, a FIR energy
density of 5 eV cm could already lead to a detection in H.E.S.S.
(assuming no other IC target field) within 50 hours of exposure and just the
CMB inverse Compton contribution would produce VHE photons at the CTA
sensitivity. We also propose models for G76.9+1.0, G310.6--1.6 and G292.0+1.8
which suggest their TeV detection in a moderate exposure for the latter two
with the current generation of Cherenkov telescopes. We analyze the possibility
that these PWNe are highly magnetized, where the low number of particles
explains the residual detection in X-rays and their lack of detection at TeV
energies.Comment: Accepted for publication in MNRA
Gamma rays from microquasars Cygnus X-1 and Cygnus X-3
Gamma-ray observations of microquasars at high and very-high energies can
provide valuable information of the acceleration processes inside the jets, the
jet-environment interaction and the disk-jet coupling. Two high-mass
microquasars have been deeply studied to shed light on these aspects: Cygnus
X-1 and Cygnus X-3. Both systems display the canonical hard and soft X-ray
spectral states of black hole transients, where the radiation is dominated by
non-thermal emission from the corona and jets and by thermal emission from the
disk, respectively. Here, we report on the detection of Cygnus X-1 above 60 MeV
using 7.5 yr of Pass8 Fermi-LAT data, correlated with the hard X-ray state. A
hint of orbital flux modulation was also found, as the source is only detected
in phases around the compact object superior conjunction. We conclude that the
high-energy gamma-ray emission from Cygnus X-1 is most likely associated with
jets and its detection allow us to constrain the production site. Moreover, we
include in the discussion the final results of a MAGIC long-term campaign on
Cygnus X-1 that reaches almost 100 hr of observations at different X-ray
states. On the other hand, during summer 2016, Cygnus X-3 underwent a flaring
activity period in radio and high-energy gamma rays, similar to the one that
led to its detection in the high-energy regime in 2009. MAGIC performed
comprehensive follow-up observations for a total of about 70 hr. We discuss our
results in a multi-wavelength context.Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC
2017), Bexco, Busan, Korea (arXiv:1708.05153
Seniority number in spin-adapted spaces and compactness of configuration interaction wave functions
This work extends the concept of seniority number, which has been widely used for classifying N-electron Slater determinants, to wave functions of N electrons and spin S, as well as to N-electron spin-adapted Hilbert spaces. We propose a spin-free formulation of the seniority number operator and perform a study on the behavior of the expectation values of this operator under transformations of the molecular basis sets. This study leads to propose a quantitative evaluation for the convergence of the expansions of the wave functions in terms of Slater determinants. The non-invariant character of the seniority number operator expectation value of a wave function with respect to a unitary transformation of the molecular orbital basis set, allows us to search for a change of basis which minimizes that expectation value. The results found in the description of wave functions of selected atoms and molecules show that the expansions expressed in these bases exhibit a more rapid convergence than those formulated in the canonical molecular orbital bases and even in the natural orbital ones.Fil: Alcoba, Diego Ricardo. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de FÃsica de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FÃsica; ArgentinaFil: Torre, Alicia. Universidad del PaÃs Vasco. Facultad de Ciencia y TecnologÃa. Departamento de QuÃmica FÃsica; España;Fil: Lain, Luis. Universidad del PaÃs Vasco. Facultad de Ciencia y TecnologÃa. Departamento de QuÃmica FÃsica; España;Fil: Massaccesi, Gustavo Ernesto. Universidad de Buenos Aires. Ciclo Básico Común; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Matemática; ArgentinaFil: Oña, Ofelia Beatriz. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico la Plata. Instituto de Investigaciones FisicoquÃmicas Teóricas y Aplicadas; Argentin
Configuration interaction wave functions: A seniority number approach
This work deals with the configuration interaction method when an N-electron Hamiltonian is projected on Slater determinants which are classified according to their seniority number values. We study the spin features of the wave functions and the size of the matrices required to formulate states of any spin symmetry within this treatment. Correlation energies associated with the wave functions arising from the seniority-based configuration interaction procedure are determined for three types of molecular orbital basis: canonical molecular orbitals, natural orbitals, and the orbitals resulting from minimizing the expectation value of the N-electron seniority number operator. The performance of these bases is analyzed by means of numerical results obtained from selected N-electron systems of several spin symmetries. The comparison of the results highlights the efficiency of the molecular orbital basis which minimizes the mean value of the seniority number for a state, yielding energy values closer to those provided by the full configuration interaction procedure.Fil: Alcoba, Diego Ricardo. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de FÃsica de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FÃsica; ArgentinaFil: Torre, Alicia. Universidad del Pais Vasco; EspañaFil: Lain, Luis . Universidad del Pais Vasco; EspañaFil: Massaccesi, Gustavo Ernesto. Universidad de Buenos Aires. Ciclo Básico Común; ArgentinaFil: Oña, Ofelia Beatriz. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico la Plata. Instituto de Investigaciones FisicoquÃmicas Teóricas y Aplicadas; Argentin
The role of asymmetric interactions on the effect of habitat destruction in mutualistic networks
Plant-pollinator mutualistic networks are asymmetric in their interactions:
specialist plants are pollinated by generalist animals, while generalist plants
are pollinated by a broad involving specialists and generalists. It has been
suggested that this asymmetric ---or disassortative--- assemblage could play an
important role in determining the equal susceptibility of specialist and
generalist plants under habitat destruction. At the core of the argument lies
the observation that specialist plants, otherwise candidates to extinction,
could cope with the disruption thanks to their interaction with generalist
pollinators. We present a theoretical framework that supports this thesis. We
analyze a dynamical model of a system of mutualistic plants and pollinators,
subject to the destruction of their habitat. We analyze and compare two
families of interaction topologies, ranging from highly assortative to highly
disassortative ones, as well as real pollination networks. We found that
several features observed in natural systems are predicted by the mathematical
model. First, there is a tendency to increase the asymmetry of the network as a
result of the extinctions. Second, an entropy measure of the differential
susceptibility to extinction of specialist and generalist species show that
they tend to balance when the network is disassortative. Finally, the
disappearance of links in the network, as a result of extinctions, shows that
specialist plants preserve more connections than the corresponding plants in an
assortative system, enabling them to resist the disruption.Comment: 14 pages, 7 figure
Bimetallic Intersection in PdFe@FeOx-C Nanomaterial for Enhanced Water Splitting Electrocatalysis
Supported Fe-doped Pd-nanoparticles (NPs) are prepared via soft transfor-mation of a PdFe-metal oraganic framework (MOF). The thus synthesized bimetallic PdFe-NPs are supported on FeOx@C layers, which are essential for developing well-defined and distributed small NPs, 2.3 nm with 35% metal loading. They are used as bifunctional nanocatalysts for the electro-catalytic water splitting process. They display superior mass activity for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), both in alkaline and acid media, compared with those obtained for benchmarking platinum HER catalyst, and ruthenium, and iridium oxide OER catalysts. PdFe-NPs also exhibit outstanding stability against sintering that can be explained by the protecting role of graphitic carbon layers provided by the organic linker of the MOF. Additionally, the superior electrocatalytic performance of the bimetallic PdFe-NPs compared with those of monometallic Pd/C NPs and FeOx points to a synergetic effect induced by Fe-Pd interactions that facilitates the water splitting reaction. This is supported by additional characterization of the PdFe-NPs prior and post electrolysis by TEM, XRD, X-ray photoelectron spectroscopy, and Raman revealing that dispersed PdFe NPs on FeOx@C promote interactions between Pd and Fe, most likely to be Pd-O-Fe active centers
Multiband study of RX J0838-2827 and XMM J083850.4-282759: A new asynchronous magnetic cataclysmic variable and a candidate transitional millisecond pulsar
Indexación: Scopus.In a search for the counterpart to the Fermi-LAT source 3FGL J0838.8-2829, we performed a multiwavelength campaign: in the X-ray band with Swift and XMM-Newton; in the infrared and optical with OAGH, ESO-NTT and IAC80; and in the radio with ATCA observations. We also used archival hard X-ray data obtained by INTEGRAL. We report on three X-ray sources consistent with the position of the Fermi-LAT source.We confirm the identification of the brightest object, RX J0838-2827, as a magnetic cataclysmic variable that we recognize as an asynchronous system (not associated with the Fermi-LAT source). RX J0838-2827 is extremely variable in the X-ray and optical bands, and timing analysis reveals the presence of several periodicities modulating its X-ray and optical emission. The most evident modulations are interpreted as being caused by the binary system orbital period of ~1.64 h and the white dwarf spin period of ~1.47 h. A strong flux modulation at ~15 h is observed at all energy bands, consistent with the beat frequency between spin and orbital periods. Optical spectra show prominent Hß, He I and He II emission lines that are Doppler-modulated at the orbital period and at the beat period. Therefore, RX J0838-2827 accretes through a disc-less configuration and could be either a strongly asynchronous polar or a rare example of a pre-polar system on its way to reaching synchronism. Regarding the other two X-ray sources, XMM J083850.4-282759 showed a variable X-ray emission, with a powerful flare lasting for ~600 s, similar to what is observed in transitional millisecond pulsars during the subluminous disc state: this observation possibly means that this source can be associated with the Fermi-LAT source. © 2017 The Authors.https://academic.oup.com/mnras/article/471/3/2902/408195
Variational determination of the two-electron reduced density matrix within the doubly occupied configuration interaction scheme: An extension to the study of open-shell systems
This work proposes to describe open-shell molecules or radicals using the framework of the doubly occupied configuration interaction (DOCI) treatments, so far limited to closed-shell system studies. The proposal is based on considering molecular systems in singlet states generated by adding extra hydrogen atoms located at infinite distance from the target radical system. The energy of this radical is obtained by subtracting the energies of the dissociated hydrogen atoms from that provided by the two-electron reduced density matrix corresponding to the singlet state system in the DOCI space, which is variationally calculated by imposing a set of N-representability conditions. This method is numerically assessed by describing potential energy curves and reduced density matrices in selected ionic and neutral open-shell systems in the doublet spin symmetry ground state.Fil: Oña, Ofelia Beatriz. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - La Plata. Instituto de Investigaciones FisicoquÃmicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones FisicoquÃmicas Teóricas y Aplicadas; ArgentinaFil: Torre, Alicia. Universidad del PaÃs Vasco; EspañaFil: Lain, Luis. Universidad del PaÃs Vasco; EspañaFil: Alcoba, Diego Ricardo. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de FÃsica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FÃsica de Buenos Aires; ArgentinaFil: RÃos, ElÃas Daniel. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - La Plata. Instituto de Investigaciones FisicoquÃmicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones FisicoquÃmicas Teóricas y Aplicadas; ArgentinaFil: Massaccesi, Gustavo Ernesto. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentin
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