740 research outputs found
Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197
Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE
J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission
(XMM-Newton) are used to study its spectrum and pulsed light curve as the
source fades from outburst. The decay is consistent with an exponential of time
constant 300 days, but not a power law as predicted in some models of sudden
deep crustal heating events. All spectra are well fitted by a blackbody plus a
steep power law, a problematic model that is commonly fitted to anomalous X-ray
pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better
motivated physically in view of the faint optical/IR fluxes, the X-ray pulse
shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting
areas that are less than or equal to the surface area of a neutron star. The
fitted temperatures remained the same while the flux declined by 46%, which can
be interpreted as a decrease in area of the emitting regions. The pulsar
continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s
s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic
field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with
both the outburst and quiescent X-ray luminosities being powered by magnetic
field decay, i.e., XTE J1810-197 is a magnetar.Comment: 10 pages, 5 figures, accepted by Ap.
Caracterização de resíduos sólidos de coleta seletiva em condomínios residenciais Estudo de Caso em Vitória - ES.
A caracterização, identificação e quantificação de resíduos sólidos é de grande importância para a elaboração dos planos de gerenciamento de resídos sólidos. Os condomínios residenciais verticalizados merecem destaque no gerenciamento, pois constituem uma importante fonte de geração de resíduos sólidos, em grande escala. Assim, o presente trabalho teve como objetivo realizar a caracterização física dos resíduos sólidos de coleta seletiva de condomínios residenciais verticalizados da cidade de Vitória ES. Foram realizados levantamentos de dados, ensaios de caracterização física dos resíduos e avaliados os resultados com relação à participação, à receita da venda dos recicláveis, à operacionalidade do programa e à redução de resíduos encaminhados ao aterro sanitário. O valor per capita médio de lixo seco encontrado foi de 0,069 kg/hab/dia. Dentre os materiais segregados o que obteve maior quantidade em peso e volume foi o papel e os menos observados foram os REEE. O papel apresentou percentual gravimétrico acima da média nacional. Os demais materiais ficaram abaixo da média do país. Não foi observado comportamento padrão semanal na geração dos resíduos, nem encontrada correlação entre o tempo de existência do programa de coleta seletiva e o valor per capita observado. Observou-se alta correlação negativa entre a geração per capita e o número de apartamentos nos condomínios, sugerindo-se que condomínios com maior número de residências têm dificuldades de disseminar a coleta seletiva em todos os apartamentos, levando a índices menores de participação. O peso específico aparente médio observado para os resíduos sólidos secos foi de 68,04 kg/m3, com rejeitos, e 62,58 kg/m3, sem rejeitos. O IRMR obtido foi de 6,53%, valor considerado médio, e o percentual de participação estimado em 19,93%. A receita potencial gerada a partir da venda dos recicláveis foi de mais de 120 mil reais por mês, garantindo a sustentabilidade econômica das associações de catadores. Detectou-se a possibilidade de ajustes na freqüência de coleta para minimizar os custos do serviço. A transferência de resíduos para associações de catadores poderia ser aumentada em 293 vezes, e tem potencial de redução de mais de 5% dos resíduos do município encaminhados a aterros sanitários
Quantum-kinetic theory of photocurrent generation via direct and phonon-mediated optical transitions
A quantum-kinetic theory of direct and phonon mediated indirect optical
transitions is developed within the framework of the non-equilibrium Green's
function formalism. After validation against the standard Fermi-Golden-Rule
approach in the bulk case, it is used in the simulation of photocurrent
generation in ultra-thin crystalline silicon p-i-n-junction devices.Comment: 12 pages, 11 figure
BATSE Observations of the Piccinotti Sample of AGN
BATSE Earth occultation data have been used to search for emission in the
20-100 keV band from all sources in the Piccinotti sample, which represents to
date the only complete 2-10 keV survey of the extragalactic sky down to a
limiting flux of 3.1 x 10^(-11) erg cm^(-2)$ s^(-1). Nearly four years of
observations have been analyzed to reach a 5sigma sensitivity level of about
7.8x 10^(-11) erg cm^(-2) s^(-1) in the band considered. Of the 36 sources in
the sample, 14 have been detected above 5sigma confidence level while marginal
detection (3<sigma<5) can be claimed for 13 sources; for 9 objects 2sigma upper
limits are reported. Comparison of BATSE results with data at higher energies
is used to estimate the robustness of our data analysis: while the detection
level of each source is reliable, the flux measurement maybe overestimated in
some sources by as much as 35%, probably due to incomplete data cleaning.
Comparison of BATSE fluxes with X-ray fluxes, obtained in the 2-10 keV range
and averaged over years, indicates that a canonical power law of photon index
1.7 gives a good description of the broad band spectra of bright AGNs and that
spectral breaks preferentially occur above 100 keV.Comment: 18 pages, 1 figure. Accepted for publication on Apj
Accurate classification of 75 counterparts of objects detected in the 54 month Palermo Swift/BAT hard X-ray catalogue
Through an optical campaign performed at 4 telescopes located in the northern
and the southern hemispheres, we have obtained optical spectroscopy for 75
counterparts of unclassified or poorly studied hard X-ray emitting objects
detected with Swift/BAT and listed in the 54 month Palermo BAT catalogue. All
these objects have also observations taken with Swift/XRT, ROSAT or Chandra
satellites which allowed us to reduce the high energy error box and pinpoint
the most likely optical counterpart/s. We find that 69 sources in our sample
are Active Galactic Nuclei (AGNs); of them, 35 are classified as type 1 (with
broad and narrow emission lines), 33 are classified as type 2 (with only narrow
emission lines) and one is an high redshift QSO; the remaining 6 objects are
galactic cataclysmic variables (CVs). Among type 1 AGNs, 32 are objects of
intermediate Seyfert type (1.2-1.9) and one is Narrow Line Seyfert 1 galaxy;
for 29 out of 35 type 1 AGNs, we have been able to estimate the central black
hole mass and the Eddington ratio. Among type 2 AGNs, two display optical
features typical of the LINER class, 3 are classified as transition objects, 1
is a starburst galaxy and 2 are instead X-ray bright, optically normal
galaxies. All galaxies classified in this work are relatively nearby objects
(0.006 - 0.213) except for one at redshift 1.137.Comment: 19 pages, 5 figures, 6 tables, accepted for publications on Astronomy
and Astrophysics, main journal. arXiv admin note: text overlap with
arXiv:1206.509
Engineering Silicon Nanocrystals: Theoretical study of the effect of Codoping with Boron and Phosphorus
We show that the optical and electronic properties of nanocrystalline silicon
can be efficiently tuned using impurity doping. In particular, we give
evidence, by means of ab-initio calculations, that by properly controlling the
doping with either one or two atomic species, a significant modification of
both the absorption and the emission of light can be achieved. We have
considered impurities, either boron or phosphorous (doping) or both (codoping),
located at different substitutional sites of silicon nanocrystals with size
ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped
nanocrystals have the lowest impurity formation energies when the two
impurities occupy nearest neighbor sites near the surface. In addition, such
systems present band-edge states localized on the impurities giving rise to a
red-shift of the absorption thresholds with respect to that of undoped
nanocrystals. Our detailed theoretical analysis shows that the creation of an
electron-hole pair due to light absorption determines a geometry distortion
that in turn results in a Stokes shift between adsorption and emission spectra.
In order to give a deeper insight in this effect, in one case we have
calculated the absorption and emission spectra going beyond the single-particle
approach showing the important role played by many-body effects. The entire set
of results we have collected in this work give a strong indication that with
the doping it is possible to tune the optical properties of silicon
nanocrystals.Comment: 14 pages 19 figure
BL Lacertae identifications in a ROSAT-selected sample of Fermi unidentified objects
The optical spectroscopic followup of 27 sources belonging to a sample of 30
high-energy objects selected by positionally cross correlating the first
Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is
presented here. It has been found or confirmed that 25 of them are BL Lacertae
objects (BL Lacs), while the remaining two are Galactic cataclysmic variables
(CVs). This strongly suggests that the sources in the first group are
responsible for the GeV emission detected with Fermi, while the two CVs most
likely represent spurious associations. We thus find an 80% a posteriori
probability that the sources selected by matching GeV and X-ray catalogs belong
to the BL Lac class. We also show suggestions that the BL Lacs selected with
this approach are probably high-synchrotron-peaked sources and in turn good
candidates for the detection of ultra-high-energy (TeV) photons from them.Comment: 16 pages, 9 figures, 4 tables, one appendix, accepted for publication
on A&A, main journal. Tables 1-3 and Figures 2-6 will only be published in
the electronic edition of the journa
Mesoscopic Analysis of Structure and Strength of Dislocation Junctions in FCC Metals
We develop a finite element based dislocation dynamics model to simulate the
structure and strength of dislocation junctions in FCC crystals. The model is
based on anisotropic elasticity theory supplemented by the explicit inclusion
of the separation of perfect dislocations into partial dislocations bounding a
stacking fault. We demonstrate that the model reproduces in precise detail the
structure of the Lomer-Cottrell lock already obtained from atomistic
simulations. In light of this success, we also examine the strength of
junctions culminating in a stress-strength diagram which is the locus of points
in stress space corresponding to dissolution of the junction.Comment: 9 Pages + 4 Figure
- …