848 research outputs found
VLBI for Gravity Probe B. II. Monitoring of the Structure of the Reference Sources 3C 454.3, B2250+194, and B2252+172
We used 8.4 GHz VLBI images obtained at up to 35 epochs between 1997 and 2005
to examine the radio structures of the main reference source, 3C 454.3, and two
secondary reference sources, B2250+194 and B2252+172, for the guide star for
the NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in
2004 May, we also obtained images at 5.0 and 15.4 GHz. The 35 8.4 GHz images
for quasar 3C 454.3 confirm a complex, evolving, core-jet structure. We
identified at each epoch a component, C1, near the easternmost edge of the core
region. Simulations of the core region showed that C1 is located, on average,
0.18 +- 0.06 mas west of the unresolved "core" identified in 43 GHz images. We
also identified in 3C 454.3 at 8.4 GHz several additional components which
moved away from C1 with proper motions ranging in magnitude between 0.9c and
5c. The detailed motions of the components exhibit two distinct bends in the
jet axis located ~3 and ~5.5 mas west of C1. The spectra between 5.0 and 15.4
GHz for the "moving" components are steeper than that for C1. The 8.4 GHz
images of B2250+194 and B2252+172, in contrast to those of 3C 454.3, reveal
compact structures. The spectrum between 5.0 and 15.4 GHz for B2250+194 is
inverted while that for B2252+172 is flat.
Based on its position near the easternmost edge of the 8.4 GHz radio
structure, close spatial association with the 43 GHz core, and relatively flat
spectrum, we believe 3C 454.3 component C1 to be the best choice for the
ultimate reference point for the GP-B guide star. The compact structures and
inverted to flat spectra of B2250+194 and B2252+172 make these objects valuable
secondary reference sourcesComment: Accepted for publication in the Astrophysical Journal Supplement
Seri
VLBI for Gravity Probe B. IV. A New Astrometric Analysis Technique and a Comparison with Results from Other Techniques
When VLBI observations are used to determine the position or motion of a
radio source relative to reference sources nearby on the sky, the astrometric
information is usually obtained via: (i) phase-referenced maps; or (ii)
parametric model fits to measured fringe phases or multiband delays. In this
paper we describe a "merged" analysis technique which combines some of the most
important advantages of these other two approaches. In particular, our merged
technique combines the superior model-correction capabilities of parametric
model fits with the ability of phase-referenced maps to yield astrometric
measurements of sources that are too weak to be used in parametric model fits.
We compare the results from this merged technique with the results from
phase-referenced maps and from parametric model fits in the analysis of
astrometric VLBI observations of the radio-bright star IM Pegasi (HR 8703) and
the radio source B2252+172 nearby on the sky. In these studies we use
central-core components of radio sources 3C 454.3 and B2250+194 as our
positional references. We obtain astrometric results for IM Peg with our merged
technique even when the source is too weak to be used in parametric model fits,
and we find that our merged technique yields superior astrometric results to
the phase-referenced mapping technique. We used our merged technique to
estimate the proper motion and other astrometric parameters of IM Peg in
support of the NASA/Stanford Gravity Probe B mission.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
Introducao, avaliacao e caracterizacao de selecoes de goiabeiras (Psidium guajava L.) na chapada do Araripe-Pernambuco: II - caracteristicas produtivas.
Uma avaliacao do comportamento de 21 selecoes de goiabeiras, cultivadas sob regime de sequeiro na Chapada do Araripe- Pernambuco, e apresentada ressaltando-se as suas caracteristicas produtivas. A producao das goiabeiras com 3 1/2 anos de idade, relativa a safra de setembro/90 a setembro/91 variou de 10,1 a 52,9 kg/planta, para selecoes White Selection of Florida 2 e Patillo 2.3, respectivamente. Ja com relacao ao numero de frutos colhidos, verifica-se uma variacao de 77 na selecao White Selection of Florida-2 ate 743 na selecao IPA B-22.1. Observou-se uma tendencia entre as selecoes estudadas, de maior peso medio dos frutos para as selecoes com o menor numero de frutos por planta
Ordering in a spin glass under applied magnetic field
Torque, torque relaxation, and magnetization measurements on a AuFe spin
glass sample are reported. The experiments carried out up to 7 T show a
transverse irreversibility line in the (H,T) plane up to high applied fields,
and a distinct strong longitudinal irreversibility line at lower fields. The
data demonstrate for that this type of sample, a Heisenberg spin glass with
moderately strong anisotropy, the spin glass ordered state survives under high
applied fields in contrast to predictions of certain "droplet" type scaling
models. The overall phase diagram closely ressembles those of mean field or
chiral models, which both have replica symmetry breaking transitions.Comment: 4 pages, 3 figures, accepted for PR
Recommended from our members
VLBI for Gravity Probe B. IV. A New Astrometric Analysis Technique and a Comparison with Results from Other Techniques
When very long baseline interferometry (VLBI) observations are used to determine the position or motion of a radio source relative to reference sources nearby on the sky, the astrometric information is usually obtained via1) phase-referenced maps or (2) parametric model fits to measured fringe phases or multiband delays. In this paper, we describe a “merged” analysis technique which combines some of the most important advantages of these other two approaches. In particular, our merged technique combines the superior model-correction capabilities of parametric model fits with the ability of phase-referenced maps to yield astrometric measurements of sources that are too weak to be used in parametric model fits. We compare the results from this merged technique with the results from phase-referenced maps and from parametric model fits in the analysis of astrometric VLBI observations of the radio-bright star IM Pegasi (HR 8703) and the radio source B2252+172 nearby on the sky. In these studies we use central-core components of radio sources 3C 454.3 and B2250+194 as our positional references. We obtain astrometric results for IM Peg with our merged technique even when the source is too weak to be used in parametric model fits, and we find that our merged technique yields astrometric results superior to the phase-referenced mapping technique. We used our merged technique to estimate the proper motion and other astrometric parameters of IM Peg in support of the NASA/Stanford Gravity Probe B mission.AstronomyPhysic
Existence of radial stationary solutions for a system in combustion theory
In this paper, we construct radially symmetric solutions of a nonlinear
noncooperative elliptic system derived from a model for flame balls with
radiation losses. This model is based on a one step kinetic reaction and our
system is obtained by approximating the standard Arrehnius law by an ignition
nonlinearity, and by simplifying the term that models radiation. We prove the
existence of 2 solutions using degree theory
Efeito da cianamida hidrogenada associada à desfolha química e manual na indução floral de pinheiras (Annona squamosa L.).
As técnicas de poda, desfolha e indução floral na pinheira (Annona squamosa L.) merecem uma especial atenção na região do Nordeste do Brasil, vista a possibilidade de se obter duas colheitas anuais. Visando avaliar a influência da cianamida hidrogenada (CH), associada à desfolha química e manual, na indução floral de pinheiras irrigadas por gotejo, foi realizado experimento com os seguintes tratamentos: testemunha absoluta; duas dosagens para desfolha química com ethephon a 1.000 ppm e 2.000 ppm e poda os ponteiros; dois tipos de desfolha (toda parte aérea da planta e 50% da parte aérea) e poda dos ponteiros; duas dosagens de CH a 1,5% e 2,0% e poda dos ponteiros; desfolha da planta e poda dos ponteiros. O delineamento experimental foi o de blocos ao acaso, com dez tratamentos, três repetições e três plantas por parcela. As desfolhas manual e química, associadas a indutores de brotação constituem uma boa alternativa para regular e aumentar a brotação de gemas e promover a frutificação. A desfolha manual em 50% da parte aérea da planta (DM2CH) e a desfolha química com ethephon a 1.000 ppm (DQICH), poda e pulverização com CH a 1,5%, apresentaram os melhores resultados. A colheita não foi antecipada nem retardada pela ação dos tratamentos utilizados.Número especial
Stochastic Hysteresis and Resonance in a Kinetic Ising System
We study hysteresis for a two-dimensional, spin-1/2, nearest-neighbor,
kinetic Ising ferromagnet in an oscillating field, using Monte Carlo
simulations and analytical theory. Attention is focused on small systems and
weak field amplitudes at a temperature below . For these restricted
parameters, the magnetization switches through random nucleation of a single
droplet of spins aligned with the applied field. We analyze the stochastic
hysteresis observed in this parameter regime, using time-dependent nucleation
theory and the theory of variable-rate Markov processes. The theory enables us
to accurately predict the results of extensive Monte Carlo simulations, without
the use of any adjustable parameters. The stochastic response is qualitatively
different from what is observed, either in mean-field models or in simulations
of larger spatially extended systems. We consider the frequency dependence of
the probability density for the hysteresis-loop area and show that its average
slowly crosses over to a logarithmic decay with frequency and amplitude for
asymptotically low frequencies. Both the average loop area and the
residence-time distributions for the magnetization show evidence of stochastic
resonance. We also demonstrate a connection between the residence-time
distributions and the power spectral densities of the magnetization time
series. In addition to their significance for the interpretation of recent
experiments in condensed-matter physics, including studies of switching in
ferromagnetic and ferroelectric nanoparticles and ultrathin films, our results
are relevant to the general theory of periodically driven arrays of coupled,
bistable systems with stochastic noise.Comment: 35 pages. Submitted to Phys. Rev. E Minor revisions to the text and
updated reference
VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3
We made VLBI observations at 8.4 GHz between 1997 and 2005 to estimate the
coordinates of the "core" component of the superluminal quasar, 3C 454.3, the
ultimate reference point in the distant universe for the NASA/Stanford
Gyroscope Relativity Mission, Gravity Probe B. These coordinates are determined
relative to those of the brightness peaks of two other compact extragalactic
sources, B2250+194 and B2252+172, nearby on the sky, and within a celestial
reference frame (CRF), defined by a large suite of compact extragalactic radio
sources, and nearly identical to the International Celestial Reference Frame 2
(ICRF2). We find that B2250+194 and B2252+172 are stationary relative to each
other, and also in the CRF, to within 1-sigma upper limits of 15 and 30
micro-arcsec/yr in RA and decl., respectively. The core of 3C 454.3 appears to
jitter in its position along the jet direction over ~0.2 mas, likely due to
activity close to the putative supermassive black hole nearby, but on average
is stationary in the CRF within 1-sigma upper limits on its proper motion of 39
micro-arcsec/yr (1.0c) and 30 micro-arcsec/yr (0.8c) in RA and decl.,
respectively, for the period 2002 - 2005. Our corresponding limit over the
longer interval, 1998 - 2005, of more importance to GP-B, is 46 and 56
micro-arcsec/yr in RA and decl., respectively. Some of 3C 454.3's jet
components show significantly superluminal motion with speeds of up to ~200
micro-arcsec/yr or 5c in the CRF. The core of 3C 454.3 thus provides for
Gravity Probe B a sufficiently stable reference in the distant universe.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
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