1,948,925 research outputs found
New aperture photometry of QSO 0957+561; application to time delay and microlensing
We present a re-reduction of archival CCD frames of the doubly imaged quasar
0957+561 using a new photometry code. Aperture photometry with corrections for
both cross contamination between the quasar images and galaxy contamination is
performed on about 2650 R-band images from a five year period (1992-1997). From
the brightness data a time delay of 424.9 +/- 1.2 days is derived using two
different statistical techniques. The amount of gravitational microlensing in
the quasar light curves is briefly investigated, and we find unambiguous
evidence of both long term and short term microlensing. We also note the
unusual circumstance regarding time delay estimates for this gravitational
lens. Estimates by different observers from different data sets or even with
the same data sets give lag estimates differing by typically 8 days, and error
bars of only a day or two. This probably indicates several complexities where
the result of each estimate depends upon the details of the calculation.Comment: 14 pages, 16 figures (several in color
Multi-Temperature Blackbody Spectra of Thin Accretion Disks With and Without a Zero-Torque Inner Boundary Condition
The standard spectral model for analyzing the soft component of thermal
emission from a thin accretion disk around a black hole is the
multi-temperature blackbody (MTB) model. The widely used implementation of this
model, which is known as ``diskbb,'' assumes nonzero torque at the inner edge
of the accretion disk. This assumption is contrary to the classic and current
literature on thin-disk accretion, which advocates the use of a zero-torque
boundary condition. Consequently, we have written code for a zero-torque model,
``ezdiskbb,'' which we compare to the nonzero-torque model diskbb by fitting
RXTE spectra of three well-known black hole binaries: 4U 1543-47, XTE
J1550-564, and GRO J1655-40. The chief difference we find is that the
zero-torque model gives a value for the inner disk radius that is about 2.2
times smaller than the value given by diskbb. This result has important
implications, especially for the determination of black-hole angular momentum
and mass accretion rate.Comment: 27 pages, 9 figures, submitted to Ap
Pressure, compressibility, and contact of the two-dimensional attractive Fermi gas
Using ab initio lattice methods, we calculate the finite temperature
thermodynamics of homogeneous two-dimensional spin-1/2 fermions with attractive
short-range interactions. We present results for the density, pressure,
compressibility, and quantum anomaly (i.e. Tan's contact) for a wide range of
temperatures and coupling strengths, focusing on the unpolarized case. Within
our statistical and systematic uncertainties, our prediction for the density
equation of state differs quantitatively from the prediction by Luttinger-Ward
theory in the strongly coupled region of parameter space, but otherwise agrees
well with it. We also compare our calculations with the second- and third-order
virial expansion, with which they are in excellent agreement in the
low-fugacity regime.Comment: 7 pages, 8 figures, including supplemental material
An Architectural Approach to Ensuring Consistency in Hierarchical Execution
Hierarchical task decomposition is a method used in many agent systems to
organize agent knowledge. This work shows how the combination of a hierarchy
and persistent assertions of knowledge can lead to difficulty in maintaining
logical consistency in asserted knowledge. We explore the problematic
consequences of persistent assumptions in the reasoning process and introduce
novel potential solutions. Having implemented one of the possible solutions,
Dynamic Hierarchical Justification, its effectiveness is demonstrated with an
empirical analysis
Structure and mixing of a transverse jet in incompressible flow
The flow field induced by a jet in incompressible cross-flow is analysed and the results compared with those obtained in a reacting water-jet experiment. It is argued that the axial vortex pair in the flow arises from the jet momentum normal to the free stream, the momentum flux being equivalent to a normal force, i.e. to a lift
Thermal and bias cycling stabilizes planar silicon devices
Terminal burn-in or baking step time in the processing of planar silicon devices is extended to reduce their inversion tendencies. The collector-base junction of the device is also cyclically biased during the burn-in
A Model for the Effectiveness of Aircraft Alerting and Warning Systems
The effectiveness of an alerting system with a single alert was analyzed. The pilot's decision behavior is modeled by the theory of signal detection and therefore accounts for different strengths of cross check information and different pilot criteria. The model includes the effects of the alerting and warning system (CAWS) error rate; the pilot's past experience with the CAWS accuracy; his reliance on the CAWS rather than independent monitoring; missed alerts; and adoption of a minimum error or Neyman-Pearson objective rather than minimum cost objective. It is showwn that for rare events: (1) the expected cost is greatly increased if the pilot ignores the a posteriori information in the existence of an alert; (2) the expected cost is insensitive to CAWS Type 1 errors; and (3) the expected cost is sensitive to CAWS type 2 errors only when the cross check information is ambiguous
Vertical copy camera system provides photographs from ERTS-1 imagery
Versatility of commercially-available camera system permits wide range of enlargement (up to 10X) and reduction (down to 1/8) to be achieved with standard lenses. Use of easily interchangeable camera backs permits photographic formats from 35 mm to 10.2 X 12.7 cm (4 x 5 in) and permits easy use of black and white and color films and Polaroid materials
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