1,360 research outputs found
Polarized Scattering in the Vicinty of Galaxies
Some bright cD galaxies in cluster cooling flows have Thomson optical depths
exceeding 0.01. A few percent of their luminosity is scattered and appears as
diffuse polarized emission. We calculate the scattering process for different
geometric combinations of luminosity sources and scattering media. We apply our
results to galaxies, with and without active nuclei, immersed in cooling flows.
We model observations of NGC 1275 and M87 (without active nuclei) in the
presence of sky and galactic background fluxes which hinder the measurement of
the scattered light at optical wavelengths. Current instruments are unable to
detect the scattered light from such objects. However, when a galaxy has an
active nucleus of roughly the same luminosity as the remainder of the galaxy in
V, both the total and polarized scattered intensity should observable on large
scales (5--30kpc), meaning intensity levels greater than 1% of the background
level. For typical AGN and galaxy spectral distributions, the scattering is
most easily detected at short (U) wavelengths. We point out that a number of
such cases will occur. We show that the radiation pattern from the central
nuclear region can be mapped using the scattering. We also show that the
scattered light can be used to measure inhomogeneities in the cooling flow.Comment: 29 pages of TEX, 14 figs, CRSR-1046, in ApJ Nov 20, 199
Radiative instabilities in simulations of spherically symmetric supernova blast waves
High-resolution simulations of the cooling regions of spherically symmetric
supernova remnants demonstrate a strong radiative instability. This
instability, whose presence is dependent on the shock velocity, causes
large-amplitude fluctuations in the shock velocity. The fluctuations begin
almost immediately after the radiative phase begins (upon shell formation) if
the shock velocity lies in the unstable range; they last until the shock slows
to speeds less than approximately 130 km/s. We find that shock-velocity
fluctuations from the reverberations of waves within the remnant are small
compared to those due to the instability. Further, we find (in plane-parallel
simulations) that advected inhomogeneities from the external medium do not
interfere with the qualitative nature of the instability-driven fluctuations.
Large-amplitude inhomogeneities may alter the phases of shock-velocity
fluctuations, but do not substantially reduce their amplitudes.Comment: 18 pages text, LaTeX/AASTeX (aaspp4); 10 figures; accepted by Ap
Effects of magnetic fields on radiatively overstable shock waves
We discuss high-resolution simulations of one-dimensional, plane-parallel
shock waves with mean speeds between 150 and 240 km/s propagating into gas with
Alfven velocities up to 40 km/s and outline the conditions under which these
radiative shocks experience an oscillatory instability in the cooling length,
shock velocity, and position of the shock front. We investigate two forms of
postshock cooling: a truncated single power law and a more realistic piecewise
power law. The degree of nonlinearity of the instability depends strongly on
the cooling power law and the Alfven Mach number: for power-law indices \alpha
< 0 typical magnetic field strengths may be insufficient either to stabilize
the fundamental oscillatory mode or to prevent the oscillations from reaching
nonlinear amplitudes.Comment: 11 text pages, LaTeX/AASTeX (aaspp4); 5 figures; accepted by Ap
Class Mobility And Reproduction For Black And White Adults In The United States: A Visualization
The relationship between where people start out in life (class origin) and where they are likely to end up (class destination) is central to any question about the fairness of contemporary society. Yet we often don’t have a good picture—literally or metaphorically—of the contours of that relationship. Further, work on class mobility in the United States often glosses over the large differences between white and Black Americans’ class positions and mobility trajectories. This visualization, based on data from the Panel Study of Income Dynamics, shows the association between occupational class origin and destination for Black and white employed Americans ages 25 to 69. Stark racial inequality, produced by the legacy and ongoing operation of white supremacy, is evident in each aspect of these figures
Optimal sequential fingerprinting: Wald vs. Tardos
We study sequential collusion-resistant fingerprinting, where the
fingerprinting code is generated in advance but accusations may be made between
rounds, and show that in this setting both the dynamic Tardos scheme and
schemes building upon Wald's sequential probability ratio test (SPRT) are
asymptotically optimal. We further compare these two approaches to sequential
fingerprinting, highlighting differences between the two schemes. Based on
these differences, we argue that Wald's scheme should in general be preferred
over the dynamic Tardos scheme, even though both schemes have their merits. As
a side result, we derive an optimal sequential group testing method for the
classical model, which can easily be generalized to different group testing
models.Comment: 12 pages, 10 figure
Ariel - Volume 12(13) Number 2
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