510 research outputs found
Neural Video Compression using GANs for Detail Synthesis and Propagation
We present the first neural video compression method based on generative
adversarial networks (GANs). Our approach significantly outperforms previous
neural and non-neural video compression methods in a user study, setting a new
state-of-the-art in visual quality for neural methods. We show that the GAN
loss is crucial to obtain this high visual quality. Two components make the GAN
loss effective: we i) synthesize detail by conditioning the generator on a
latent extracted from the warped previous reconstruction to then ii) propagate
this detail with high-quality flow. We find that user studies are required to
compare methods, i.e., none of our quantitative metrics were able to predict
all studies. We present the network design choices in detail, and ablate them
with user studies.Comment: First two authors contributed equally. ECCV Camera ready versio
Line emission from optically thick relativistic accretion tori
We calculate line emission from relativistic accretion tori around Kerr black
holes and investigate how the line profiles depend on the viewing inclination,
spin of the central black hole, parameters describing the shape of the tori,
and spatial distribution of line emissivity on the torus surface. We also
compare the lines with those from thin accretion disks. Our calculations show
that lines from tori and lines from thin disks share several common features.
In particular, at low and moderate viewing inclination angles they both have
asymmetric double-peaked profiles with a tall, sharp blue peak and a shorter
red peak which has an extensive red wing. At high viewing inclination angles
they both have very broad, asymmetric lines which can be roughly considered
single-peaked. Torus and disk lines may show very different red and blue line
wings, but the differences are due to the models for relativistic tori and
disks having differing inner boundary radii. Self-eclipse and lensing play some
role in shaping the torus lines, but they are effective only at high
inclination angles. If inner and outer radii of an accretion torus are the same
as those of an accretion disk, their line profiles show substantial differences
only when inclination angles are close to 90 degrees, and those differences
manifest mostly at the central regions of the lines instead of the wings.Comment: 14 pages, 17 figures. Accepted to A&
The ACS Virgo Cluster Survey III. Chandra and HST Observations of Low-Mass X-Ray Binaries and Globular Clusters in M87
The ACIS instrument on board the Chandra X-ray Observatory has been used to
carry out the first systematic study of low-mass X-ray binaries (LMXBs) in M87.
We identify 174 X-ray point-sources, of which ~150 are likely LMXBs. This LMXB
catalog is combined with deep F475W and F850LP images taken with ACS on HST to
examine the connection between LMXBs and globular clusters in M87. Of the 1688
globular clusters in our catalog, f_X = 3.6 +- 0.5% contain a LMXB and we find
that the metal-rich clusters are 3 +- 1 times more likely to harbor a LMXB than
their metal-poor counterparts. In agreement with previous findings for other
galaxies, we find that brighter, more metal-rich clusters are more likely to
contain a LMXB. For the first time, however, we are able to demonstrate that
the probability, p_X, that a given cluster will contain a LMXB depends
sensitively on the dynamical properties of the host cluster. Specifically, we
use the HST images to measure the half-light radius, concentration index and
central density, \rho_0, for each globular, and define a parameter, \Gamma,
which is related to the tidal capture and binary-neutron star exchange rate.
Our preferred form for p_X is then p_X \propto \Gamma \rho_0^{-0.42\pm0.11}
(Z/Z_{\odot})^{0.33\pm0.1}. We argue that if the form of p_X is determined by
dynamical processes, then the observed metallicity dependence is a consequence
of an increased number of neutron stars per unit mass in metal-rich globular
clusters. Finally, we find no compelling evidence for a break in the luminosity
distribution of resolved X-ray point sources. Instead, the LMXB luminosity
function is well described by a power law with an upper cutoff at L_X ~ 10^39
erg/s. (abridged)Comment: 23 pages, 21 figures. Accepted for publication in ApJ. Also available
at http://www.physics.rutgers.edu/~pcote/acs/publications.htm
General relativistic radiative transfer: formulation and emission from structured tori around black holes
We construct a general relativistic radiative transfer (RT) formulation,
applicable to particles with or without mass in astrophysical settings. Derived
from first principles, the formulation is manifestly covariant. Absorption and
emission, as well as relativistic, geometrical and optical depth effects are
treated self-consistently. The RT formulation can handle 3D geometrical
settings and structured objects with variations and gradients in the optical
depths across the objects and along the line-of-sight. The presence of mass
causes the intensity variation along the particle bundle ray to be reduced by
an aberration factor. We apply the formulation and demonstrate RT calculations
for emission from accretion tori around rotating black holes, considering two
cases: idealised optically thick tori that have a sharply defined emission
boundary surface, and structured tori that allow variations in the absorption
coefficient and emissivity within the tori. Intensity images and emission
spectra of these tori are calculated. Geometrical effects, such as
lensing-induced self-occulation and multiple-image contribution are far more
significant in accretion tori than geometrically thin accretion disks.
Optically thin accretion tori emission line profiles are distinguishable from
the profiles of lines from optically thick accretion tori and optically thick
geometrically thin accretion disks. Line profiles of optically thin accretion
tori have a weaker dependence on viewing inclination angle than those of the
optically thick accretion tori or accretion disks, especially at high viewing
inclination angles. Limb effects are present in accretion tori with finite
optical depths. Finally, in accretion flows onto relativistic compact objects,
gravitationally induced line resonance can occur. This resonance occurs easily
in 3D flows, but not in 2D flows, such as a thin accretion disk around a black
hole.Comment: 13 pages, 10 figures, Accepted for publication in Astronomy and
Astrophysic
Evolution of the Most Massive Galaxies to z ~ 0.6: II. The link between radio AGN activity and star formation
We analyze the optical spectra of massive (log M*/Msun > 11.4) radio-loud
galaxies at z~0.2 and z~0.6. By comparing stellar population parameters of
these radio-loud samples with radio-quiet control samples, we investigate how
the presence of a radio-emitting jet relates to the recent star formation
history of the host galaxy. We also investigate how the emission-line
properties of the radio galaxies evolve with redshift by stacking their
spectra. Our main results are the following. (1) Both at low and at high
redshift, half as many radio-loud as radio-quiet galaxies have experienced
significant star formation in the past Gyr. (2) The Balmer absorption line
properties of massive galaxies that have experienced recent star formation show
that star formation occurred as a burst in many of these systems. (3) Both the
radio and the emission-line luminosity of radio AGN evolve significantly with
redshift. However, radio galaxies with similar stellar population parameters,
have similar emission-line properties both at high- and at low-redshift. These
results suggest that massive galaxies experience cyclical episodes of gas
accretion, star formation and black hole growth, followed by the production of
a radio jet that shuts down further activity. The behaviour of galaxies with
log M*/Msun > 11.4 is the same at z = 0.6 as it is at z = 0.2, except that
higher redshift galaxies experience more star formation and black hole growth
and produce more luminous radio jets during each accretion cycle.Comment: 14 pages, 12 figures, submitted to MNRA
Changes in the long term intensity variations in Cyg X-2 and LMC X-3
We report the detection of changes in the long-term intensity variations in
two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the
long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi
X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning
modulation collimator of HEAO 1. It is found that in the light curves of both
the sources, obtained with these instruments at various times over the last 30
years, more than one periodic or quasi-periodic component is always present.
The multiple prominent peaks in the periodograms have frequencies unrelated to
each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days,
and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are
also observed in LMC X-3. The various strong peaks in the periodograms of LMC
X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and
328 days (observed with Ginga-ASM). The present results, when compared with the
earlier observations of periodicities in these two systems, demonstrate the
absence of any stable long period. The 78 day periodicity detected earlier in
Cyg X-2 was probably due to the short time base in the RXTE data that were
used, and the periodicity of 198 days in LMC X-3 was due to a relatively short
duration of observation with HEAO 1.Comment: 11 pages, 7 postscript figures include
Unpaid labour and territorial extraction in digital value networks
Production in knowledge and data-intensive industries is powered by work that can, in theory, be done from anywhere, via cloudwork platforms. Cloudwork platforms govern data value chains in distinct ways to concentrate power and extract value at the global scale. We argue that unpaid labour is a systemic mechanism of accumulation in these digital value networks. In this paper we demonstrate how it is tied to platform business models and facilitated by elements of platform governance including monopsony power, a high degree of spatial flexibility in sourcing labour, regulatory unaccountability and digital enclosure. We draw on a survey of 699 workers on 14 platforms in 74 countries to show that unpaid labour is an engine of South–North value extraction, and workers in the global South perform more unpaid labour than counterparts in the global North. Our findings have important ramifications our understanding of the changing international division of labour and platform capitalism
Spin-polarized current amplification and spin injection in magnetic bipolar transistors
The magnetic bipolar transistor (MBT) is a bipolar junction transistor with
an equilibrium and nonequilibrium spin (magnetization) in the emitter, base, or
collector. The low-injection theory of spin-polarized transport through MBTs
and of a more general case of an array of magnetic {\it p-n} junctions is
developed and illustrated on several important cases. Two main physical
phenomena are discussed: electrical spin injection and spin control of current
amplification (magnetoamplification). It is shown that a source spin can be
injected from the emitter to the collector. If the base of an MBT has an
equilibrium magnetization, the spin can be injected from the base to the
collector by intrinsic spin injection. The resulting spin accumulation in the
collector is proportional to , where is the proton
charge, is the bias in the emitter-base junction, and is the
thermal energy. To control the electrical current through MBTs both the
equilibrium and the nonequilibrium spin can be employed. The equilibrium spin
controls the magnitude of the equilibrium electron and hole densities, thereby
controlling the currents. Increasing the equilibrium spin polarization of the
base (emitter) increases (decreases) the current amplification. If there is a
nonequilibrium spin in the emitter, and the base or the emitter has an
equilibrium spin, a spin-valve effect can lead to a giant magnetoamplification
effect, where the current amplifications for the parallel and antiparallel
orientations of the the equilibrium and nonequilibrium spins differ
significantly. The theory is elucidated using qualitative analyses and is
illustrated on an MBT example with generic materials parameters.Comment: 14 PRB-style pages, 10 figure
Ram pressure stripping of the cool core of the Ophiuchus Cluster
(abridged) We report results from a Chandra study of the central regions of
the nearby, X-ray bright, Ophiuchus Cluster (z = 0.03), the second-brightest
cluster in the sky. Our study reveals a dramatic, close-up view of the
stripping and potential destruction of a cool core within a rich cluster. The
X-ray emission from the Ophiuchus Cluster core exhibits a comet-like morphology
extending to the north, driven by merging activity, indicative of ram-pressure
stripping caused by rapid motion through the ambient cluster gas. A cold front
at the southern edge implies a velocity of 1000200 km/s (M~0.6). The X-ray
emission from the cluster core is sharply peaked. As previously noted, the peak
is offset by 4 arcsec (~2 kpc) from the optical center of the associated cD
galaxy, indicating that ram pressure has slowed the core, allowing the
relatively collisionless stars and dark matter to carry on ahead. The cluster
exhibits the strongest central temperature gradient of any massive cluster
observed to date: the temperature rises from 0.7 keV within 1 kpc of the
brightness peak, to 10 keV by 30 kpc. A strong metallicity gradient is also
observed within the same region. This supports a picture in which the outer
parts of the cool core have been stripped by ram-pressure due to its rapid
motion. The cooling time of the innermost gas is very short, ~5
yrs. Within the central 10 kpc radius, multiple small-scale fronts and a
complex thermodynamic structure are observed, indicating significant motions.
Beyond the central 50 kpc, and out to a radius ~150 kpc, the cluster appears
relatively isothermal and has near constant metallicity. The exception is a
large, coherent ridge of enhanced metallicity observed to trail the cool core,
and which is likely to have been stripped from it.Comment: Accepted to MNRAS. 11 pages, 9 figure
On the ability of spectroscopic SZ effect measurements to determine the temperature structure of galaxy clusters
(abridged) We explore in this paper the ability of spatially resolved
spectroscopic measurements of the SZ effect (SZE) to determine the temperature
profile of galaxy clusters. We derive a general formalism for the thermal SZE
in galaxy clusters with a non-uniform temperature profile that can be applied
to both cool-core clusters and non-cool core cluster with an isothermal or
non-isothermal temperature structure. We derive an inversion technique through
which the electron distribution function can be extracted from spectroscopic
SZE observations over a wide frequency range. We study the fitting procedure to
extract the cluster temperature from a set of simulated spatially resolved
spectroscopic SZE observations in different bands of the spectrum, from 100 to
450 GHz. The results of our analysis for three different cluster prototypes
(A2199 with a low-temperature cool core, Perseus with a relatively
high-temperature cool core, Ophiuchus with an isothermal temperature
distribution) provide both the required precision of the SZE observations and
the optimal frequency bands for a determination of the cluster temperature
similar or better than that obtainable from X-ray observations. The precision
of SZE-derived temperature is also discussed for the outer regions of clusters.
We also study the possibility to extract, from our method, the parameters
characterizing the non-thermal SZE spectrum of the relativistic plasma
contained in the lobes of radio galaxies as well as the spectrum of
relativistic electrons co-spatially distributed with the thermal plasma in
clusters with non-thermal phenomena. We find that the next generation SZE
experiments with spectroscopic capabilities can provide precise temperature
distribution measurements (...)Comment: Submitted to Astronomy & Astrophysic
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