3,097 research outputs found
Quantitative in-situ analysis of collector adsorption reactions by FTIR internal reflection spectroscopy
Journal ArticleResearchers in flotation chemistry have long sought to describe collector adsorption phenomena in greater detail including kinetics, orientation and interactions between adsorbed species. Now a quantitative description can be made in-situ using FTIR internal reflection spectroscopy. Details of the adsorption process at the surface of mineral crystals, machined into internal reflection elements, can be described from first principles, provided the refractive indices, incident angle, and wavelength of light are known
Approximate Inference for Constructing Astronomical Catalogs from Images
We present a new, fully generative model for constructing astronomical
catalogs from optical telescope image sets. Each pixel intensity is treated as
a random variable with parameters that depend on the latent properties of stars
and galaxies. These latent properties are themselves modeled as random. We
compare two procedures for posterior inference. One procedure is based on
Markov chain Monte Carlo (MCMC) while the other is based on variational
inference (VI). The MCMC procedure excels at quantifying uncertainty, while the
VI procedure is 1000 times faster. On a supercomputer, the VI procedure
efficiently uses 665,000 CPU cores to construct an astronomical catalog from 50
terabytes of images in 14.6 minutes, demonstrating the scaling characteristics
necessary to construct catalogs for upcoming astronomical surveys.Comment: accepted to the Annals of Applied Statistic
Actitudes y conductas públicas ante la COVID-19 en Estados Unidos: estudio de un caso en orden a la comprensión de un sistema político polarizado
How do citizens in a polarized political system react to an unexpected emergency like the COVID-19 pandemic and how do citizens process conflicting polarized narratives to formulate a public policy view of the threat of the pandemic? The emergence of the COVID-19 pandemic is a health emergency unlike anything in the United States since the polio epidemic 70 years ago, but the political climate of the U.S. in the 1950’s was far more centrist and consensual than the deep divisions observed today. This paper will utilize data from a 35-year longitudinal study of Generation X young adults (now in their mid-40’s) and a three-decade time series of national U.S. surveys to examine information acquisition behaviors to understand the new threat. Our analysis of the last 35 years of Generation X finds that polarized ideological partisanship was the strongest single predictor of individual votes in the 2020 election, but that individuals with a higher level of understanding of the coronavirus were more critical of the Trump Administration’s handling of the COVID-19 pandemic and were more likely to vote for Biden than Trump. A parallel analysis of a national probability sample of U.S. adults in 2020 found the same pattern of influence from ideological partisanship, coronavirus understanding, and assessment of the Trump Administration’s handling of the pandemic. The results indicate that knowledge and understanding can provide a critical balancing effect in an evenly divided polarized political system.¿Cómo reacciona la ciudadanía en un sistema político polarizado ante una emergencia como la pandemia de la COVID-19?, ¿cómo procesa la ciudadanía las narrativas polarizadas que están en conflicto?, y ¿qué imagen se forman de la gestión política de la amenaza de la pandemia? En EE. UU, hay que retrotraerse a la epidemia de la polio de hace 70 años para encontrar una emergencia sanitaria como la pandemia de la COVID-19. No obstante, hay importantes diferencias; en la década de 1950, el clima político de los EE.UU. era mucho más uniforme y consensuado que el actual, profundamente dividido y polarizado. Este trabajo utiliza datos de un estudio longitudinal realizado durante 35 años en personas jóvenes de la Generación X (ahora ya en la cuarentena) y datos provenientes de encuestas realizadas en Estados Unidos durante tres décadas, con el propósito de examinar los patrones de adquisición de la información en la comprensión de una nueva amenaza. Nuestro análisis de los últimos 35 años de la Generación X muestra que, en las elecciones de 2020, el factor predictivo del voto individual que tuvo más fuerza fue una ideología política polarizada, pero, aquellas personas que contaban con una mejor comprensión del coronavirus fueron más críticas con la gestión de la pandemia de la COVID-19 que realizó la administración de Donald Trump y estuvieron más predispuestas a votar por Joe Biden que a votar por Trump. Un análisis paralelo de una muestra probabilística representativa de personas adultas estadounidenses en 2020 reveló el mismo patrón de influencia del partidismo ideológico, la comprensión del coronavirus y la evaluación de la administración de Trump. Los resultados muestran que el conocimiento y la comprensión pueden proporcionar un efecto moderador crítico en un sistema político polarizado y dividido
The new X-ray transient SAX J1711.6-3808: decoupling between its 3-20 keV luminosity and its state transitions
We present a study of the correlated spectral and timing behavior of the new
X-ray transient SAX J1711.6-3808 during its 2001 outburst using data obtained
with the RXTE. We also investigate the correlations between those source
properties and the 3-20 keV X-ray luminosity. The behavior of the source during
the observations can be divided into two distinct state types. During the hard
state, the energy spectra are relatively hard and can be described by only a
power-law component, and the characteristic frequencies (i.e., the frequency of
the 1-7 Hz QPOs observed for the first time in this source) in the power
spectra are low. However, during the ``soft'' state, the spectra are
considerably softer (in addition to the power-law component, a soft component
is necessary to fit the spectra) and the frequencies are the highest observed.
Remarkably, this distinction into two separate states cannot be extrapolated to
also include the 3-20 keV X-ray luminosity. Except for one observation, this
luminosity steadily decreased but the hard state was observed both at the
highest and lowest observed luminosities. In contrast, the soft state occurred
only at intermediate luminosities. This clearly demonstrates that the state
behavior of SAX J1711.6-3808 is decoupled from its X-ray luminosity and that if
the X-ray luminosity traces the accretion rate in SAX J1711.6-3808, then the
state transitions are not good accretion rate indicators, or vice versa. The
data of SAX J1711.6-3808 does not allow us to conclusively determine its exact
nature. The source resembles both neutron star and black hole systems when they
have low luminosities. We discuss our results with respect to the correlated
timing and spectral behavior observed in other LMXBs and the implications of
our results on the modeling of the outburst light curves of X-ray transients.Comment: Accepted for publication in ApJ Main Journal, 13 September 200
The hard quiescent spectrum of the neutron-star X-ray transient EXO 1745-248 in the globular cluster Terzan 5
We present a Chandra observation of the globular cluster Terzan 5 during
times when the neutron-star X-ray transient EXO 1745-248 located in this
cluster was in its quiescent state. We detected the quiescent system with a
(0.5-10 keV) luminosity of ~2 x 10^{33} ergs/s. This is similar to several
other neutron-star transients observed in their quiescent states. However, the
quiescent X-ray spectrum of EXO 1745--48 was dominated by a hard power-law
component instead of the soft component that usually dominates the quiescent
emission of other neutron-star X-ray transients. This soft component could not
conclusively be detected in EXO 1745-248 and we conclude that it contributed at
most 10% of the quiescent flux in the energy range 0.5-10 keV. EXO 1745-248 is
only the second neutron-star transient whose quiescent spectrum is dominated by
the hard component (SAX J1808.4-3658 is the other one). We discuss possible
explanations for this unusual behavior of EXO 1745-248, its relationship to
other quiescent neutron-star systems, and the impact of our results on
understanding quiescent X-ray binaries. We also discuss the implications of our
results on the way the low-luminosity X-ray sources in globular clusters are
classified.Comment: Accepted by ApJ Main Journal, September 22, 2004. Figure 2 is a color
figur
On the Progenitor System of the Type Iax Supernova 2014dt in M61
We present pre-explosion and post-explosion Hubble Space Telescope images of
the Type Iax supernova (SN Iax) 2014dt in M61. After astrometrically aligning
these images, we do not detect any stellar sources at the position of the SN in
the pre-explosion images to relatively deep limits (3 sigma limits of M_F438W >
-5.0 mag and M_F814W > -5.9 mag). These limits are similar to the luminosity of
SN 2012Z's progenitor system (M_F435W = -5.43 +/- 0.15 and M_F814W = -5.24 +/-
0.16 mag), the only probable detected progenitor system in pre-explosion images
of a SN Iax, and indeed, of any white dwarf supernova. SN 2014dt is consistent
with having a C/O white-dwarf primary/helium-star companion progenitor system,
as was suggested for SN 2012Z, although perhaps with a slightly smaller or
hotter donor. The data are also consistent with SN 2014dt having a low-mass red
giant or main-sequence star companion. The data rule out main-sequence stars
with M_init > 16 M_sun and most evolved stars with M_init > 8 M_sun as being
the progenitor of SN 2014dt. Hot Wolf-Rayet stars are also allowed, but the
lack of nearby bright sources makes this scenario unlikely. Because of its
proximity (D = 12 Mpc), SN 2014dt is ideal for long-term monitoring, where
images in ~2 years may detect the companion star or the luminous bound remnant
of the progenitor white dwarf.Comment: 5 pages, 3 figures, submitted to ApJ
An X-Ray View of the Jet-Cycle in the Radio Loud AGN 3C120
We present a study of the central engine in the broad-line radio galaxy 3C120
using a multi-epoch analysis of a deep XMM-Newton observation and two deep
Suzaku pointings (in 2012). In order to place our spectral data into the
context of the disk-disruption/jet-ejection cycles displayed by this object, we
monitor the source in the UV/X-ray bands, and in the radio band. We find three
statistically acceptable spectral models, a disk-reflection model, a jet-model
and a jet+disk model. Despite being good descriptions of the data, the
disk-reflection model violates the radio constraints on the inclination, and
the jet-model has a fine-tuning problem, requiring a jet contribution exceeding
that expected. Thus, we argue for a composite jet+disk model. Within the
context of this model, we verify the basic predictions of the jet-cycle
paradigm, finding a truncated/refilling disk during the Suzaku observations and
a complete disk extending down to the innermost stable circular orbit (ISCO)
during the XMM-Newton observation. The idea of a refilling disk is further
supported by the detection of the ejection of a new jet knot approximately one
month after the Suzaku pointings. We also discover a step-like event in one of
the Suzaku pointings in which the soft band lags the hard band. We suggest that
we are witnessing the propagation of a disturbance from the disk into the jet
on a timescale set by the magnetic field.Comment: 14 pages, 10 figures, accepted for publication in Ap
A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: too cold a neutron star?
After more than a decade of actively accreting at about a tenth of the
Eddington critical mass accretion rate, the neutron-star X-ray transient KS
1731-260 returned to quiescence in early 2001. We present a Chandra/ACIS-S
observation taken several months after this transition. We detected the source
at an unabsorbed flux of ~2 x 10^{-13} erg/cm^2/s (0.5-10 keV). For a distance
of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 x 10^{33} erg/s and a
bolometric luminosity approximately twice that. This quiescent luminosity is
very similar to that of the other quiescent neutron star systems. However, if
this luminosity is due to the cooling of the neutron star, this low luminosity
may indicate that the source spends at least several hundreds of years in
quiescence in between outbursts for the neutron star to cool. If true, then it
might be the first such X-ray transient to be identified and a class of
hundreds of similar systems may be present in the Galaxy. Alternatively,
enhanced neutrino cooling could occur in the core of the neutron star which
would cool the star more rapidly. However, in that case the neutron star in KS
1731-260 would be more massive than those in the prototypical neutron star
transients (e.g., Aql X-1 or 4U 1608-52).Comment: Accepted for publicaton in ApJ letters, 13 September 200
The magnetic nature of disk accretion onto black holes
Although disk accretion onto compact objects - white dwarfs, neutron stars,
and black holes - is central to much of high energy astrophysics, the
mechanisms which enable this process have remained observationally elusive.
Accretion disks must transfer angular momentum for matter to travel radially
inward onto the compact object. Internal viscosity from magnetic processes and
disk winds can in principle both transfer angular momentum, but hitherto we
lacked evidence that either occurs. Here we report that an X-ray-absorbing wind
discovered in an observation of the stellar-mass black hole binary GRO J1655-40
must be powered by a magnetic process that can also drive accretion through the
disk. Detailed spectral analysis and modeling of the wind shows that it can
only be powered by pressure generated by magnetic viscosity internal to the
disk or magnetocentrifugal forces. This result demonstrates that disk accretion
onto black holes is a fundamentally magnetic process.Comment: 15 pages, 2 color figures, accepted for publication in Nature.
Supplemental materials may be obtained by clicking
http://www.astro.lsa.umich.edu/~jonmm/nature1655.p
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