3,132 research outputs found
VCBART: Bayesian trees for varying coefficients
Many studies have reported associations between later-life cognition and
socioeconomic position in childhood, young adulthood, and mid-life. However,
the vast majority of these studies are unable to quantify how these
associations vary over time and with respect to several demographic factors.
Varying coefficient (VC) models, which treat the covariate effects in a linear
model as nonparametric functions of additional effect modifiers, offer an
appealing way to overcome these limitations. Unfortunately, state-of-the-art VC
modeling methods require computationally prohibitive parameter tuning or make
restrictive assumptions about the functional form of the covariate effects.
In response, we propose VCBART, which estimates the covariate effects in a VC
model using Bayesian Additive Regression Trees. With simple default
hyperparameter settings, VCBART outperforms existing methods in terms of
covariate effect estimation and prediction. Using VCBART, we predict the
cognitive trajectories of 4,167 subjects from the Health and Retirement Study
using multiple measures of socioeconomic position and physical health. We find
that socioeconomic position in childhood and young adulthood have small effects
that do not vary with age. In contrast, the effects of measures of mid-life
physical health tend to vary with respect to age, race, and marital status. An
R package implementing VC-BART is available at
https://github.com/skdeshpande91/VCBAR
Repurposing the (super)crip: media representations of disability at the Rio 2016 Paralympic Games
Swift captures the spectrally evolving prompt emission of GRB 070616
The origins of Gamma-ray Burst prompt emission are currently not well
understood and in this context long, well-observed events are particularly
important to study. We present the case of GRB 070616, analysing the
exceptionally long-duration multipeaked prompt emission, and later afterglow,
captured by all the instruments on-board Swift and by Suzaku WAM. The high
energy light curve remained generally flat for several hundred seconds before
going into a steep decline. Spectral evolution from hard to soft is clearly
taking place throughout the prompt emission, beginning at 285 s after the
trigger and extending to 1200 s. We track the movement of the spectral peak
energy, whilst observing a softening of the low energy spectral slope. The
steep decline in flux may be caused by a combination of this strong spectral
evolution and the curvature effect. We investigate origins for the spectral
evolution, ruling out a superposition of two power laws and considering instead
an additional component dominant during the late prompt emission. We also
discuss origins for the early optical emission and the physics of the
afterglow. The case of GRB 070616 clearly demonstrates that both broadband
coverage and good time resolution are crucial to pin down the origins of the
complex prompt emission in GRBs.Comment: 13 pages, 11 figures (2 in colour), MNRAS accepte
Multiwavelength XMM-Newton observations of the Laor et al. sample of PG quasars
We present XMM-Newton/EPIC spectra for the Laor et al. sample of Palomar
Green quasars. We find that a power-law provides a reasonable fit to the 2-5
keV region of the spectra. Excess soft X-ray emission below 2 keV is present
for all objects, with the exception of those known to contain a warm absorber.
A single power-law is, however, a poor fit to the 0.3-10.0 keV spectrum and
instead we find that a simple model, consisting of a broken power-law (plus an
iron line), provides a reasonable fit in most cases. The equivalent width of
the emission line is constrained in just twelve objects but with low (<2 sigma)
significance in most cases. For the sources whose spectra are well-fit by the
broken power-law model, we find that various optical and X-ray line and
continuum parameters are well-correlated; in particular, the power-law photon
index is well-correlated with the FWHM of the Hbeta line and the photon indices
of the low and high energy components of the broken power-law are
well-correlated with each other. These results suggest that the 0.3-10 keV
X-ray emission shares a common (presumably non-thermal) origin, as opposed to
suggestions that the soft excess is directly produced by thermal disc emission
or via an additional spectral component. We present XMM-Newton/OM data which we
combine with the X-ray spectra so as to produce broad-band spectral energy
distributions, free from uncertainties due to long-term variability in
non-simultaneous data. Fitting these optical-UV spectra with a Comptonized disc
model indicates that the soft X-ray excess is independent of the accretion
disc, confirming our interpretation of the tight correlation between the hard
and soft X-ray spectra.Comment: Accepted for publication in MNRA
Gas and dust properties in the afterglow spectra of GRB 050730
We present early WHT ISIS optical spectroscopy of the afterglow of gamma-ray
burst GRB 050730. The spectrum shows a DLA system with the highest measured
hydrogen column to date: N(HI) = 22.1 +/- 0.1 at the third-highest GRB redshift
z = 3.968. Our analysis of the Swift XRT X-ray observations of the early
afterglow show X-ray flares accompanied by decreasing X-ray absorption. From
both the optical and the X-ray spectra we constrain the dust and gas properties
of the host galaxy. We find the host to be a low metallicity galaxy, with low
dust content. Much of the X-ray absorbing gas is situated close to the GRB,
whilst the HI absorption causing the DLA is most likely located further out.Comment: 5 pages, 2 figures. Accepted for A&A Letter
Universal law of fractionation for slightly polydisperse systems
By perturbing about a general monodisperse system, we provide a complete description of two-phase equilibria in any system which is slightly polydisperse in some property (e.g., particle size, charge, etc.). We derive a universal law of fractionation which is corroborated by comprehensive experiments on a model colloid-polymer mixture. We furthermore predict that phase separation is an effective method of reducing polydispersity only for systems with a skewed distribution of the polydisperse property
Gamma-Ray Burst Afterglows as Probes of Environment and Blastwave Physics II: The Distribution of p and Structure of the Circumburst Medium
We constrain blastwave parameters and the circumburst media of a subsample of BeppoSAX Gamma-Ray Bursts. For this sample we derive the values of the injected electron energy distribution index, p, and the density structure index of the circumburst medium, k, from simultaneous spectral fits to their X-ray, optical and nIR afterglow data. The spectral fits have been done in count space and include the effects of metallicity, and are compared with the previously reported optical and X-ray temporal behaviour. Assuming the fireball model, we can find a mean value of p for the sample as a whole of 2.035. A statistical analysis Of the distribution demonstrates that the p values in this sample are inconsistent with a single universal value for p at the 3sigma level or greater. This approach provides us with a measured distribution of circumburst density structures rather than considering only the cases of k = 0 (homogeneous) and k = 2 (wind-like). We find five GRBs for which k can be well constrained, and in four of these cases the circumburst medium is clearly wind-like. The fifth source has a value of 0 less than or equal to k less than or equal to 1, consistent with a homogeneous circumburst medium
Optical Classification of Gamma-Ray Bursts in the Swift Era
We propose a new method for the classification of optically dark gamma-ray
bursts (GRBs), based on the X-ray and optical-to-X-ray spectral indices of GRB
afterglows, and utilizing the spectral capabilities of Swift. This method
depends less on model assumptions than previous methods, and can be used as a
quick diagnostic tool to identify optically sub-luminous bursts. With this
method we can also find GRBs that are extremely bright at optical wavelengths.
We show that the previously suggested correlation between the optical darkness
and the X-ray/gamma-ray brightness is merely an observational selection effect.Comment: 6 pages, 3 figures; accepted for publication in Ap
Spectroscopy of the optical afterglow of GRB 021004: Origin of the blue-shifted hydrogen lines
We present spectra of the afterglow of GRB 021004 taken with
WHT ISIS and VLT FORS1 at three epochs spanning 0.49â6.62 days after the burst. Alongside absorption lines from the host galaxy, we identify absorption in HI, SiIV and CIV with blueshifts of up to 2800 km sâ1 from the explosion centre which we assume originates close to the progenitor. We investigate the origin of the outflowing material and evaluate various possible progenitor models, in particular a
binary progenitor consisting of a Wolf-Rayet star and hydrogen-rich companion
- âŠ