1,531 research outputs found
Advances in Bayesian Time Series Modeling and the Study of Politics: Theory Testing, Forecasting, and Policy Analysis."
Bayesian approaches to the study of politics are increasingly popular. But Bayesian approaches to modeling multiple time series have not been critically evaluated. This is in spite of the potential value of these models in international relations, political economy, and other fields of our discipline. We review recent developments in Bayesian multi-equation time series modeling in theory testing, forecasting, and policy analysis. Methods for constructing Bayesian measures of uncertainty of impulse responses (Bayesian shape error bands) are explained. A reference prior for these models that has proven useful in short-and medium-term forecasting in macroeconomics is described. Once modified to incorporate our experience analyzing political data and our theories, this prior can enhance our ability to forecast over the short and medium terms complex political dynamics like those exhibited by certain international conflicts. In addition, we explain how contingent Bayesian forecasts can be constructed, contingent Bayesian forecasts that embody policy counterfactuals. The value of these new Bayesian methods is illustrated in a reanalysis of the Israeli-Palestinian conflict of the 1980s
Synthesizing Political Zero-Shot Relation Classification via Codebook Knowledge, NLI, and ChatGPT
Recent supervised models for event coding vastly outperform pattern-matching
methods. However, their reliance solely on new annotations disregards the vast
knowledge within expert databases, hindering their applicability to
fine-grained classification. To address these limitations, we explore zero-shot
approaches for political event ontology relation classification, by leveraging
knowledge from established annotation codebooks. Our study encompasses both
ChatGPT and a novel natural language inference (NLI) based approach named ZSP.
ZSP adopts a tree-query framework that deconstructs the task into context,
modality, and class disambiguation levels. This framework improves
interpretability, efficiency, and adaptability to schema changes. By conducting
extensive experiments on our newly curated datasets, we pinpoint the
instability issues within ChatGPT and highlight the superior performance of
ZSP. ZSP achieves an impressive 40% improvement in F1 score for fine-grained
Rootcode classification. ZSP demonstrates competitive performance compared to
supervised BERT models, positioning it as a valuable tool for event record
validation and ontology development. Our work underscores the potential of
leveraging transfer learning and existing expertise to enhance the efficiency
and scalability of research in the field.Comment: Preprin
The Sloan Digital Sky Survey Reverberation Mapping Project: Velocity Shifts of Quasar Emission Lines
Quasar emission lines are often shifted from the systemic velocity due to
various dynamical and radiative processes in the line-emitting region. The
level of these velocity shifts depends both on the line species and on quasar
properties. We study velocity shifts for the line peaks of various narrow and
broad quasar emission lines relative to systemic using a sample of 849 quasars
from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. The
coadded (from 32 epochs) spectra of individual quasars have sufficient
signal-to-noise ratio (SNR) to measure stellar absorption lines to provide
reliable systemic velocity estimates, as well as weak narrow emission lines.
The sample also covers a large dynamic range in quasar luminosity (~2 dex),
allowing us to explore potential luminosity dependence of the velocity shifts.
We derive average line peak velocity shifts as a function of quasar luminosity
for different lines, and quantify their intrinsic scatter. We further quantify
how well the peak velocity can be measured for various lines as a function of
continuum SNR, and demonstrate there is no systematic bias in the line peak
measurements when the spectral quality is degraded to as low as SNR~3 per SDSS
pixel. Based on the observed line shifts, we provide empirical guidelines on
redshift estimation from [OII]3728, [OIII]5008, [NeV]3426, MgII, CIII],
HeII1640, broad Hbeta, CIV, and SiIV, which are calibrated to provide unbiased
systemic redshifts in the mean, but with increasing intrinsic uncertainties of
46, 56, 119, 205, 233, 242, 400, 415, and 477 km/s, in addition to the
measurement uncertainties. These more realistic redshift uncertainties are
generally much larger than the formal uncertainties reported by the redshift
pipelines for spectroscopic quasar surveys, and demonstrate the infeasibility
of measuring quasar redshifts to better than ~200 km/s with only broad lines.Comment: matched to the published version; minor changes and conclusions
unchange
X-ray Insights Into Interpreting CIV Blueshifts and Optical/UV Continua
We present 0.5-8.0 keV Chandra observations of six bright quasars that
represent extrema in quasar emission-line properties -- three quasars each with
small and large blueshifts of the CIV emission line with respect to the
systemic redshift of the quasars. Supplemented with seven archival Chandra
observations of quasars that met our selection criteria, we investigate the
origin of this emission-line phenomenon in the general context of the structure
of quasars. We find that the quasars with the largest CIV blueshifts show
evidence, from joint-spectral fitting, for intrinsic X-ray absorption (N_H ~
10^22 cm^-2). Given the lack of accompanying CIV absorption, this gas is likely
to be highly ionized, and may be identified with the shielding gas in the
disk-wind paradigm. Furthermore, we find evidence for a correlation of
alpha_uv, the ultraviolet spectral index, with the hardness of the X-ray
continuum; an analysis of independent Bright Quasar Survey data from the
literature supports this conclusion. This result points to intrinsically red
quasars having systematically flatter hard X-ray continua without evidence for
X-ray absorption. We speculate on the origins of these correlations of X-ray
properties with both CIV blueshift and alpha_uv and discuss the implications
for models of quasar structure.Comment: 9 figs, 25 pages, AASTeX; accepted for publication in A
The Sloan Digital Sky Survey Reverberation Mapping Project: Ensemble Spectroscopic Variability of Quasar Broad Emission Lines
We explore the variability of quasars in the MgII and Hbeta broad emission
lines and UV/optical continuum emission using the Sloan Digital Sky Survey
Reverberation Mapping project (SDSS-RM). This is the largest spectroscopic
study of quasar variability to date: our study includes 29 spectroscopic epochs
from SDSS-RM over months, containing 357 quasars with MgII and 41 quasars
with Hbeta . On longer timescales, the study is also supplemented with
two-epoch data from SDSS-I/II. The SDSS-I/II data include an additional
quasars with MgII and 572 quasars with Hbeta. The MgII emission line is
significantly variable ( 10% on 100-day timescales), a necessary
prerequisite for its use for reverberation mapping studies. The data also
confirm that continuum variability increases with timescale and decreases with
luminosity, and the continuum light curves are consistent with a damped
random-walk model on rest-frame timescales of days. We compare the
emission-line and continuum variability to investigate the structure of the
broad-line region. Broad-line variability shows a shallower increase with
timescale compared to the continuum emission, demonstrating that the broad-line
transfer function is not a -function. Hbeta is more variable than MgII
(roughly by a factor of ), suggesting different excitation mechanisms,
optical depths and/or geometrical configuration for each emission line. The
ensemble spectroscopic variability measurements enabled by the SDSS-RM project
have important consequences for future studies of reverberation mapping and
black hole mass estimation of quasars.Comment: 20 pages, 25 figures. ApJ accepted: minor revisions following referee
repor
Chandra Observations of SDSS J1004+4112: Constraints on the Lensing Cluster and Anomalous X-Ray Flux Ratios of the Quadruply Imaged Quasar
We present results from Chandra observations of SDSS J1004+4112, a strongly
lensed quasar system with a maximum image separation of 15". All four bright
images of the quasar, as well as resolved X-ray emission originating from the
lensing cluster, are clearly detected. The emission from the lensing cluster
extends out to approximately 1.5 arcmin. We measure the bolometric X-ray
luminosity and temperature of the lensing cluster to be 4.7e44 erg s^-1 and 6.4
keV, consistent with the luminosity-temperature relation for distant clusters.
The mass estimated from the X-ray observation shows excellent agreement with
the mass derived from gravitational lensing. The X-ray flux ratios of the
quasar images differ markedly from the optical flux ratios, and the combined
X-ray spectrum of the images possesses an unusually strong Fe Kalpha emission
line, both of which are indicative of microlensing.Comment: 9 pages, 5 figures. Accepted for publication in ApJ. Version with
high-quality color figures at
http://cosmic.riken.jp/ota/publications/index.htm
A Population of X-ray Weak Quasars: PHL 1811 Analogs at High Redshift
We report the results from Chandra and XMM-Newton observations of a sample of
10 type 1 quasars selected to have unusual UV emission-line properties (weak
and blueshifted high-ionization lines; strong UV Fe emission) similar to those
of PHL 1811, a confirmed intrinsically X-ray weak quasar. These quasars were
identified by the Sloan Digital Sky Survey at high redshift (z~2.2); eight are
radio quiet while two are radio intermediate. All of the radio-quiet PHL 1811
analogs are notably X-ray weak by a mean factor of ~13. These sources lack
broad absorption lines and have blue UV/optical continua, suggesting they are
intrinsically X-ray weak. However, their average X-ray spectrum appears to be
harder than those of typical quasars, which may indicate the presence of heavy
intrinsic X-ray absorption. Our radio-quiet PHL 1811 analogs support a
connection between an X-ray weak spectral energy distribution and PHL 1811-like
UV emission lines; this connection provides an economical way to identify X-ray
weak type 1 quasars. The fraction of radio-quiet PHL 1811 analogs in the
radio-quiet quasar population is estimated to be < 1.2%. We have investigated
correlations between relative X-ray brightness and UV emission-line properties
for a sample combining radio-quiet PHL 1811 analogs, PHL 1811, and typical type
1 quasars. These correlation analyses suggest that PHL 1811 analogs may have
extreme wind-dominated broad emission-line regions. Observationally,
radio-quiet PHL 1811 analogs appear to be a subset (~30%) of radio-quiet
weak-line quasars. The existence of a subset of quasars in which
high-ionization "shielding gas" covers most of the BELR, but little more than
the BELR, could potentially unify the PHL 1811 analogs and WLQs. The two
radio-intermediate PHL 1811 analogs are X-ray bright. One of them appears to
have jet-dominated X-ray emission, while the nature of the other remains
unclear.Comment: ApJ accepted; 25 pages, 11 figures and 8 table
The Sloan Digital Sky Survey Reverberation Mapping Project: Technical Overview
The Sloan Digital Sky Survey Reverberation Mapping project (SDSS-RM) is a
dedicated multi-object RM experiment that has spectroscopically monitored a
sample of 849 broad-line quasars in a single 7 deg field with the SDSS-III
BOSS spectrograph. The RM quasar sample is flux-limited to i_psf=21.7 mag, and
covers a redshift range of 0.1<z<4.5. Optical spectroscopy was performed during
2014 Jan-Jul dark/grey time, with an average cadence of ~4 days, totaling more
than 30 epochs. Supporting photometric monitoring in the g and i bands was
conducted at multiple facilities including the CFHT and the Steward Observatory
Bok telescopes in 2014, with a cadence of ~2 days and covering all lunar
phases. The RM field (RA, DEC=14:14:49.00, +53:05:00.0) lies within the CFHT-LS
W3 field, and coincides with the Pan-STARRS 1 (PS1) Medium Deep Field MD07,
with three prior years of multi-band PS1 light curves. The SDSS-RM 6-month
baseline program aims to detect time lags between the quasar continuum and
broad line region (BLR) variability on timescales of up to several months (in
the observed frame) for ~10% of the sample, and to anchor the time baseline for
continued monitoring in the future to detect lags on longer timescales and at
higher redshift. SDSS-RM is the first major program to systematically explore
the potential of RM for broad-line quasars at z>0.3, and will investigate the
prospects of RM with all major broad lines covered in optical spectroscopy.
SDSS-RM will provide guidance on future multi-object RM campaigns on larger
scales, and is aiming to deliver more than tens of BLR lag detections for a
homogeneous sample of quasars. We describe the motivation, design and
implementation of this program, and outline the science impact expected from
the resulting data for RM and general quasar science.Comment: 25 pages, submitted to ApJS; project website at http://www.sdssrm.or
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