15,669 research outputs found
Binscatter Regressions
We introduce the \texttt{Stata} (and \texttt{R}) package \textsf{Binsreg},
which implements the binscatter methods developed in
\citet*{Cattaneo-Crump-Farrell-Feng_2019_Binscatter}. The package includes the
commands \texttt{binsreg}, \texttt{binsregtest}, and \texttt{binsregselect}.
The first command (\texttt{binsreg}) implements binscatter for the regression
function and its derivatives, offering several point estimation, confidence
intervals and confidence bands procedures, with particular focus on
constructing binned scatter plots. The second command (\texttt{binsregtest})
implements hypothesis testing procedures for parametric specification and for
nonparametric shape restrictions of the unknown regression function. Finally,
the third command (\texttt{binsregselect}) implements data-driven number of
bins selectors for binscatter implementation using either quantile-spaced or
evenly-spaced binning/partitioning. All the commands allow for covariate
adjustment, smoothness restrictions, weighting and clustering, among other
features. A companion \texttt{R} package with the same capabilities is also
available
On Binscatter
Binscatter is very popular in applied microeconomics. It provides a flexible,
yet parsimonious way of visualizing and summarizing large data sets in
regression settings, and it is often used for informal evaluation of
substantive hypotheses such as linearity or monotonicity of the regression
function. This paper presents a foundational, thorough analysis of binscatter:
we give an array of theoretical and practical results that aid both in
understanding current practices (i.e., their validity or lack thereof) and in
offering theory-based guidance for future applications. Our main results
include principled number of bins selection, confidence intervals and bands,
hypothesis tests for parametric and shape restrictions of the regression
function, and several other new methods, applicable to canonical binscatter as
well as higher-order polynomial, covariate-adjusted and smoothness-restricted
extensions thereof. In particular, we highlight important methodological
problems related to covariate adjustment methods used in current practice. We
also discuss extensions to clustered data. Our results are illustrated with
simulated and real data throughout. Companion general-purpose software packages
for \texttt{Stata} and \texttt{R} are provided. Finally, from a technical
perspective, new theoretical results for partitioning-based series estimation
are obtained that may be of independent interest
Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey
detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z
> 2, indicating an apparent increase in angular diameter or a decrease in flux
density of the most compact components of these sources, relative to their
extended emission. This can result from intrinsic source size effects or
scatter broadening in the Intergalactic Medium (IGM), in excess of the expected
(1+z)^0.5 angular diameter scaling of brightness temperature limited sources
due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations
and data analysis for a sample of 140 compact, flat-spectrum sources which may
allow us to determine the origin of this angular diameter-redshift relation by
exploiting their different wavelength dependences. In addition to using ISS as
a cosmological probe, the observations provide additional insight into source
morphologies and the characteristics of ISS. As in the MASIV Survey, the
variability of the sources is found to be significantly correlated with
line-of-sight H-alpha intensities, confirming its link with ISS. For 25
sources, time delays of about 0.15 to 3 days are observed between the
scintillation patterns at both frequencies, interpreted as being caused by a
shift in core positions when probed at different optical depths. Significant
correlation is found between ISS amplitudes and source spectral index; in
particular, a large drop in ISS amplitudes is observed at spectral indices of <
-0.4 confirming that steep spectrum sources scintillate less. We detect a
weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the
mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2
sources relative to the z < 2 sources, as opposed to the factor of 3 decrease
observed at 4.9 GHz. This suggests scatter broadening in the IGM.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical
Journa
Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
The luminosity distance vs. redshift law is now measured using supernovae and
gamma ray bursts, and the angular size distance is measured at the surface of
last scattering by the CMB and at z = 0.35 by baryon acoustic oscillations. In
this paper this data is fit to models for the equation of state with w = -1, w
= const, and w(z) = w_0+w_a(1-a). The last model is poorly constrained by the
distance data, leading to unphysical solutions where the dark energy dominates
at early times unless the large scale structure and acoustic scale constraints
are modified to allow for early time dark energy effects. A flat LambdaCDM
model is consistent with all the data.Comment: 19 pages Latex with 8 Postscript figure files. A new reference and
constraint, w vs w' contour plots updated. Version accepted by the the Ap
B and I-band optical micro-variability observations of the BL Lac objects S5 2007+777 and 3C371
We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8
nights, respectively, during various observing runs in 2001, 2002 and 2004. The
observations resulted in almost evenly sampled light curves, 6-9 hours long. We
do not detect any flares within the observed light curves, but we do observe
small amplitude, significant variations, in both bands, on time scales of hours
and days. The average variability amplitude on time scales of minutes/hours is
2.5% and 1-1.5% in the case of S5 2007+777 and 3C371, respectively. The average
amplitudes increase to 5-12% and 4-6%, respectively, on time scales of days. We
find that the B and I band variations are highly correlated, on both short and
long time scales. During the 2004 observations, which resulted in the longest
light curves, we observe two well defined flux-decay and rising trends in the
light curves of both objects. When the flux decays, we observe significant
delays, with the B band flux decaying faster than the flux in the I band. As a
result, we also observe significant, flux related spectral variations as well.
The flux-spectral relation is rather complicated, with loop-like structures
forming during the flux evolution. The presence of spectral variations imply
that the observed variability is not caused by geometric effects. On the other
hand, our results are fully consistent with the hypothesis that the observed
variations are caused by perturbations which affect different regions in the
jet of the sources.Comment: Accepted for publication in Astronomy and Astrophysic
Variability of the soft X-ray excess in IRAS 13224-3809
We study the soft excess variability of the narrow line Seyfert 1 galaxy IRAS
13224-3809. We considered all five archival XMM-Newton observations, and we
applied the 'flux-flux plot' (FFP) method. We found that the flux-flux plots
were highly affected by the choice of the light curves' time bin size, most
probably because of the fast and large amplitude variations, and the intrinsic
non-linear flux--flux relations in this source. Therefore, we recommend that
the smallest bin-size should be used in such cases. Hence, We constructed FFPs
in 11 energy bands below 1.7 keV, and we considered the 1.7-3 keV band, as
being representative of the primary emission. The FFPs are reasonably well
fitted by a 'power-law plus a constant' model. We detected significant positive
constants in three out of five observations. The best-fit slopes are flatter
than unity at energies below keV, where the soft excess is
strongest. This suggests the presence of intrinsic spectral variability. A
power-law-like primary component, which is variable in flux and spectral slope
(as ) and a soft-excess component, which varies
with the primary continuum (as ),
can broadly explain the FFPs. In fact, this can create positive `constants',
even when a stable spectral component does not exist. Nevertheless, the
possibility of a stable, soft--band constant component cannot be ruled out, but
its contribution to the observed 0.2-1 keV band flux should be less than %. The model constants in the FFPs were consistent with zero in one
observation, and negative at energies below 1 keV in another. It is hard to
explain these results in the context of any spectral variability scenario, but
they may signify the presence of a variable, warm absorber in the source.Comment: Accepted for publication in A&A, 10 pages, 7 figure
Velocity dispersion in N-body simulations of CDM models
This work reports on a study of the spatially coarse-grained velocity
dispersion in cosmological N-body simulations (OCDM and Lambda-CDM models) as a
function of time (redshifts z=0-4) and of the coarsening length (0.6-20 Mpc/h).
The main result is the discovery of a polytropic relationship I_1 ~ rho^{2-eta}
between the velocity-dispersion kinetic energy density of the coarsening cells,
I_1, and their mass density, rho. The exponent eta, dependent on time and
coarsening scale, is a compact measure of the deviations from the naive virial
prediction eta_virial=0. This relationship supports the ``polytropic
assumption'' which has been employed in theoretical models for the growth of
cosmological structure by gravitational instability.Comment: Minor, unimportant changes. Matches published versio
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