1,435 research outputs found
Finding the brightest galactic bulge microlensing events with a small aperture telescope and image subtraction
Following the suggestion of Gould and Depoy (1998) we investigate the feasibility of studying the brightest microlensing events towards the Galactic bulge using a small aperture (â 10 cm) telescope. We used one of the HAT telescopes to obtain 151 expos
An Attempt to Reproduce the SchwarzâHora Effect
Several attempts were made to observe the modulation of an electron beam by a laser (the SchwarzâHora effect). These were not successful and possible reasons are reported
The Optical Afterglow of GRB 011211
We present early-time optical photometry and spectroscopy of the optical
afterglow of the gamma-ray burst GRB 011211. The spectrum of the optical
afterglow contains several narrow metal lines which are consistent with the
burst occurring at a redshift of 2.140 +/- 0.001. The optical afterglow decays
as a power law with a slope of alpha = 0.83 +/- 0.04 for the first
approximately two days after the burst at which time there is evidence for a
break. The slope after the break is at least 1.4. There is evidence for rapid
variations in the R-band light approximately 0.5 days after the burst. These
variations suggest that there are density fluctuations near the gamma-ray burst
on spatial scales of approximately 40--125 AU. The magnitude of the break in
the light curve, the spectral slope, and the rate of decay in the optical,
suggest that the burst expanded into an ambient medium that is homogeneous on
large scales. We estimate that the local particle density is between
approximately 0.1 and 10 cm^{-3} and that the total gamma-ray energy in the
burst was 1.2--1.9 x 10^{50} erg. This energy is smaller than, but consistent
with, the ``standard'' value of (5 +/- 2) x 10^{50} erg. Comparing the observed
color of the optical afterglow with predictions of the standard beaming model
suggests that the rest-frame V-band extinction in the host galaxy is less than
approximately 0.03 mag.Comment: 17 pages, 4 figures, AASTeX 5.02, to appear in AJ Referee's report
incorporated, minor changes in the tex
Lifting the Dusty Veil With Near- and Mid-Infrared Photometry: III. Two-Dimensional Extinction Maps of the Galactic Midplane Using the Rayleigh-Jeans Color Excess Method
We provide new, high-resolution A(Ks) extinction maps of the heavily reddened
Galactic midplane based on the Rayleigh-Jeans Color Excess ("RJCE") method.
RJCE determines star-by-star reddening based on a combination of near- and
mid-infrared photometry. The new RJCE-generated maps have 2 x 2 arcmin pixels
and span some of the most severely extinguished regions of the Galaxy -- those
covered with Spitzer+IRAC imaging by the GLIMPSE-I, -II, -3D, and Vela-Carina
surveys, from 256<l<65 deg and, in general, for |b| <= 1-1.5 deg (extending up
to |b|<=4 deg in the bulge). Using RJCE extinction measurements, we generate
dereddened color-magnitude diagrams and, in turn, create maps based on main
sequence, red clump, and red giant star tracers, each probing different
distances and thereby providing coarse three-dimensional information on the
relative placement of dust cloud structures. The maps generated from red giant
stars, which reach to ~18-20 kpc, probe beyond most of the Milky Way extinction
in most directions and provide close to a "total Galactic extinction" map -- at
minimum they provide high angular resolution maps of lower limits on A(Ks).
Because these maps are generated directly from measurements of reddening by the
very dust being mapped, rather than inferred on the basis of some less direct
means, they are likely the most accurate to date for charting in detail the
highly patchy differential extinction in the Galactic midplane. We provide
downloadable FITS files and an IDL tool for retrieving extinction values for
any line of sight within our mapped regions.Comment: 23 pages, 5 figures, accepted for publication in ApJ
Rotation and Turbulence of the Hot ICM in Galaxy Clusters
Cosmological simulations of galaxy clusters typically find that the weight of
a cluster at a given radius is not balanced entirely by the thermal gas
pressure of the hot ICM, with theoretical studies emphasizing the role of
random turbulent motions to provide the necessary additional pressure support.
Using a set of high-resolution, hydrodynamical simulations of galaxy clusters
that include radiative cooling and star formation, we find instead that in the
most relaxed clusters rotational support exceeds that from random turbulent
motions for radii 0.1 - 0.5 r_500, and that the observed clusters are much
rounder than the simulated, relaxed clusters within ~ 0.4 r_500. Moreover,
while the observed clusters display an average ellipticity profile that does
not vary significantly with radius, the ellipticity of the relaxed CDM clusters
declines markedly with increasing radius, suggesting that the ICM of the
observed clusters rotates less rapidly than that of the relaxed CDM clusters
out to ~ 0.6 r_500. We also find the ellipticity profile of a simulated cluster
without radiative cooling is in much better agreement with the observations,
implying that over-cooling has a substantial impact on the gas dynamics and
morphology out to larger radii than previously recognized. It also suggests
that the 10%-20% systematic errors from non-thermal gas pressure support
reported for simulated cluster masses, obtained from fitting simulated X-ray
data over large radial ranges within r_500, may need to be revised downward.
These results demonstrate the utility of X-ray ellipticity profiles as a probe
of ICM rotation and over-cooling which should be used to constrain future
cosmological cluster simulations.Comment: 13 pages, 8 figures, accepted for publication in Ap
On the Baryon Fractions in Clusters and Groups of Galaxies
We present the baryon fractions of 2MASS groups and clusters as a function of
cluster richness using total and gas masses measured from stacked ROSAT X-ray
data and stellar masses estimated from the infrared galaxy catalogs. We detect
X-ray emission even in the outskirts of clusters, beyond r_200 for richness
classes with X-ray temperatures above 1 keV. This enables us to more accurately
determine the total gas mass in these groups and clusters. We find that the
optically selected groups and clusters have flatter temperature profiles and
higher stellar-to-gas mass ratios than the individually studied, X-ray bright
clusters. We also find that the stellar mass in poor groups with temperatures
below 1 keV is comparable to the gas mass in these systems. Combining these
results with individual measurements for clusters, groups, and galaxies from
the literature, we find a break in the baryon fraction at ~1 keV. Above this
temperature, the baryon fraction scales with temperature as f_b \propto
T^0.20\pm0.03. We see significantly smaller baryon fractions below this
temperature, and the baryon fraction of poor groups joins smoothly onto that of
systems with still shallower potential wells such as normal and dwarf galaxies
where the baryon fraction scales with the inferred velocity dispersion as f_b
\propto \sigma^1.6. The small scatter in the baryon fraction at any given
potential well depth favors a universal baryon loss mechanism and a preheating
model for the baryon loss. The scatter is, however, larger for less massive
systems. Finally, we note that although the broken power-law relation can be
inferred from data points in the literature alone, the consistency between the
baryon fractions for poor groups and massive galaxies inspires us to fit the
two categories of objects (galaxies and clusters) with one relation.Comment: 21 pages, 5 figures, ApJ in pres
SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy
While performing a survey to detect recoiling supermassive black holes, we
have identified an unusual source having a projected offset of 800 pc from a
nearby dwarf galaxy. The object, SDSS J113323.97+550415.8, exhibits broad
emission lines and strong variability. While originally classified as a
supernova (SN) because of its nondetection in 2005, we detect it in recent and
past observations over 63 yr and find over a magnitude of rebrightening in the
last 2 years. Using high-resolution adaptive optics observations, we constrain
the source emission region to be <12 pc and find a disturbed host-galaxy
morphology indicative of recent merger activity. Observations taken over more
than a decade show narrow [O III] lines, constant ultraviolet emission, broad
Balmer lines, a constant putative black hole mass over a decade of observations
despite changes in the continuum, and optical emission-line diagnostics
consistent with an active galactic nucleus (AGN). However, the optical spectra
exhibit blueshifted absorption, and eventually narrow Fe II and [Ca II]
emission, each of which is rarely found in AGN spectra. While this peculiar
source displays many of the observational properties expected of a potential
black hole recoil candidate, some of the properties could also be explained by
a luminous blue variable star (LBV) erupting for decades since 1950, followed
by a Type IIn SN in 2001. Interpreted as an LBV followed by a SN analogous to
SN 2009ip, the multi-decade LBV eruptions would be the longest ever observed,
and the broad Halpha emission would be the most luminous ever observed at late
times (>10 yr), larger than that of unusually luminous supernovae such as SN
1988Z, suggesting one of the most extreme episodes of pre-SN mass loss ever
discovered.Comment: Accepted for publication in MNRA
Lifting the Dusty Veil With Near- and Mid-Infrared Photometry: I. Description and Applications of the Rayleigh-Jeans Color Excess Method
The Milky Way (MW) remains a primary laboratory for understanding the
structure and evolution of spiral galaxies, but typically we are denied clear
views of MW stellar populations at low Galactic latitudes because of extinction
by interstellar dust. However, the combination of 2MASS near-infrared (NIR) and
Spitzer-IRAC mid-infrared (MIR) photometry enables a powerful method for
determining the line of sight reddening to any star: the sampled wavelengths
lie in the Rayleigh-Jeans part of the spectral energy distribution of most
stars, where, to first order, all stars have essentially the same intrinsic
color. Thus, changes in stellar NIR-MIR colors due to interstellar reddening
are readily apparent, and (under an assumed extinction law) the observed colors
and magnitudes of stars can be easily and accurately restored to their
intrinsic values, greatly increasing their usefulness for Galactic structure
studies. In this paper we explore this "Rayleigh-Jeans Color Excess" (RJCE)
method and demonstrate that use of even a simple variant of the RJCE method
based on a single reference color, (H-[4.5um]), can rather accurately remove
dust effects from previously uninterpretable 2MASS color-magnitude diagrams of
stars in fields along the heavily reddened Galactic mid-plane, with results far
superior to those derived from application of other dereddening methods. We
also show that "total" Galactic midplane extinction looks rather different from
that predicted using 100um emission maps from the IRAS/ISSA and COBE/DIRBE
instruments as presented by Schlegel et al. Instead, the Galactic mid-plane
extinction strongly resembles the distribution of 13-CO (J=1->0) emission.
Future papers will focus on refining the RJCE method and applying the technique
to understand better not only dust and its distribution, but the distribution
of stars intermixed with the dust in the low-latitude Galaxy.Comment: Accepted to ApJ; 21 pages, 17 figure
Impact of Homogeneous Strain On Uranium Vacancy Diffusion In Uranium Dioxide
We present a detailed mechanism of, and the effect of homogeneous strains on, the migration of uranium vacancies in UO2. Vacancy migration pathways and barriers are identified using density functional theory and the effect of uniform strain fields are accounted for using the dipole tensor approach. We report complex migration pathways and noncubic symmetry associated with the uranium vacancy in UO2 and show that these complexities need to be carefully accounted for to predict the correct diffusion behavior of uranium vacancies. We show that under homogeneous strain fields, only the dipole tensor of the saddle with respect to the minimum is required to correctly predict the change in the energy barrier between the strained and the unstrained case. Diffusivities are computed using kinetic Monte Carlo simulations for both neutral and fully charged state of uranium single and divacancies. We calculate the effect of strain on migration barriers in the temperature range 800â1800 K for both vacancy types. Homogeneous strains as small as 2% have a considerable effect on diffusivity of both single and divacancies of uranium, with the effect of strain being more pronounced for single vacancies than divacancies. In contrast, the response of a given defect to strain is less sensitive to changes in the charge state of the defect. Further, strain leads to anisotropies in the mobility of the vacancy and the degree of anisotropy is very sensitive to the nature of the applied strain field for strain of equal magnitude. Our results suggest that the influence of strain on vacancy diffusivity will be significantly greater when single vacancies dominate the defect structure, such as sintering, while the effects will be much less substantial under irradiation conditions where divacancies dominate
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