2,519 research outputs found
Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma
We have carried out Very Large Array (VLA) continuum observations to study
the variability of Sgr A* at 43 GHz (=7mm) and 22 GHz
(=13mm). A low level of flare activity has been detected with a
duration of 2 hours at these frequencies, showing the peak flare
emission at 43 GHz leading the 22 GHz peak flare by to 40 minutes. The
overall characteristics of the flare emission are interpreted in terms of the
plasmon model of Van der Laan (1966) by considering the ejection and
adiabatically expansion of a uniform, spherical plasma blob due to flare
activity. The observed peak of the flare emission with a spectral index
of =1.6 is consistent with the prediction that the peak
emission shifts toward lower frequencies in an adiabatically-expanding
self-absorbed source. We present the expected synchrotron light curves for an
expanding blob as well as the peak frequency emission as a function of the
energy spectral index constrained by the available flaring measurements in
near-IR, sub-millimeter, millimeter and radio wavelengths. We note that the
blob model is consistent with the available measurements, however, we can not
rule out the jet of Sgr A*. If expanding material leaves the gravitational
potential of Sgr A*, the total mass-loss rate of nonthermal and thermal
particles is estimated to be M yr. We
discuss the implication of the mass-loss rate since this value matches closely
with the estimated accretion rate based on polarization measurements.Comment: Revised with new Figures 1 and 2, 17 pages, 4 figures, ApJ (in press
B3 0003+387: AGN Marked Large-Scale Structure at z=1.47?
We present evidence for a significant overdensity of red galaxies, as much as
a factor of 14 over comparable field samples, in the field of the z=1.47 radio
galaxy B3 0003+387. The colors and luminosities of the brightest red galaxies
are consistent with their being at z>0.8. The radio galaxy and one of the red
galaxies are separated by 5" and show some evidence of a possible interaction.
However, the red galaxies do not show any strong clustering around the radio
galaxy nor around any of the brighter red galaxies. The data suggest that we
are looking at a wall or sheet of galaxies, possibly associated with the radio
galaxy at z=1.47. Spectroscopic redshifts of these red galaxies will be
necessary to confirm this large-scale structure.Comment: 19 pages, 7 figures, LaTeX2e/AASTeX v5.0.2. The full photometric
catalog is included as a separate deluxetable file. To appear in the
Astronomical Journal (~Nov 00
Flaring Activity of Sgr A*: Expanding Hot Blobs
Sgr A* is considered to be a massive black hole at the Galactic center and is
known to be variable in radio, millimeter, near-IR and X-rays. Recent
multi-wavelength observing campaigns show a simultaneous X-ray and near-IR
flare, as well as sub-millimeter and near-IR flares from Sgr A*. The flare
activity is thought to be arising from the innermost region of Sgr A*. We have
recently argued that the duration of flares in near-IR and submillimeter
wavelengths implies that the burst of emission expands and cools on a dynamical
time scale before the flares leave Sgr A*. The detection of radio flares with a
time delay in the range of 20 and 40 minutes between 7 and 12mm peak emission
implies adiabatic expansion of a uniform, spherical hot blob due to flare
activity. We suspect that this simple outflow picture shows some of the
characteristics that are known to take place in microquasars, thus we may learn
much from comparative study of Sgr A* and its environment vs. microquasars.Comment: 10 pages, 5 figures, to be published in IV Microquasar Workshop:
Microquasars and Beyond, September 18-22 2006, Como, Ital
Selective Carbon-Carbon Bond Cleavage of Cyclopropylamine Derivatives.
This review summarizes synthetic developments reported from 1987 to 2019 that exploit C-C single bond cleavage of cyclopropylamine-based systems. The synthetic and mechanistic aspects of key methodologies are highlighted, and examples where aminocyclopropanes are exploited as key intermediates in multistep synthesis are also discussed. The review encompasses cases where aminocyclopropanes participate in polar reactions, pericyclic processes, radical-based reactions, and C-C bond activations
The Deficit of Distant Galaxy Clusters in the RIXOS X-ray Survey
Clusters of galaxies are the largest gravitationally bound systems and
therefore provide an important way of studying the formation and evolution of
the large scale structure of the Universe. Cluster evolution can be inferred
from observations of the X-ray emission of the gas in distant clusters, but
interpreting these data is not straightforward. In a simplified view, clusters
grow from perturbations in the matter distribution: their intracluster gas is
compressed and shock-heated by the gravitational collapse. The resulting
X-ray emission is determined by the hydrostatic equilibrium of the gas in the
changing gravitational potential. However, if processes such as radiative
cooling or pre-collapse heating of the gas are important, then the X-ray
evolution will be strongly influenced by the thermal history of the gas. Here
we present the first results from a faint flux-limited sample of X-ray selected
clusters compiled as part of the ROSAT International X-ray and Optical Survey
(RIXOS). Very few distant clusters have been identified. Most importantly,
their redshift distribution appears to be inconsistent with simple models based
on the evolution of the gravitational potential. Our results suggest that
radiative cooling or non-gravitational heating of the intracluster gas must
play an important role in the evolution of clusters.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Simulating the Hot X-ray Emitting Gas in Elliptical Galaxies
We study the chemo-dynamical evolution of elliptical galaxies and their hot
X-ray emitting gas using high-resolution cosmological simulations. Our Tree
N-body/SPH code includes a self-consistent treatment of radiative cooling, star
formation, supernovae feedback, and chemical enrichment. We present a series of
LCDM cosmological simulations which trace the spatial and temporal evolution of
heavy element abundance patterns in both the stellar and gas components of
galaxies. X-ray spectra of the hot gas are constructed via the use of the
vmekal plasma model, and analysed using XSPEC with the XMM EPN response
function. Simulation end-products are quantitatively compared with the
observational data in both the X-ray and optical regime. We find that radiative
cooling is important to interpret the observed X-ray luminosity, temperature,
and metallicity of the interstellar medium of elliptical galaxies. However,
this cooled gas also leads to excessive star formation at low redshift, and
therefore results in underlying galactic stellar populations which are too blue
with respect to observations.Comment: 6 pages, 3 figures, to appear in the proceedings of "The IGM/Galaxy
Connection - The Distribution of Baryons at z=0", ed. M. Putman & J.
Rosenberg; High resolution version is available at
http://astronomy.swin.edu.au/staff/dkawata/research/papers.htm
X-ray Spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectral properties of
sources detected in the RIXOS survey, that is nearly complete down to a flux
limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing
sources with low count rate spectral slopes estimated using simple hardness
ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray
data using statistical techniques appropriate to the Poisson regime which
removes the effects of this bias. We have then applied this technique to the
RIXOS survey to study the spectral properties of the sample. For the AGN we
find an average energy index of 1.05+-0.05 with no evidence for spectral
evolution with redshift. Individual AGN are shown to have a range of properties
including soft X-ray excesses and intrinsic absorption. Narrow Emission Line
Galaxies also seem to fit to a power-law spectrum, which may indicate a
non-thermal origin for their X-ray emission. We infer that most of the clusters
in the sample have a bremsstrahlung temperature >3 keV, although some show
evidence for a cooling flow. The stars deviate strongly from a power-law model
but fit to a thermal model. Finally, we have analysed the whole RIXOS sample
containing 1762 sources. We find that the mean spectral slope of the sources
hardens at lower fluxes in agreement with results from other samples. However,
a study of the individual sources demonstrates that the hardening of the mean
is caused by the appearance of a population of very hard sources at the lowest
fluxes. This has implications for the nature of the soft X-ray background.Comment: 31,LaTeX file, 2 PS files with Table 2 and 22 PS figures. MNRAS in
pres
Double Distribution of Dark Matter Halos with respect to Mass and Local Overdensity
We present a double distribution function of dark matter halos, with respect
to both object mass and local over- (or under-) density. This analytical tool
provides a statistical treatment of the properties of matter surrounding
collapsed objects, and can be used to study environmental effects on
hierarchical structure formation. The size of the "local environment" of a
collapsed object is defined to depend on the mass of the object. The
Press-Schechter mass function is recovered by integration of our double
distribution over the density contrast. We also present a detailed treatment of
the evolution of overdensities and underdensities in Einstein-deSitter and flat
LCDM universes, according to the spherical evolution model. We explicitly
distinguish between true and linearly extrapolated overdensities and provide
conversion relations between the two quantities.Comment: 25 pages, 10 figures, comments welcom
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