2,570 research outputs found
The Effect of Variability on the Estimation of Quasar Black Hole Masses
We investigate the time-dependent variations of ultraviolet (UV) black hole
mass estimates of quasars in the Sloan Digital Sky Survey (SDSS). From SDSS
spectra of 615 high-redshift (1.69 < z < 4.75) quasars with spectra from two
epochs, we estimate black hole masses, using a single-epoch technique which
employs an additional, automated night-sky-line removal, and relies on UV
continuum luminosity and CIV (1549A) emission line dispersion. Mass estimates
show variations between epochs at about the 30% level for the sample as a
whole. We determine that, for our full sample, measurement error in the line
dispersion likely plays a larger role than the inherent variability, in terms
of contributing to variations in mass estimates between epochs. However, we use
the variations in quasars with r-band spectral signal-to-noise ratio greater
than 15 to estimate that the contribution to these variations from inherent
variability is roughly 20%. We conclude that these differences in black hole
mass estimates between epochs indicate variability is not a large contributer
to the current factor of two scatter between mass estimates derived from low-
and high-ionization emission lines.Comment: 76 pages, 15 figures, 2 (long) tables; Accepted for publication in
ApJ (November 10, 2007
The racist bodily imaginary: the image of the body-in-pieces in (post)apartheid culture
This paper outlines a reoccurring motif within the racist imaginary of (post)apartheid culture: the black body-in-pieces. This disturbing visual idiom is approached from three conceptual perspectives. By linking ideas prevalent in Frantz Fanonâs description of colonial racism with psychoanalytic concepts such as Lacanâs notion of the corps morcelĂ©, the paper offers, firstly, an account of the black body-in-pieces as fantasmatic preoccupation of the (post)apartheid imaginary. The role of such images is approached, secondly, through the lens of affect theory which eschews a representational âreadingâ of such images in favour of attention to their asignifying intensities and the role they play in effectively constituting such bodies. Lastly, Judith Butlerâs discussion of war photography and the conditions of grievability introduces an ethical dimension to the discussion and helps draw attention to the unsavory relations of enjoyment occasioned by such images
Image Coaddition with Temporally Varying Kernels
Large, multi-frequency imaging surveys, such as the Large Synaptic Survey
Telescope (LSST), need to do near-real time analysis of very large datasets.
This raises a host of statistical and computational problems where standard
methods do not work. In this paper, we study a proposed method for combining
stacks of images into a single summary image, sometimes referred to as a
template. This task is commonly referred to as image coaddition. In part, we
focus on a method proposed in previous work, which outlines a procedure for
combining stacks of images in an online fashion in the Fourier domain. We
evaluate this method by comparing it to two straightforward methods through the
use of various criteria and simulations. Note that the goal is not to propose
these comparison methods for use in their own right, but to ensure that
additional complexity also provides substantially improved performance
Characterization of soil organic matter in aggregates and size-density fractions by solid state C-13 CPMAS NMR spectroscopy.
Understanding the changes in soil organic matter (SOM) composition during aggregate formation is crucial to explain the stabilization of SOM in aggregates. The objectives of this study were to investigate (i) the composition of SOM associated with different aggregates and size-density fractions and (ii) the role of selective preservation in determining the composition of organic matter in aggregate and size-density fractions. Surface soil samples were collected from an Alfisol on the Northern Tablelands of NSW, Australia with contrasting land uses native pasture, crop-pasture rotation and woodland. Solid state 13C cross-polarization and magic angle spinning (CPMAS) Nuclear Magnetic Resonance (NMR) spectroscopy was used to determine the SOM composition in macroaggregates (250-2000 ”m), microaggregates (53-250 ”m), and <53 ”m fraction. The chemical composition of light fraction (LF), coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM) and mineral associated soil organic matter (mSOM) were also determined. The major constituent of SOM of aggregate size fractions was O-alkyl carbon, which represented 44-57% of the total signal acquired, whereas alkyl carbon contributed 16-27%. There was a progressive increase in alkyl carbon content with decrease in aggregate size. Results suggest that SOM associated with <53 ”m fraction was at a more advanced stage of decomposition than that of macroaggregates and microaggregates. The LF and cPOM were dominated by O-alkyl carbon while alkyl carbon content was high in fPOM and mSOM. Interestingly, the relative change in O-alkyl, alkyl and aromatic carbon between aggregates and SOM fractions revealed that microbial synthesis and decomposition of organic matter along with selective preservation of alkyl and aromatic carbon plays a significant role in determining the composition of organic matter in aggregates
The 3-53 keV Spectrum of the Quasar 1508+5714: X-rays from z = 4.3
We present a high-quality X-ray spectrum in the 3--53 keV rest-frame band of
the radio-loud quasar 1508+5714, by far the brightest known X-ray source at z >
4. A simple power-law model with an absorption column density equal to the
Galactic value in the direction of the source provides an excellent and fully
adequate fit to the data; the measured power-law photon index Gamma = 1.42
(+0.13,-0.10). Upper limits to Fe K alpha line emission and Compton-reflection
components are derived. We offer evidence for both X-ray and radio variability
in this object and provide the first contemporaneous radio spectrum (alpha =
-0.25). The data are all consistent with a picture in which the emission from
this source is dominated by a relativistically beamed component in both the
X-ray and radio bands.Comment: 8 pages, TeX, 2 postscript figures; to appear in ApJ Letter
Optical monitoring of the z=4.40 quasar Q 2203+292
We report Cousins R-band monitoring of the high-redshift (z=4.40) radio quiet
quasar Q 2203+292 from May 1999 to October 2007. The quasar shows maximum
peak-to-peak light curve amplitude of ~0.3 mag during the time of our
monitoring, and ~0.9 mag when combined with older literature data. The rms of a
fit to the light curve with a constant is 0.08 mag and 0.2 mag, respectively.
The detected changes are at ~3-sigma level. The quasar was in a stable state
during the recent years and it might have undergone a brightening event in the
past. The structure function analysis concluded that the object shows
variability properties similar to those of the lower redshift quasars. We set a
lower limit to the Q 2203+292 broad line region mass of 0.3-0.4 M_odot.
Narrow-band imaging search for redshifted Ly_alpha from other emission line
objects at the same redshift shows no emission line objects in the quasar
vicinity.Comment: 9 pages, 8 figures, accepted for publication in MNRA
ESO for GOODS' sake
Currently public ESO data sets pertinent to the CDFS/GOODS field are briefly
illustrated along with an indication on how to get access to them. Future ESO
plans for complementing the GOODS database with optical/IR imaging and optical
spectroscopy are also described.Comment: 6 pages, 2 figures, to appear in the proceedings of the ESO/USM
Workshop "The Mass of Galaxies at Low and High Redshift" (Venice, Italy,
October 2001), eds. R. Bender and A. Renzin
Red Parkes-Quasars: Evidence for Soft X-ray Absorption
The Parkes Half-Jansky Flat Spectrum Sample contains a large number of
sources with unusually red optical-to-near-infrared continua. If this is to be
interpreted as extinction by dust in the line-of-sight, then associated
material might also give rise to absorption in the soft X-ray regime. This
hypothesis is tested using broadband (0.1-2.4 keV) data from the {\it ROSAT}
All-Sky Survey provided by Siebert et al. (1998). Significant (
confidence level) correlations between optical (and near-infrared)--to--soft
X-ray continuum slope and optical extinction are found in the data, consistent
with absorption by material with metallicity and a range in gas-to-dust ratio
as observed in the local ISM. Under this simple model, the soft X-rays are
absorbed at a level consistent with the range of extinctions (
magnitudes) implied by the observed optical reddening. Excess X-ray absorption
by warm (ionised) gas, (ie. a `warm absorber') is not required.Comment: 23 pages of text, 3 figures, to appear in Jan 10 (1999) issue of The
Astrophysical Journa
Exact quantization of a PT-symmetric (reversible) Li\'enard-type nonlinear oscillator
We carry out an exact quantization of a PT symmetric (reversible) Li\'{e}nard
type one dimensional nonlinear oscillator both semiclassically and quantum
mechanically. The associated time independent classical Hamiltonian is of
non-standard type and is invariant under a combined coordinate reflection and
time reversal transformation. We use von Roos symmetric ordering procedure to
write down the appropriate quantum Hamiltonian. While the quantum problem
cannot be tackled in coordinate space, we show how the problem can be
successfully solved in momentum space by solving the underlying Schr\"{o}dinger
equation therein. We obtain explicitly the eigenvalues and eigenfunctions (in
momentum space) and deduce the remarkable result that the spectrum agrees
exactly with that of the linear harmonic oscillator, which is also confirmed by
a semiclassical modified Bohr-Sommerfeld quantization rule, while the
eigenfunctions are completely different.Comment: 10 pages, 1 figure, Fast Track Communicatio
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