989 research outputs found
Adaptive Filters Revisited - RFI Mitigation in pulsar observations
Pulsar detection and timing experiments are applications where adaptive
filters seem eminently suitable tools for radio-frequency-interference (RFI)
mitigation. We describe a novel variant which works well in field trials of
pulsar observations centred on an observing frequency of 675 MHz, a bandwidth
of 64 MHz and with 2-bit sampling. Adaptive filters have generally received bad
press for RFI mitigation in radio astronomical observations with their most
serious drawback being a spectral echo of the RFI embedded in the filtered
signals. Pulsar observations are intrinsically less sensitive to this as they
operate in the (pulsar period) time domain. The field trials have allowed us to
identify those issues which limit the effectiveness of the adaptive filter. We
conclude that adaptive filters can significantly improve pulsar observations in
the presence of RFI.Comment: Accepted for publication in Radio Scienc
A z=0.9 supercluster of X-ray luminous, optically-selected, massive galaxy clusters
We report the discovery of a compact supercluster structure at z=0.9. The
structure comprises three optically-selected clusters, all of which are
detected in X-rays and spectroscopically confirmed to lie at the same redshift.
The Chandra X-ray temperatures imply individual masses of ~5x10^14 Msun. The
X-ray masses are consistent with those inferred from optical--X-ray scaling
relations established at lower redshift. A strongly-lensed z~4 Lyman break
galaxy behind one of the clusters allows a strong-lensing mass to be estimated
for this cluster, which is in good agreement with the X-ray measurement.
Optical spectroscopy of this cluster gives a dynamical mass in good agreement
with the other independent mass estimates. The three components of the
RCS2319+00 supercluster are separated from their nearest neighbor by a mere <3
Mpc in the plane of the sky and likely <10 Mpc along the line-of-sight, and we
interpret this structure as the high-redshift antecedent of massive (~10^15
Msun) z~0.5 clusters such as MS0451.5-0305.Comment: ApJ Letters accepted. 5 pages in emulateapj, 3 figure
Correlations of Richness and Global Properties in Galaxy Clusters
We measure the optical richness of galaxy clusters from the CNOC1 cluster
redshift survey using the galaxy-cluster center correlation amplitude B_gc. We
show that the B_gc values measured using photometric catalogs are consistent
with those derived from redshift catalogs, indicating that richness can be
measured reliably from photometric data alone, even at moderate redshifts of
\~0.6. We establish the correlations between optical richness and other
important attributes of a galaxy cluster, such as velocity dispersion, mass,
radius, and X-ray temperature and luminosity. We find that the scaling
relations of these quantities with richness are entirely consistent with those
derived by assuming a simple mass density profile at 0.5 Mpc of rho~r^{-1.8}.
The excellent correlations between B_gc and velocity dispersion and X-ray
temperature allow one to use richness, an easily measurable quantity using
relatively shallow optical imaging data alone, as a predictor of these
quantities at moderate redshifts. The B_gc parameter can be used to estimate
the velocity dispersion of a cluster to a precision of approximately 15%
(~+/-100 km/s), and X-ray temperature to about 20%. Similar correlations, but
with larger scatter, are also obtained between richness and the characteristic
radius and mass of the clusters. We compare the relative merits of B_gc, T_x,
and L_x as predictors of the dynamical mass, and find that they are comparable,
providing estimates at an accuracy of ~30%. We also perform similar analyses of
correlations between richness and velocity dispersion, T_x and L_x with a
sample of low-redshift Abell clusters and find consistent results, but with
larger scatter, which may be the result of a less homogeneous database, or
sample-dependent effects.Comment: 18 pages. Accepted; to appear in ApJ, March 200
A morphological algorithm for improving radio-frequency interference detection
A technique is described that is used to improve the detection of
radio-frequency interference in astronomical radio observatories. It is applied
on a two-dimensional interference mask after regular detection in the
time-frequency domain with existing techniques. The scale-invariant rank (SIR)
operator is defined, which is a one-dimensional mathematical morphology
technique that can be used to find adjacent intervals in the time or frequency
domain that are likely to be affected by RFI. The technique might also be
applicable in other areas in which morphological scale-invariant behaviour is
desired, such as source detection. A new algorithm is described, that is shown
to perform quite well, has linear time complexity and is fast enough to be
applied in modern high resolution observatories. It is used in the default
pipeline of the LOFAR observatory.Comment: Accepted for publication in A&
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Diffusion MR Characteristics Following Concurrent Radiochemotherapy Predicts Progression-Free and Overall Survival in Newly Diagnosed Glioblastoma.
The standard of care for newly diagnosed glioblastoma (GBM) is surgery, then radiotherapy (RT) with concurrent temozolomide (TMZ), followed by adjuvant TMZ. We hypothesized patients with low diffusivity measured using apparent diffusion coefficient (ADC) histogram analysis evaluated after RT+TMZ, prior to adjuvant TMZ, would have a significantly shorter progression-free (PFS) and overall survival (OS). To test this hypothesis we evaluated 120 patients with newly diagnosed GBM receiving RT+TMZ followed by adjuvant TMZ. MRI was performed after completion of RT+TMZ, prior to initiation of adjuvant TMZ. A double Gaussian mixed model was used to describe the ADC histograms within the enhancing tumor, where ADCL and ADCH were defined as the mean ADC value of the lower and higher Gaussian distribution, respectively. An ADCL value of 1.0 um2/ms and ADCH value of 1.6 um2/ms were used to stratify patients into high and low risk categories. Results suggest patients with low ADCL had significantly shorter PFS (Cox Hazard Ratio = 0.12, P = 0.0006). OS was significantly shorter with low ADCL tumors, showing a median OS of 407 vs. 644 days (Cox Hazard Ratio = 0.31, P = 0.047). ADCH was not predictive of PFS or OS when accounting for age and ADCL. In summary, newly diagnosed glioblastoma patients with low ADCL after completion of RT+TMZ are likely to progress and die earlier than patients with higher ADCL. Results suggest ADC histogram analysis may be useful for patient risk stratification following completion of RT+TMZ
Validation of vessel size imaging (VSI) in high-grade human gliomas using magnetic resonance imaging, image-guided biopsies, and quantitative immunohistochemistry.
To evaluate the association between a vessel size index (VSIMRI) derived from dynamic susceptibility contrast (DSC) perfusion imaging using a custom spin-and-gradient echo echoplanar imaging (SAGE-EPI) sequence and quantitative estimates of vessel morphometry based on immunohistochemistry from image-guided biopsy samples. The current study evaluated both relative cerebral blood volume (rCBV) and VSIMRI in eleven patients with high-grade glioma (7 WHO grade III and 4 WHO grade IV). Following 26 MRI-guided glioma biopsies in these 11 patients, we evaluated tissue morphometry, including vessel density and average radius, using an automated procedure based on the endothelial cell marker CD31 to highlight tumor vasculature. Measures of rCBV and VSIMRI were then compared to histological measures. We demonstrate good agreement between VSI measured by MRI and histology; VSIMRI = 13.67 μm and VSIHistology = 12.60 μm, with slight overestimation of VSIMRI in grade III patients compared to histology. rCBV showed a moderate but significant correlation with vessel density (r = 0.42, p = 0.03), and a correlation was also observed between VSIMRI and VSIHistology (r = 0.49, p = 0.01). The current study supports the hypothesis that vessel size measures using MRI accurately reflect vessel caliber within high-grade gliomas, while traditional measures of rCBV are correlated with vessel density and not vessel caliber
The Radio Sky at Meter Wavelengths: m-Mode Analysis Imaging with the Owens Valley Long Wavelength Array
A host of new low-frequency radio telescopes seek to measure the 21-cm
transition of neutral hydrogen from the early universe. These telescopes have
the potential to directly probe star and galaxy formation at redshifts , but are limited by the dynamic range they can achieve
against foreground sources of low-frequency radio emission. Consequently, there
is a growing demand for modern, high-fidelity maps of the sky at frequencies
below 200 MHz for use in foreground modeling and removal. We describe a new
widefield imaging technique for drift-scanning interferometers,
Tikhonov-regularized -mode analysis imaging. This technique constructs
images of the entire sky in a single synthesis imaging step with exact
treatment of widefield effects. We describe how the CLEAN algorithm can be
adapted to deconvolve maps generated by -mode analysis imaging. We
demonstrate Tikhonov-regularized -mode analysis imaging using the Owens
Valley Long Wavelength Array (OVRO-LWA) by generating 8 new maps of the sky
north of with 15 arcmin angular resolution, at frequencies
evenly spaced between 36.528 MHz and 73.152 MHz, and 800 mJy/beam thermal
noise. These maps are a 10-fold improvement in angular resolution over existing
full-sky maps at comparable frequencies, which have angular resolutions . Each map is constructed exclusively from interferometric observations
and does not represent the globally averaged sky brightness. Future
improvements will incorporate total power radiometry, improved thermal noise,
and improved angular resolution -- due to the planned expansion of the OVRO-LWA
to 2.6 km baselines. These maps serve as a first step on the path to the use of
more sophisticated foreground filters in 21-cm cosmology incorporating the
measured angular and frequency structure of all foreground contaminants.Comment: 27 pages, 18 figure
Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots
We examine the multiple exciton population dynamics in PbS quantum dots by
ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We
simultaneously probe the first three excitonic transitions over a broad
spectral range. Transient spectra show the presence of first order bleach of
absorption for the 1S_h-1S_e transition and second order bleach along with
photoinduced absorption band for 1P_h-1P_e transition. We also report evidence
of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures
of carrier multiplication (multiexcitons for the single absorbed photon) from
analysis of the first and second order bleaches, in the limit of low absorbed
photon numbers (~ 10^-2), at pump energies from two to four times the
semiconductor band gap. The multiexciton generation efficiency is discussed
both in terms of a broadband global fit and the ratio between early- to
long-time transient absorption signals.. Analysis of population dynamics shows
that the bleach peak due to the biexciton population is red-shifted respect the
single exciton one, indicating a positive binding energy.Comment: 16 pages, 5 figure
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