470 research outputs found
On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy
We compare the photometric accuracy of spectra and images in flares observed
with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI)}spacecraft. We
test the accuracy of the photometry by comparing the photon fluxes obtained in
different energy ranges from the spectral-fitting software SPEX with those
fluxes contained in the images reconstructed with the Clean, MEM, MEM-Vis,
Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the
fidelity of source geometries, and spatial spectra reconstructed with the five
image reconstruction algorithms. We investigate the effects of grid selection,
pixel size, field-of-view, and time intervals on the quality of image
reconstruction. The detailed parameters and statistics are provided in an
accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean
have a robust convergence behavior and a photometric accuracy in the order of a
few percents, while MEM does not converge optimally for large degrees of
freedom (for large field-of-views and/or small pixel sizes), and MEM-Vis
suffers in the case of time-variable sources. This comparative study documents
the current status of the RHESSI spectral and imaging software, one year after
launch.Comment: 2 Figures, full version on
http://www.lmsal.com/~aschwand/eprints/2003_photo/index.htm
The Focusing Optics X-ray Solar Imager (FOXSI)
FOXSI is a direct-imaging, hard X-ray (HXR) telescope optimized for solar
flare observations. It detects hot plasma and energetic electrons in and near
energy release sites in the solar corona via bremsstrahlung emission, measuring
both spatial structure and particle energy distributions. It provides two
orders of magnitude faster imaging spectroscopy than previously available,
probing physically relevant timescales (<1s) never before accessible to address
fundamental questions of energy release and efficient particle acceleration
that have importance far beyond their solar application (e.g., planetary
magnetospheres, flaring stars, accretion disks). FOXSI measures not only the
bright chromospheric X-ray emission where electrons lose most of their energy,
but also simultaneous emission from electrons as they are accelerated in the
corona and propagate along magnetic field lines. FOXSI detects emission from
high in the tenuous corona, where previous instruments have been blinded by
nearby bright features and will fully characterizes the accelerated electrons
and hottest plasmas as they evolve in energy, space, and time to solve the
mystery of how impulsive energy release leads to solar eruptions, the primary
drivers of space weather at Earth, and how those eruptions are energized and
evolve.Comment: White paper submitted to the Decadal Survey for Solar and Space
Physics (Heliophysics) 2024-2033; 14 pages, 4 figures, 1 tabl
Chromospheric Jet and Growing "Loop" Observed by Hinode: New Evidence of Fan-Spine Magnetic Topology Resulting From Flux Emergence
We present observations of a chromospheric jet and growing "loop" system that
show new evidence of a fan-spine topology resulting from magnetic flux
emergence. This event, occurring in an equatorial coronal hole on 2007 February
9, was observed by the Hinode Solar Optical Telescope in the Ca II H line in
unprecedented detail. The predecessor of the jet is a bundle of fine material
threads that extend above the chromosphere and appear to rotate about the
bundle axis at ~50 km/s (period <200 s). These rotations or transverse
oscillations propagate upward at velocities up to 786 km/s. The bundle first
slowly and then rapidly swings up, with the transition occurring at the onset
of an A4.9 flare. A loop expands simultaneously in these two phases (velocity:
16-135 km/s). Near the peak of the flare, the loop appears to rupture;
simultaneous upward ejecta and mass downflows faster than free-fall appear in
one of the loop legs. The material bundle then swings back in a whiplike manner
and develops into a collimated jet, which is orientated along the inferred open
field lines with transverse oscillations continuing at slower rates. Some
material falls back along smooth streamlines, showing no more oscillations. At
low altitudes, the streamlines bifurcate at presumably a magnetic null point
and bypass an inferred dome, depicting an inverted-Y geometry. These
streamlines closely match in space the late Ca II H loop and X-ray flare loop.
These observations are consistent with the model that flux emergence in an
open-field region leads to magnetic reconnection, forming a jet and fan-spine
topology. We propose that the material bundle and collimated jet represent the
outer spine in quasi-static and eruptive stages, respectively, and the growing
loop is a 2D projection of the 3D fan surface.Comment: Accepted by ApJ, 2010 December 8, 16 pages, 15 figures, 2 tables;
followup study of an earlier ApJ Letter,
http://adsabs.harvard.edu/abs/2009ApJ...707L..37
Fundamentals of impulsive energy release in the corona
It is essential that there be coordinated and co-optimized observations in
X-rays, gamma-rays, and EUV during the peak of solar cycle 26 (~2036) to
significantly advance our understanding of impulsive energy release in the
corona. The open questions include: What are the physical origins of
space-weather events? How are particles accelerated at the Sun? How is
impulsively released energy transported throughout the solar atmosphere? How is
the solar corona heated? Many of the processes involved in triggering, driving,
and sustaining solar eruptive events -- including magnetic reconnection,
particle acceleration, plasma heating, and energy transport in magnetized
plasmas -- also play important roles in phenomena throughout the Universe. This
set of observations can be achieved through a single flagship mission or, with
foreplanning, through a combination of major missions (e.g., the previously
proposed FIERCE mission concept).Comment: White paper submitted to the Decadal Survey for Solar and Space
Physics (Heliophysics) 2024-2033; 5 pages, 1 figur
Temporal Variations of Skin Pigmentation in C57Bl/6 Mice Affect Optical Bioluminescence Quantitation
ABSTRACT
PURPOSE: Depilation-induced skin pigmentation in C57Bl/6 mice is a known occurrence, and presents a unique problem for quantitative optical imaging of small animals, especially for bioluminescence. The work reported here quantitatively investigated the optical attenuation of bioluminescent light due to melanin pigmentation in the skin of transgenic C57B1/6 mice, modified such that luciferase expression is under the transcription control of a physiologically and pharmacologically inducible gene.
PROCEDURE: Both in vivo and ex vivo experiments were performed to track bioluminescence signal attenuation through different stages of the mouse hair growth cycle. Simultaneous reflectance measurements were collected in vivo to estimate melanin levels.
RESULTS: Biological variability of skin pigmentation was found to dramatically affect collected bioluminescent signal emerging through the skin of the mice. When compared to signal through skin with no pigmentation, the signal through highly-pigmented skin was attenuated an average of 90%. Correlation of reflectance signals to bioluminescence signal loss forms the basis of the proposed correction method. We observed, however, that variability in tissue composition, which results in inconsistent reflectance spectra, limits the accuracy of the correction method but can be improved by incorporating more complex analysis.
CONCLUSION: Skin pigmentation is a significant variable in bioluminescent imaging, and should be considered in experimental design and implementation for longitudinal studies, and especially when sensitivity to small signal changes, or differences among animals, is required
Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV
The performance of muon reconstruction, identification, and triggering in CMS
has been studied using 40 inverse picobarns of data collected in pp collisions
at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection
criteria covering a wide range of physics analysis needs have been examined.
For all considered selections, the efficiency to reconstruct and identify a
muon with a transverse momentum pT larger than a few GeV is above 95% over the
whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4,
while the probability to misidentify a hadron as a muon is well below 1%. The
efficiency to trigger on single muons with pT above a few GeV is higher than
90% over the full eta range, and typically substantially better. The overall
momentum scale is measured to a precision of 0.2% with muons from Z decays. The
transverse momentum resolution varies from 1% to 6% depending on pseudorapidity
for muons with pT below 100 GeV and, using cosmic rays, it is shown to be
better than 10% in the central region up to pT = 1 TeV. Observed distributions
of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO
X-ray emission from the Sombrero galaxy: discrete sources
We present a study of discrete X-ray sources in and around the
bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival
Chandra observations with a total exposure of ~200 ks. With a detection limit
of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30
kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler
et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS
observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray
binaries (LMXBs). We quantify the differential luminosity functions (LFs) for
both the detected GC and field LMXBs, whose power-low indices (~1.1 for the
GC-LF and ~1.6 for field-LF) are consistent with previous studies for
elliptical galaxies. With precise sky positions of the GCs without a detected
X-ray source, we further quantify, through a fluctuation analysis, the GC LF at
fainter luminosities down to 1E35 erg/s. The derived index rules out a
faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent
findings in several elliptical galaxies and the bulge of M31. On the other
hand, the 2-6 keV unresolved emission places a tight constraint on the field
LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101
sources in the halo of Sombrero. The presence of these sources cannot be
interpreted as galactic LMXBs whose spatial distribution empirically follows
the starlight. Their number is also higher than the expected number of cosmic
AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray
surveys. We suggest that either the cosmic X-ray background is unusually high
in the direction of Sombrero, or a distinct population of X-ray sources is
present in the halo of Sombrero.Comment: 11 figures, 5 tables, ApJ in pres
Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV
The performance of muon reconstruction, identification, and triggering in CMS
has been studied using 40 inverse picobarns of data collected in pp collisions
at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection
criteria covering a wide range of physics analysis needs have been examined.
For all considered selections, the efficiency to reconstruct and identify a
muon with a transverse momentum pT larger than a few GeV is above 95% over the
whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4,
while the probability to misidentify a hadron as a muon is well below 1%. The
efficiency to trigger on single muons with pT above a few GeV is higher than
90% over the full eta range, and typically substantially better. The overall
momentum scale is measured to a precision of 0.2% with muons from Z decays. The
transverse momentum resolution varies from 1% to 6% depending on pseudorapidity
for muons with pT below 100 GeV and, using cosmic rays, it is shown to be
better than 10% in the central region up to pT = 1 TeV. Observed distributions
of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO
Azimuthal anisotropy of charged particles at high transverse momenta in PbPb collisions at sqrt(s[NN]) = 2.76 TeV
The azimuthal anisotropy of charged particles in PbPb collisions at
nucleon-nucleon center-of-mass energy of 2.76 TeV is measured with the CMS
detector at the LHC over an extended transverse momentum (pt) range up to
approximately 60 GeV. The data cover both the low-pt region associated with
hydrodynamic flow phenomena and the high-pt region where the anisotropies may
reflect the path-length dependence of parton energy loss in the created medium.
The anisotropy parameter (v2) of the particles is extracted by correlating
charged tracks with respect to the event-plane reconstructed by using the
energy deposited in forward-angle calorimeters. For the six bins of collision
centrality studied, spanning the range of 0-60% most-central events, the
observed v2 values are found to first increase with pt, reaching a maximum
around pt = 3 GeV, and then to gradually decrease to almost zero, with the
decline persisting up to at least pt = 40 GeV over the full centrality range
measured.Comment: Replaced with published version. Added journal reference and DO
Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy
A search for new physics is performed in events with two same-sign isolated
leptons, hadronic jets, and missing transverse energy in the final state. The
analysis is based on a data sample corresponding to an integrated luminosity of
4.98 inverse femtobarns produced in pp collisions at a center-of-mass energy of
7 TeV collected by the CMS experiment at the LHC. This constitutes a factor of
140 increase in integrated luminosity over previously published results. The
observed yields agree with the standard model predictions and thus no evidence
for new physics is found. The observations are used to set upper limits on
possible new physics contributions and to constrain supersymmetric models. To
facilitate the interpretation of the data in a broader range of new physics
scenarios, information on the event selection, detector response, and
efficiencies is provided.Comment: Published in Physical Review Letter
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