3,505 research outputs found
A High-Resolution Hubble Space Telescope Study of Apparent Lyman Continuum Leakers at
We present follow-up observations of 16
candidate LyC emitters in the HS1549+1919 field. With these data, we
obtain high spatial-resolution photometric redshifts of all sub-arcsecond
components of the LyC candidates in order to eliminate foreground contamination
and identify robust candidates for leaking LyC emission. Of the 16 candidates,
we find one object with a robust LyC detection that is not due to foreground
contamination. This object (MD5) resolves into two components; we refer to the
LyC-emitting component as MD5b. MD5b has an observed 1500\AA\ to 900\AA\
flux-density ratio of , compatible with
predictions from stellar population synthesis models. Assuming minimal IGM
absorption, this ratio corresponds to a relative (absolute) escape fraction of
% (%). The stellar
population fit to MD5b indicates an age of Myr, which is in the
youngest 10% of the sample and the youngest third of typical
Lyman break galaxies, and may be a contributing factor to its LyC detection. We
obtain a revised, contamination-free estimate for the comoving specific
ionizing emissivity at , indicating (with large uncertainties) that
star-forming galaxies provide roughly the same contribution as QSOs to the
ionizing background at this redshift. Our results show that foreground
contamination prevents ground-based LyC studies from obtaining a full
understanding of LyC emission from star-forming galaxies. Future
progress in direct LyC searches is contingent upon the elimination of
foreground contaminants through high spatial-resolution observations, and upon
acquisition of sufficiently deep LyC imaging to probe ionizing radiation in
high-redshift galaxies.Comment: 31 pages, 5 tables, 19 figures. Accepted to ApJ. Version with
full-resolution figures is available at:
http://www.astro.ucla.edu/~aes/Mostardi_HST_LyC.pd
Quasi-Periodic Pulsations during the Impulsive and Decay phases of an X-class Flare
Quasi-periodic pulsations (QPP) are often observed in X-ray emission from
solar flares. To date, it is unclear what their physical origins are. Here, we
present a multi-instrument investigation of the nature of QPP during the
impulsive and decay phases of the X1.0 flare of 28 October 2013. We focus on
the character of the fine structure pulsations evident in the soft X-ray time
derivatives and compare this variability with structure across multiple
wavelengths including hard X-ray and microwave emission. We find that during
the impulsive phase of the flare, high correlations between pulsations in the
thermal and non-thermal emissions are seen. A characteristic timescale of ~20s
is observed in all channels and a second timescale of ~55s is observed in the
non-thermal emissions. Soft X-ray pulsations are seen to persist into the decay
phase of this flare, up to 20 minutes after the non-thermal emission has
ceased. We find that these decay phase thermal pulsations have very small
amplitude and show an increase in characteristic timescale from ~40s up to
~70s. We interpret the bursty nature of the co-existing multi-wavelength QPP
during the impulsive phase in terms of episodic particle acceleration and
plasma heating. The persistent thermal decay phase QPP are most likely
connected with compressive MHD processes in the post-flare loops such as the
fast sausage mode or the vertical kink mode.Comment: 7 pages, 4 figures, 1 tabl
Precipitation changes in a GCM resulting from the indirect effects of anthropogenic aerosols
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94948/1/grl13844.pd
Photoreceptor Inner Segment Morphology in Best Vitelliform Macular Dystrophy
PURPOSE
To characterize outer retina structure in best vitelliform macular dystrophy (BVMD) and to determine the effect of macular lesions on overlying and adjacent photoreceptors.
METHODS
Five individuals with BVMD were followed prospectively with spectral domain optical coherence tomography and confocal and nonconfocal split-detector adaptive optics scanning light ophthalmoscopy (AOSLO). The AOSLO cone photoreceptor mosaic images were obtained within and around retinal lesions. Cone density was measured inside and outside lesions. In 2 subjects, densities were compared with published measurements acquired ∼2.5 years before. One subject was imaged 3 times over a 5-month period.
RESULTS
The AOSLO imaging demonstrated that photoreceptor morphology within BVMD retinal lesions was highly variable depending on the disease stage, with photoreceptor structure present even in advanced disease. The AOSLO imaging was repeatable even in severe disease over short-time and long-time intervals. Photoreceptor density was normal in retinal areas immediately adjacent to lesions and stable over ∼2.5 years. Mobile disk-like structures possibly representing subretinal macrophages were also observed.
CONCLUSION
Combined confocal and nonconfocal split-detector AOSLO imaging reveals substantial variability within clinical lesions in all stages of BVMD. Longitudinal cellular photoreceptor imaging could prove a powerful tool for understanding disease progression and monitoring emerging therapeutic treatment response in inherited degenerations such as BVMD
The Thermal Properties of Solar Flares Over Three Solar Cycles Using GOES X-ray Observations
Solar flare X-ray emission results from rapidly increasing temperatures and
emission measures in flaring active region loops. To date, observations from
the X-Ray Sensor (XRS) onboard the Geostationary Operational Environmental
Satellite (GOES) have been used to derive these properties, but have been
limited by a number of factors, including the lack of a consistent background
subtraction method capable of being automatically applied to large numbers of
flares. In this paper, we describe an automated temperature and emission
measure-based background subtraction method (TEBBS), which builds on the
methods of Bornmann (1990). Our algorithm ensures that the derived temperature
is always greater than the instrumental limit and the pre-flare background
temperature, and that the temperature and emission measure are increasing
during the flare rise phase. Additionally, TEBBS utilizes the improved
estimates of GOES temperatures and emission measures from White et al. (2005).
TEBBS was successfully applied to over 50,000 solar flares occurring over
nearly three solar cycles (1980-2007), and used to create an extensive catalog
of the solar flare thermal properties. We confirm that the peak emission
measure and total radiative losses scale with background subtracted GOES X-ray
flux as power-laws, while the peak temperature scales logarithmically. As
expected, the peak emission measure shows an increasing trend with peak
temperature, although the total radiative losses do not. While these results
are comparable to previous studies, we find that flares of a given GOES class
have lower peak temperatures and higher peak emission measures than previously
reported. The resulting TEBBS database of thermal flare plasma properties is
publicly available on Solar Monitor (www.solarmonitor.org/TEBBS/) and will be
available on Heliophysics Integrated Observatory (www.helio-vo.eu)
Large-area submillimeter resolution CdZnTe strip detector for astronomy
We report the first performance measurements of a sub-millimeter CdZnTe strip detector developed as a prototype for space-borne astronomical instruments. Strip detector arrays can be used to provide two-dimensional position resolution with fewer electronic channels than pixellated arrays. Arrays of this type and other candidate technologies are under investigation for the position-sensitive backplane detector for a coded-aperture telescope operating in the range of 30 - 300 keV. The prototype is a 1.4 mm thick, 64 multiplied by 64 stripe CdZnTe array of 0.375 mm pitch in both dimensions, approximately one square inch of sensitive area. Pulse height spectra in both single and orthogonal stripe coincidence mode were recorded at several energies. The results are compared to slab- and pixel-geometry detector spectra. The room-temperature energy resolution is less than 10 keV (FWHM) for 122 keV photons with a peak-to-valley ratio greater than 5:1. The response to photons with energies up to 662 keV appears to be considerably improved relative to that of previously reported slab and pixel detectors. We also show that strip detectors can yield spatial and energy resolutions similar to those of pixellated arrays with the same dimensions. Electrostatic effects on the pulse heights, read-out circuit complexity, and issues related to design of space borne instruments are also discussed
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Virtual Surgical Planning in Subscapular System Free Flap Reconstruction of Midface Defects.
OBJECTIVES: Reconstruction of the midface has many inherent challenges, including orbital support, skull base reconstruction, optimizing midface projection, separation of the nasal cavity and dental rehabilitation. Subscapular system free flaps (SF) have sufficient bone stock to support complex reconstruction and the option of separate soft tissue components. This study analyzes the effect of virtual surgical planning (VSP) in SF for midface on subsite reconstruction, bone segment contact and anatomic position.
MATERIALS AND METHODS: Retrospective cohort of patients with midface defects that underwent SF reconstruction at a single tertiary care institution.
RESULTS: Nine cases with VSP were compared to fourteen cases without VSP. VSP was associated with a higher number of successfully reconstructed subunits (5.9 vs 4.2, 95% CI of mean difference 0.31-3.04, p = 0.018), a higher number of successful bony contact between segments (2.2 vs 1.4, 95% CI of mean difference 0.0-1.6, p = 0.050), and a higher percent of segments in anatomic position (100% vs 71%, 95% CI of mean difference 2-55%, p = 0.035). When postoperative bone position after VSP reconstruction was compared to preoperative scans, the difference in anteroposterior, vertical and lateral projection compared to the preoperative \u27ideal\u27 bone position was82% of measurements. There were no flap losses.
CONCLUSION: VSP may augment SF reconstruction of the midface by allowing for improved subunit reconstruction, bony segment contact and anatomically correct bone segment positioning. VSP can be a useful adjunct for complex midface reconstruction and the benefits should be weighed against cost
Development of an orthogonal-stripe CdZnTe gamma radiation imaging spectrometer
We report performance measurements of a sub-millimeter resolution CdZnTe strip detector developed as a prototype for astronomical instruments operating with good efficiency in the 30-300 keV photon energy range. The prototype is a 1.4 mm thick, 64×64 contact stripe CdZnTe array of 0.375 mm pitch in both dimensions. Pulse height spectra were recorded in orthogonal-stripe coincidence mode which demonstrate room-temperature energy resolution \u3c10 keV (FWHM) for 122 keV photons with a peak-to-valley ratio \u3e5:1. Good response is also demonstrated at higher energies using a coplanar grid readout configuration. Spatial resolution capabilities finer than the stripe pitch are demonstrated. We present the image of a 133Ba source viewed through a collimator slit produced by a 4×4 stripe detector segment. Charge signals from electron and hole collecting contacts are also discussed
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