3,281 research outputs found
The Spatial Frequency Content of Urban and Indoor Environments as a Potential Risk Factor for Myopia Development
To examine the hypothesis that the spatial frequency spectra of urban and indoor environments differ from the natural environment in ways that may promote the development of myopia. Methods: A total of 814 images were analyzed from three datasets; University of California Berkeley (UCB), University of Texas (UT), and Botswana (UPenn). Images were processed in Matlab (Mathworks Inc) to map the camera color characteristics to human cone sensitivities. From the photopic luminance images generated, two-dimensional spatial frequency (SF) spectra were calculated and converted to one-dimensional spectra by rotational averaging. The spatial filtering profile of a 0.4 Bangerter foil, which has been shown to induce myopia experimentally, was also determined. Results: The SF slope for natural scenes followed the recognized 1/fα relationship with mean slopes of -1.08, -0.90, and -1.04 for the UCB, UT and UPenn image sets, respectively. Indoor scenes had a significantly steeper slope (-1.48, UCB; -1.52, UT; P \u3c 0.0001). Urban environments showed an intermediate slope (-1.29, UCB; -1.22, UT) that was significantly different from the slopes derived from the natural scenes (P \u3c 0.0001). The change in SF content between natural outdoor scenes and indoors was comparable to that induced by a 0.4 Bangerter foil, which reduced the SF slope of a natural scene from -0.88 to -1.47. Conclusions: Compared to natural outdoor images, man-made outdoor and indoor environments have spatial frequency characteristics similar to those known to induce form-deprivation myopia in animal models. The spatial properties of the man-made environment may be one of the missing drivers of the human myopia epidemic
Multiwavelength study of RX J2015.6+3711: a magnetic cataclysmic variable with a 2-hr spin period
The X-ray source RX J2015.6+3711 was discovered by ROSAT in 1996 and recently
proposed to be a cataclysmic variable (CV). Here we report on an XMM-Newton
observation of RX J2015.6+3711 performed in 2014, where we detected a coherent
X-ray modulation at a period of 7196+/-11 s, and discovered other significant
(>6sigma) small-amplitude periodicities which we interpret as the CV spin
period and the sidebands of a possible ~12 hr periodicity, respectively. The
0.3-10 keV spectrum can be described by a power law (Gamma = 1.15+/-0.04) with
a complex absorption pattern, a broad emission feature at 6.60+/-0.01 keV, and
an unabsorbed flux of (3.16+/-0.05)x10^{-12} erg/s/cm^2. We observed a
significant spectral variability along the spin phase, which can be ascribed
mainly to changes in the density of a partial absorber and the power law
normalization. Archival X-ray observations carried out by the Chandra
satellite, and two simultaneous X-ray and UV/optical pointings with Swift,
revealed a gradual fading of the source in the soft X-rays over the last 13
years, and a rather stable X-ray-to-optical flux ratio (F_X/F_V ~1.4-1.7).
Based on all these properties, we identify this source with a magnetic CV, most
probably of the intermediate polar type. The 2 hr spin period makes RX
J2015.6+3711 the second slowest rotator of the class, after RX J0524+4244
("Paloma", P_spin~2.3 hr). Although we cannot unambiguously establish the true
orbital period with these observations, RX J2015.6+3711 appears to be a key
system in the evolution of magnetic CVs.Comment: 11 pages, 8 figures, accepted for publication on MNRA
SenseCluster for exploring large data repositories
Exploring and making sense of large data repositories has become a daunting task. This is especially the case for end users who often have limited access to the data due to the complexity of the retrieval process and limited availability of IT support for developing custom queries and reports based on the data. Consequently, traditional interfaces are no longer meeting these requirements. Instead, novel interfaces are required to fully support the sense making process. In this paper, we followed a design science approach and introduced a query clustering system (Sense Cluster) that could serve as a quick exploration tool for making better sense of large data repositories. We also present an evaluation of the effectiveness of our artifact using cognitive walkthroughs
Chandra Confirmation of a Pulsar Wind Nebula in DA 495
As part of a multiwavelength study of the unusual radio supernova remnant DA
495, we present observations made with the Chandra X-ray Observatory. Imaging
and spectroscopic analysis confirms the previously detected X-ray source at the
heart of the annular radio nebula, establishing the radiative properties of two
key emission components: a soft unresolved source with a blackbody temperature
of 1 MK consistent with a neutron star, surrounded by a nonthermal nebula 40''
in diameter exhibiting a power-law spectrum with photon index Gamma =
1.6+/-0.3, typical of a pulsar wind nebula. The implied spin-down luminosity of
the neutron star, assuming a conversion efficiency to nebular flux appropriate
to Vela-like pulsars, is ~10^{35} ergs/s, again typical of objects a few tens
of kyr old. Morphologically, the nebular flux is slightly enhanced along a
direction, in projection on the sky, independently demonstrated to be of
significance in radio polarization observations; we argue that this represents
the orientation of the pulsar spin axis. At smaller scales, a narrow X-ray
feature is seen extending out 5'' from the point source, a distance consistent
with the sizes of resolved wind termination shocks around many Vela-like
pulsars. Finally, we argue based on synchrotron lifetimes in the estimated
nebular magnetic field that DA 495 represents a rare pulsar wind nebula in
which electromagnetic flux makes up a significant part, together with particle
flux, of the neutron star's wind, and that this high magnetization factor may
account for the nebula's low luminosity.Comment: 26 pages, 5 figures, AASTeX preprint style. Accepted for publication
in The Astrophysical Journa
Detection of thermal X-ray emission in the halo of the plerionic supernova remnant G21.5-0.9
The detection of a soft thermal X-ray component in the spectrum of a bright
knot in the halo of the plerion G21.5-0.9 is reported. Using a collisional
ionization equilibrium model for an hot optically thin plasma, a temperature
kev, a mass of 0.3--1.0 M and a density of 1.6-6
cm is derived. The spectral analysis suggests a possible overabundance
of Silicon with respect to the solar value in the knot; if this will be
confirmed this object may be a clump of shocked ejecta.Comment: 6 pages, 4 figures, Adv.Sp.Res. in press, proc of COSPAR Session E1.4
"Young Neutron Stars and Supernova Remnants",
http://www.astropa.unipa.it/Library/OAPA_preprints/ns.ps.g
Wave Function Microscopy of Quasibound Atomic States
In the 1980s Demkov, Kondratovich, and Ostrovsky and Kondratovich and
Ostrovsky proposed an experiment based on the projection of slow electrons
emitted by a photoionized atom onto a position-sensitive detector. In the case
of resonant excitation, they predicted that the spatial electron distribution
on the detector should represent nothing else but a magnified image of the
projection of a quasibound electronic state. By exciting lithium atoms in the
presence of a static electric field, we present in this Letter the first
experimental photoionization wave function microscopy images where signatures
of quasibound states are evident. Characteristic resonant features, such as
(i) the abrupt change of the number of wave function nodes across a resonance
and (ii) the broadening of the outer ring of the image (associated with
tunneling ionization), are observed and interpreted via wave packet
propagation simulations and recently proposed resonance tunneling mechanisms.
The electron spatial distribution measured by our microscope is a direct
macroscopic image of the projection of the microscopic squared modulus of the
electron wave that is quasibound to the atom and constitutes the first
experimental realization of the experiment proposed 30 years ago
The Deepest Radio Study of the Pulsar Wind Nebula G21.5-0.9: Still No Evidence for the Supernova Shell
We report on sensitive new 1.4-GHz VLA radio observations of the pulsar wind
nebula G21.5-0.9, powered by PSR J1833-1034, and its environs. Our observations
were targeted at searching for the radio counterpart of the shell-like
structure seen surrounding the pulsar wind nebula in X-rays. Some such radio
emission might be expected as the ejecta from the <~ 1000 yr old supernova
expand and interact with the surrounding medium. We find, however, no radio
emission from the shell, and can place a conservative 3-sigma upper limit on
its 1-GHz surface brightness of 7 x 10^-22 W/m^2/Hz/sr, comparable to the
lowest limits obtained for radio emission from shells around other pulsar-wind
nebulae. Our widefield radio image also shows the presence of two extended
objects of low-surface brightness. We re-examine previous 327-MHz images, on
which both the new objects are visible. We identify the first, G21.64-0.84, as
a new shell-type supernova remnant, with a diameter of ~13' and an unusual
double-shell structure. The second, G21.45-0.59, ~1' in diameter, is likely an
HII region.Comment: 8 Pages, submitted to MNRA
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