6,314 research outputs found
Design and fabrication of metallic thermal protection systems for aerospace vehicles
A program was conducted to develop a lightweight, efficient metallic thermal protection system (TPS) for application to future shuttle-type reentry vehicles, advanced space transports, and hypersonic cruise vehicles. Technical requirements were generally derived from the space shuttle. A corrugation-stiffened beaded-skin TPS design was used as a baseline. The system was updated and modified to incorporate the latest technology developments and design criteria. The primary objective was to minimize mass for the total system
BeppoSAX Observations of 1-Jy BL Lacertae Objects - II
We present new BeppoSAX LECS and MECS observations, covering the energy range
0.1 - 10 keV (observer's frame), of four BL Lacertae objects selected from the
1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which
we interpret as inverse Compton emission. One object shows evidence for a
low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the
synchrotron component merging into the inverse Compton one around ~ 2 keV. A
variable synchrotron tail would explain why the ROSAT spectra of our sources
are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The
broad-band spectral energy distributions fully confirm this picture and model
fits using a synchrotron inverse Compton model allow us to derive the physical
parameters (intrinsic power, magnetic field, etc.) of our sources. By combining
the results of this paper with those previously obtained on other sources we
present a detailed study of the BeppoSAX properties of a well-defined
sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s)
1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous
radio and X-ray powers for the 1-Jy sources in which the X-ray band is
dominated by inverse Compton emission, which points to a strong link between
X-ray and radio emission components in these objects.Comment: 13 pages, 6 figures. Accepted for publication in MNRAS. Postscript
file also available at http://www.stsci.edu/~padovani/xrayspectra_papers.htm
Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We
analysed a sample of 20 Swift observations, taken between 2013 April and
August, and two Chandra observations taken in 2001 September and 2004 June. We
found only marginal hints of a limited number of dips in the light curve,
previously reported to occur in this source, although the uncertainties due to
the low counting statistics of the data are large. The Chandra and Swift
spectra showed clearly spectral variability which resembles those observed in
other ULXs. We can account for this spectral variability in terms of changes in
both the normalization and intrinsic column density of a two-components model
consisting of a blackbody (for the soft component) and a multicolour accretion
disc (for the hard component). We discuss the possibility that strong outflows
ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA
Amorphous ferromagnetism and re-entrant magnetic glassiness in SmMoO: new insights into the electronic phase diagram of pyrochlore molybdates
We discuss the magnetic properties of a SmMoO single
crystal as investigated by means of different experimental techniques. In the
literature, a conventional itinerant ferromagnetic state is reported for the
Mo sublattice below K. However, our results of dc
magnetometry, muon spin spectroscopy (SR) and high-harmonics magnetic
ac susceptibility unambiguously evidence highly disordered conditions in this
phase, in spite of the crystalline and chemical order. This disordered magnetic
state shares several common features with amorphous ferromagnetic alloys. This
scenario for SmMoO is supported by the anomalously high
values of the critical exponents, as mainly deduced by a scaling analysis of
our dc magnetization data and confirmed by the other techniques. Moreover,
SR detects a significant static magnetic disorder at the microscopic
scale. At the same time, the critical divergence of the third-harmonic
component of the ac magnetic susceptibility around K leads to
additional evidence towards the glassy nature of this magnetic phase. Finally,
the longitudinal relaxation of spin polarization (also supported by
results of ac susceptibility) evidences re-entrant glassy features similar to
amorphous ferromagnets.Comment: 15 pages, 13 figure
Exploiting tree shadows on snow for estimating forest basal area using Landsat data
Basal area (BA) is a basic structural and ecological attribute of forests that is often used to describe forest composition, estimate volume of wood, and guide management decisions. BA is the sum of cross-sectional area of trees measured at 1.37 m above ground surface, per unit area, and is most commonly measured in-situ. The objective of this study was to supply estimates of BA for oak woodlands and savannas on the 12,828.5 ha Sherburne National Wildlife Refuge in Central Minnesota to guide management efforts. We used winter and summer Landsat imagery, combined with field measurements, to assess the potential for improving forest BA estimates by taking advantage of the high spectral contrast between sunlit snow, forest canopy elements, and shadows projected onto snow ground cover. We explained up to 90% of measured variation in BA using partial least squares regression models calibrated using single- and multiple-date winter Landsat data (R2 = 0.898, RMSE = 2.79 m2ha− 1), which performed better than models calibrated using summer imagery (R2 = 0.762, RMSE = 3.85 m2ha− 1). Success of the winter-based BA models may be driven, in part, by potential geometric/allometric relationships between cast shadow and forest BA, but definitive proof of this is a topic for future research. This method of BA estimation is not refuge-specific and may be extended for regional use to manage oak forest wherever winter snow coverage is consistent. Additional research is needed to determine the degree of robustness to variations in the empirical relationship between BA and tree shading patterns across different forest functional types
A multi-wavelength study of the evolution of Early-Type Galaxies in Groups: the ultraviolet view
ABRIDGED- The UV-optical color magnitude diagram (CMD) of rich galaxy groups
is characterised by a well developed Red Sequence (RS), a Blue Cloud (BC) and
the so-called Green Valley (GV). Loose, less evolved groups of galaxies likely
not virialized yet may lack a well defined RS. This is actually explained in
the framework of galaxy evolution. We are focussing on understanding galaxy
migration towards the RS, checking for signatures of such a transition in their
photometric and morphological properties. We report on the UV properties of a
sample of ETGs galaxies inhabiting the RS. The analysis of their structures, as
derived by fitting a Sersic law to their UV luminosity profiles, suggests the
presence of an underlying disk. This is the hallmark of dissipation processes
that still must have a role in the evolution of this class of galaxies. SPH
simulations with chemo-photometric implementations able to match the global
properties of our targets are used to derive their evolutionary paths through
UV-optical CDM, providing some fundamental information such as the crossing
time through the GV, which depends on their luminosity. The transition from the
BC to the RS takes several Gyrs, being about 3-5 Gyr for the the brightest
galaxies and more long for fainter ones, if it occurs. The photometric study of
nearby galaxy structures in UV is seriously hampered by either the limited FoV
of the cameras (e.g in HST) or by the low spatial resolution of the images (e.g
in the GALEX). Current missions equipped with telescopes and cameras sensitive
to UV wavelengths, such as Swift-UVOT and Astrosat-UVIT, provide a relatively
large FoV and better resolution than the GALEX. More powerful UV instruments
(size, resolution and FoV) are obviously bound to yield fundamental advances in
the accuracy and depth of the surface photometry and in the characterisation of
the galaxy environment.Comment: 12 pages, 6 figures: accepted for publication in Astrophysics & Space
Science as contributions to the workshop: "UV astronomy, the needs and the
means
The two Ultraluminous X-ray sources in the galaxy NGC 925
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC
925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality,
simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first
glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because
of its high X-ray luminosity ( erg s) and its
spectral/temporal features, a closer inspection shows that its properties are
more similar to those of a typical super-Eddington accreting stellar black hole
and we classify it as a `broadened disc' ultraluminous X-ray source. Based on
the physical interpretation of this spectral state, we suggest that ULX-1 is
seen at small inclination angles, possibly through the evacuated cone of a
powerful wind originating in the accretion disc. The spectral classification of
ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington
rates as none of its spectral/temporal properties can be associated to either
the soft or hard state of Galactic accreting black hole binaries.Comment: Accepted on MNRAS with very minor comments, 7 pages, 5 figures, 1
tabl
Optimization of grazing incidence mirrors and its application to surveying X-ray telescopes
Grazing incidence mirrors for X-ray astronomy are usually designed in the
parabola-hyperbola (Wolter I) configuration. This design allows for optimal
images on-axis, which however degrade rapidly with the off-axis angle. Mirror
surfaces described by polynomia (with terms higher than order two), have been
put forward to improve the performances over the field of view. Here we present
a refined procedure aimed at optimizing wide-field grazing incidence telescopes
for X-ray astronomy. We improve the angular resolution over existing
(wide-field) designs by ~ 20%. We further consider the corrections for the
different plate scale and focal plane curvature of the mirror shells, which
sharpen by another ~ 20% the image quality. This results in a factor of ~ 2
reduction in the observing time needed to achieve the same sensitivity over
existing wide-field designs and of ~ 5 over Wolter I telescopes. We demonstrate
that such wide-field X-ray telescopes are highly advantageous for deep surveys
of the X-ray sky.Comment: 8 pages 4 figures. Accepted for publication on A&A (macro included
An important role for Myb-MuvB and its target gene KIF23 in a mouse model of lung adenocarcinoma
The conserved Myb-MuvB (MMB) multiprotein complex has an important role in transcriptional activation of mitotic genes. MMB target genes are overexpressed in several different cancer types and their elevated expression is associated with an advanced tumor state and a poor prognosis. This suggests that MMB could contribute to tumorigenesis by mediating overexpression of mitotic genes. However, although MMB has been extensively characterized biochemically, the requirement for MMB in tumorigenesis in vivo has not been investigated. Here we demonstrate that MMB is required for tumor formation in a mouse model of lung cancer driven by oncogenic K-RAS. We also identify a requirement for the mitotic kinesin KIF23, a key target gene of MMB, in tumorigenesis. RNA interference-mediated depletion of KIF23 inhibited lung tumor formation in vivo and induced apoptosis in lung cancer cell lines. Our results suggest that inhibition of KIF23 could be a strategy for treatment of lung cancer
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