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81.
Patrizia A. Caraveo Giovanni F. Bignami Joachim E. Trümper 《Astronomy and Astrophysics Review》1996,7(3):209-216
Summary. As of today, seven X-ray sources have been tentatively identified as radio-quiet, isolated neutron stars. The family appears
to be a rapidly growing one, although not all the objects have been identified with the same degree of certainty. The most
convincing example of radio quiet pulsar is certainly Geminga, the neutron star nature of which, proposed in 1983 on the basis
of its similarity with the Vela pulsar, has been firmly established with the discovery of its X and pulsation. Four more neutron star candidates, originally found in the Einstein data, have been confirmed by ROSAT, which
has added to the list two more entries. All this is not the result of an unbiased search. The seven sources were not selected
at random: four are inside supernova remnants, an obvious place to search for isolated neutron stars, while the remaining
three were singled out because of some peculiarity. Intense -ray emission in the case of Geminga, very high X-ray counting rate for RXJ185635-3754, or being the brightest unidentified
source in the Einstein medium sensitivity survey, MS 0317-6647. In spite of the limited number of objects and of the observational
biases, these seven radio quiet neutron star candidates add valuable pieces of information to the observational panorama of
known pulsars. Their properties, inferred from the X-ray emission, offer a coherent picture, pointing towards thermally emitting,
cooling neutron stars.
Received: April 1, 1996 相似文献
82.
83.
Salvatore Erminio Modoni Giuseppe Spagnoli Giovanni Arciero Michela Mascolo Maria Cristina Ochmański Maciej 《Acta Geotechnica》2022,17(11):5073-5087
Acta Geotechnica - Plasticity of clays makes Deep Soil Mixing (DSM) problematic due to the tendency of the material to congest the rotating blades, reduce mixing efficiency and remain... 相似文献
84.
Elastic collisions in ion irradiation experiments: A mechanism for space weathering of silicates 总被引:1,自引:0,他引:1
Ion irradiation experiments have been performed on silicates (bulk samples) rich of olivine, pyroxene, and serpentine to simulate the effects of space weathering induced on asteroids by solar wind ions. We have used different ions (H+, He+, Ar+, Ar2+) having different energies (from 60 to 400 keV) to weather the samples, probed by Raman spectroscopy and UV-vis-NIR reflectance spectroscopy. All the irradiated materials have shown reddening and darkening of reflectance spectra in the 0.25-2.7 μm spectral range. We have found that the increase of the spectral slope of the continuum across the 1-μm band is strongly related with the number of displacements caused by colliding ions because of elastic collisions with the target nuclei. The spectral slopes have been compared, at increasing ion fluence, with those from irradiated Epinal meteorite. We show that formation of nuclear displacements by solar wind ion irradiation is a physical mechanism that reddens the asteroidal surfaces on a time-scale lower than 106 years. 相似文献
85.
Magnetic secular variation is recorded as a quasi-rhythmical process of uncertain origin. Rhythmic changes recorded in the Scisti a Fucoidi in the central Apennines of Italy, pelagic sediments of Mid-Cretaceous (Late Albian) age, show strong correspondence to patterns of orbital variation. In the same strata cyclic magnetic variations are found with peaks in the same range (105, 40, 26,19 kyr). These results appear to support earlier suggestions that secular variations may be related to these astronomical patterns. 相似文献
86.
Diffuse CO<Subscript>2</Subscript> degassing at Vesuvio,Italy 总被引:1,自引:0,他引:1
Francesco?FrondiniEmail author Giovanni?Chiodini Stefano?Caliro Carlo?Cardellini Domenico?Granieri Guido?Ventura 《Bulletin of Volcanology》2004,66(7):642-651
At Vesuvio, a significant fraction of the rising hydrothermal–volcanic fluids is subjected to a condensation and separation process producing a CO2–rich gas phase, mainly expulsed through soil diffuse degassing from well defined areas called diffuse degassing structures (DDS), and a liquid phase that flows towards the outer part of the volcanic cone. A large amount of thermal energy is associated with the steam condensation process and subsequent cooling of the liquid phase. The total amount of volcanic–hydrothermal CO2 discharged through diffuse degassing has been computed through a sequential Gaussian simulation (sGs) approach based on several hundred accumulation chamber measurements and, at the time of the survey, amounted to 151 t d–1. The steam associated with the CO2 output, computed assuming that the original H2O/CO2 ratio of hydrothermal fluids is preserved in fumarolic effluents, is 553 t d–1, and the energy produced by the steam condensation and cooling of the liquid phase is 1.47×1012 J d–1 (17 MW). The location of the CO2 and temperature anomalies show that most of the gas is discharged from the inner part of the crater and suggests that crater morphology and local stratigraphy exert strong control on CO2 degassing and subsurface steam condensation. The amounts of gas and energy released by Vesuvio are comparable to those released by other volcanic degassing areas of the world and their estimates, through periodic surveys of soil CO2 flux, can constitute a useful tool to monitor volcanic activity.Editorial responsibility: H. Shinohara 相似文献
87.
Frequency of Boundary-Layer-Top Fluctuations in Convective and Stable Conditions Using Laser Remote Sensing 总被引:1,自引:0,他引:1
Giovanni Martucci Renaud Matthey Valentin Mitev Hans Richner 《Boundary-Layer Meteorology》2010,135(2):313-331
The planetary boundary-layer (PBL) height is determined with high temporal and altitude resolution from lidar backscatter
profiles. Then, the frequencies of daytime thermal updrafts and downdrafts and of nighttime gravity waves are obtained applying
a fast Fourier transform on the temporal fluctuation of the PBL height. The principal frequency components of each spectrum
are related to the dominant processes occurring at the daytime and nighttime PBL top. Two groups of cases are selected for
the study: one group combines daytime cases, measured in weak horizontal wind conditions and dominated by convection. The
cases show higher updraft and downdraft frequencies for the shallow, convective boundary layer and lower frequencies for a
deep PBL. For cases characterized by strong horizontal winds, the frequencies directly depend on the wind speed. The temporal
variation of the PBL height is determined also in the likely presence of lee waves. For nighttime cases, the main frequency
components in the spectra do not show a real correlation with the nocturnal PBL height. Altitude fluctuations of the top of
the nocturnal boundary layer are observed even though the boundary layer is statically stable. These oscillations are associated
with the wind shear effect and with buoyancy waves at the PBL top. 相似文献
88.
Giovanni Federico Gronchi 《Celestial Mechanics and Dynamical Astronomy》2009,103(4):301-326
Charlier’s theory (1910) provides a geometric interpretation of the occurrence of multiple solutions in Laplace’s method of
preliminary orbit determination, assuming geocentric observations. We introduce a generalization of this theory allowing to
take into account topocentric observations, that is observations made from the surface of the rotating Earth. The generalized
theory works for both Laplace’s and Gauss’ methods. We also provide a geometric definition of a curve that generalizes Charlier’s
limiting curve, separating regions with a different number of solutions. The results are generically different from Charlier’s:
they may change according to the value of a parameter that depends on the observations. 相似文献
89.
90.
The landscape setting of estuaries varies widely and is an important aspect of determining how they evolve. This paper focusses on alluvial estuaries in river valleys and how they respond to sea level rise. We examine the implications of marine transgression, as a response to sea level rise, where the estuary moves upwards and landwards to maintain its position in the tidal frame (so-called stratigraphic rollover). Here we encapsulate such kinematic movement of the estuary morphology using a ‘morphokinematic’ model, to assess the potential response to sea level rise and sediment supply. The model of the estuary form includes a single convergent channel, intertidal and surrounding floodplains (the valley) and allows the relative importance of the space available for deposition of sediments, the accommodation space, to be investigated as a function of rates of sea level rise and sediment supply. The transgression of the system is determined using a sediment mass balance, taking account of any supply from the river and marine environment. Model results confirm that the transgression distance, measured as the distance the entity moves landward, varies in proportion to the change in accommodation space, which mainly depends on the floodplain area. As the size of the floodplain reduces, the transgression distance is less and the system becomes much more sensitive to changes in the rate of sea level rise or changes in sediment supply. The greater demand for sediment when a floodplain is present results in greater cannibalization of the estuary form (i.e. greater landward movement) to meet the sediment demand. When the floodplain is disconnected from the estuary, the synergistic relationship is lost and the accommodation space increases. The capacity for restoration will depend on the availability of sediment and the prevailing rate of sea level rise. 相似文献