共查询到20条相似文献，搜索用时 671 毫秒
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P.A. Caraveo 《Astronomische Nachrichten》2008,329(2):191-193
The unprecedented harvest of X‐ray photons detected from dozens of isolated neutron stars has made it possible to glimpse at their emission mechanisms as well as at their emission geometry. Rotating hot spot(s), superimposed to the global thermal emission from the neutron star surface, are seen from several objects, allowing to probe the stars' external heating sources. Non‐thermal emission is also seen to vary as the stars rotate. Moreover, absorption features have been detected in the spectra of several objects, allowing to probe (tentatively) the stars' magnetic fields. Spectacular tails, trailing the stars' supersonic motion, trace the boundaries of the relativist winds streaming from the star's magnetosphere. Apart from classical radio pulsar and certified radio‐quiet neutron stars, XMM‐Newton has devoted significant observation time to the enigmatic central compact objects, presumably isolated neutron stars shining at the center of their supernova remnants. Far from showing a unifying behaviour, XMM‐Newton data have unveiled a surprising diversity. Understanding the reason(s) behind such diversity is the challenge for the next decade of X‐ray observations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Abstract— We explore the orbital dynamics of Earth‐crossing objects with the intent to understand the time scales under which an “orbital stream” of material could produce time‐correlated meteorite falls. These “meteoroid streams” have been suggested to be associated with three well‐known meteorite‐dropping fireballs (Innisfree, Peekskill, and P?íbram). We have performed two different analyses of the statistical significance of the “orbital similarity,” in particular calculating how often orbits of the same level of similarity would come from a random sample. Secondly, we have performed extremely detailed numerical integrations related to these three cases, and we find that if they were streams of objects in similar orbits, then they would become “decoherent” (in the sense that the day‐of‐fall of meteorites of these streams become almost random) on time scales of 104–105 yr. Thus, an extremely recent breakup would be required, much more recent that the cosmic ray exposure ages of the recovered falls in each case. We conclude that orbital destruction is too efficient to allow the existence of long‐lived meteoroid streams and that the statistical evidence for such streams is insufficient; random fall patterns show comparable levels of clustering. 相似文献
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R. L. Dowden 《Planetary and Space Science》1966,14(12):1273-1274
Occasionally micropulsation dynamic spectra show a series of nosed tones in place of the more usual purely rising tones. These are explained in some detail if, at times, some 3–10 per cent of the ionic content in the outer magnetosphere is helium (H6+). Although this is one or two orders of magnitude greater than expected from diffusive equilibrium theory, it is in broad agreement with recent Ogo-A measurements. 相似文献
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Abstract— In the early morning hours of December 13, 2002, a bright Geminid fireball with an absolute magnitude of ?9.2 ± 0.5 was observed from Southern Saskatchewan, Canada. The fireball displayed distinct small‐scale oscillations in brightness, or flickering, indicative of the parent meteoroid being both non‐spherical and rotating. Using the light curve derived from a calibrated radiometer, we determine a photometric mass of 0.429 ± 0.15 kg for the meteoroid, and we estimate from its initial rotation rate of some 6 Hz that the meteoroid was ejected from the parent body (3200) Phaethon some 2500 ± 500 years ago. We find that 70% of Geminid fireballs brighter than magnitude ?3 display distinct flickering effects, a value that is in stark contrast to the 18% flickering rate exhibited by sporadic fireballs. The high coincidence of flickering and the deep atmospheric penetration of Geminid fireballs are suggestive of Geminid meteoroids having a highly resilient structure, a consequence, we suggest, of their having suffered a high degree of thermal processing. The possibility of Gemind material surviving atmospheric ablation and being sampled is briefly discussed, but the likelihood of collecting and identifying any such material is admittedly very small. 相似文献
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The reconstruction of the solar activity during some years of the 18th century is poorly known because there are scarce sunspot observations. The aim of this short contribution is to present a “lost” sunspot observation realized by the Portuguese scientist Sanches Dorta during his observation of the solar eclipse of 1785 from Rio de Janeiro (Brazil). This record was not included in the database compiled by Hoyt and Schatten (1998). We present new estimations of the solar activity during 1785. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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We present results of a study of the so-called “stickiness” regions where orbits in mappings and dynamical systems stay for very long times near an island and then escape to the surrounding chaotic region. First we investigated the standard map in the form xi+1 = xi+yi+1 and yi+1 = yi+K/2π · sin(2πxi) with a stochasticity parameter K = 5, where only two islands of regular motion survive. We checked now many consecutive points—for special initial conditions of the mapping—stay within a certain region around the island. For an orbit on an invariant curve all the points remain forever inside this region, but outside the “last invariant curve” this number changes significantly even for very small changes in the initial conditions. In our study we found out that there exist two regions of “sticky” orbits around the invariant curves: A small region I confined by Cantori with small holes and an extended region II is outside these cantori which has an interesting fractal character. Investigating also the Sitnikov-Problem where two equally massive primary bodies move on elliptical Keplerian orbits, and a third massless body oscillates through the barycentre of the two primaries perpendicularly to the plane of the primaries—a similar behaviour of the stickiness region was found. Although no clearly defined border between the two stickiness regions was found in the latter problem the fractal character of the outer region was confirmed. 相似文献
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The experiment with 10K-80 aboard the INTER-BALL-2 (which detects protons with energies > 7, 27–41, 41–58,
58–88, 88–180 and 180–300 MeV) registered six events of the solar energetic particle (SEP)
increase. These events are during the initial rise phase of the 23rd solar activity cycle. Solar flares with the SEP generation
are accompanied by coronal mass ejection (CME). Here we analyze the dynamics of the differential energy spectrum at different
phases of the SEP increase. 相似文献
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Wolf U. Reimold Christian Koeberl Cedric Wright Iain McDonald 《Meteoritics & planetary science》2001,36(5):747-749
Abstract— Gorter and Glikson (2000) proposed that the Fohn‐1 structure in the Timor Sea north of Australia probably is an impact structure. Examination of their evidence reveals that it is almost impossible to confirm any of their conclusions, as the authors do not provide sufficient detail of their experimental methods. Problems with the presentation of both seismic and geochemical data cast further doubt on their conclusions. Absence of evidence for shock metamorphism and non‐chondritic siderophile element patterns in the few samples analysed make it likely that Fohn‐1 is neither a probable, nor even a possible, impact structure. 相似文献
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The general relativistic model of Cooperstock and Tieu, which attempts to fit rotation curves of spiral galaxies without invoking dark matter, is tested empirically using observations of the Milky Way. In particular, predictions for the mass density in the solar neighbourhood and the vertical density distribution at the position of the Sun are compared with observations. It is shown that the model of Cooperstock and Tieu, which was so constructed that it gives an excellent fit of the observed rotation curve, singularly fails to reproduce the observed local mass density and the vertical density profile of the Milky Way. 相似文献
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William F. Read 《Meteoritics & planetary science》1972,7(4):417-428
With the exception of a distinctive olivine-bronzite chondrite recognized by H. H. Nininger: “Ness County (1938)”, most of the numerous stony meteorites found in Ness County, Kansas, have been lumped together under the designation “Ness County (1894)”. It is the purpose of this paper to show that at least three separate falls are covered by this name 相似文献
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John W. Morgan 《Meteoritics & planetary science》1995,30(3):352-353
Abstract— Jones (1994) demonstrated that fractional crystallization of IIIAB iron meteorites may result in a log Au vs. log Ni plot of constant slope even though kAu and kNi change. Jones' example is a special case, however, and does not necessarily describe behavior by other elements or in other metallic magmas. 相似文献
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The very-low-frequency emission known as “chorus” has been studied for four Australian recording stations. The phenomenon exhibits a maximum of occurrence at about geomagnetic latitude 60° and occurs only about one-tenth as frequently at geomagnetic latitude 45°. It was never definitely observed at geomagnetic latitude 35°. Marked diurnal variations are present with morning peaks for all stations and also an evening peak at Adelaide and Hobart. Analysis of the diurnal variations suggests that two different mechanisms are involved. The average strength of chorus increases with increasing value of K-index at lower latitude stations and shows a maximum at moderate values of the index for geomagnetic latitude 61°. This is interpreted as meaning that the region of maximum average-chorus-strength moves towards lower latitudes at times of magnetic disturbance. 相似文献
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