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1.
Since October 1990, 3 weeks after the launch of the Ulysses spacecraft, the dust detector onboard recorded impacts of cosmic dust particles. Besides dust impacts, the detector recorded noise from a variety of sources. So far, a very rigid scheme had been applied to eliminate noise from impact data. The data labeled “big” dust impacts previously led to the identification of interstellar dust and of dust streams from Jupiter. The analysis presented here is concerned with data of signals of small amplitudes which are strongly contaminated by noise. Impacts identified in this data set are called “small” impacts. It is shown that dust impacts can be clearly distinguished from noise for most of the events due to the multi-coincidence characteristics of the instrument. 516 “small” impacts have been identified. For an additional 119 events, strong arguments can be given that they are probably small dust impacts. Thereby, the total number of dust impacts increases from 333 to 968 in the time period from 28 October 1990 to 31 December 1992. This increase permits a better statistical analysis, especially of the Jupiter dust streams which consist mostly of small and fast particles. Additional dust streams have been identified between the already known streams before and after Jupiter flyby. The dependence of the deflection from the Jupiter direction, the stream intensity and width on Jupiter distance support the assertion that they have been emitted from the Jovian system. The masses of the 635 “small” dust particles range from 6 × 10−17 to 3 × 10−10 g with a mean value of 1 × 10−12 g, which compares to a range from 1 × 10−16 to 4 × 10−9 g with a mean value of 2 × 10−11 g for the previously identified 333 “big” dust particles.  相似文献   

2.
We present the results of observation of the Geminga pulsar carried out in the TeV energy band during the 6 year period spanning 2000–2006 using the Pachmarhi Array of Cherenkov Telescopes (PACT). A long stretch of data, new computer codes and the “Tempo” package have been used in the present analysis. We have searched for evidence of pulsed emission of γ-rays from the Geminga pulsar using the post-glitch pulsar elements obtained by Jackson and Halpern from X-ray/γ-ray satellite data. We do not see any significant evidence for pulsed emission from the Geminga pulsar at a threshold energy of 825 GeV. In this paper we present our results on the light curve in the TeV energy band, set an upper limit on the time averaged flux of γ-rays, and compare our results with other ground based observations.  相似文献   

3.
From 1978 December to 1979 May, we made spectrographic observations of φPer, ζTau and κDra using our Schmidt cassegrain grating spectrograph. Values of electron density, rotational speed and variation have been obtained and a comparison between ζTau and the supergiant βOri carried out. We found that, over our observed period, θPer showed clear shell activities, ζTau was in a relatively calm phase, its shell slowly expanding, while κDra had all the signs of a “classical” Be star.  相似文献   

4.
The inversion of helioseismic modes leads to the sound velocity inside the Sun with a precision of about 0.1 per cent. Comparisoons of solar models with the “seismic sun” represent powerful tools to test the physics: depth of the convection zone, equation of state, opacities, element diffusion processes and mixing inside the radiative zone. We now have evidence that microscopic diffusion (element segregation) does occur below the convection zone, leading to a mild helium depletion in the solar outer layers. Meanwhile this process must be slowed down by some macroscopic effect, presumably rotation-induced mixing. The same mixing is also responsible for the observed lithium depletion. On the other hand, the observations of beryllium and helium 3 impose specific constraints on the depth of this mildly mixed zone. Helioseismology also gives information on the internal solar rotation: while differential rotation exists in the convection zone, solid rotation prevails in the radiative zone, and the transition layer (the so-called “tachocline”) is very small. These effects are discussed, together with the astrophysical constraints on the solar neutrino fluxes.  相似文献   

5.
By applying the fireball model of γ-ray burst with a central pulsar, the radiation fluxes of the afterglows of two γ-ray bursts, GRB970228 and GRB000301c, are calculated. The results of the calculation agree very well with the observations. The differing characters of the light curves with a “break” in the optical waveband R of the afterglows of two bursts are interpreted, in terms of differing pulsar parameter values.  相似文献   

6.
With an instrument on board the Japanese satellite EXOS-A electron temperatures of more than 1000 K above the “normal” values have been observed in the night-time topside F-region above the geographic region where the total magnetic field is below 20000 nT. Simultaneously enhanced wave emissions on 45 kHz, 2 MHz and 3 MHz were found and an increase in particle precipitation. The observations are described in detail and several mechanism are discussed which can explain the results.  相似文献   

7.
The time scale of the response of the high-latitude dayside ionospheric flow to changes in the North-South component of the interplanetary magnetic field (IMF) has been investigated by examining the time delays between corresponding sudden changes. Approximately 40 h of simultaneous IMF and ionospheric flow data have been examined, obtained by the AMPTE-UKS and -IRM spacecraft and the EISCAT “Polar” experiment, respectively, in which 20 corresponding sudden changes have been identified. Ten of these changes were associated with southward turnings of the IMF, and 10 with northward turnings. It has been found that the corresponding flow changes occurred simultaneously over the whole of the “Polar” field-of-view, extending more than 2° in invariant latitude, and that the ionospheric response delay following northward turnings is the same as that following southward turnings, though the form of the response is different in the two cases. The shortest response time, 5.5 ± 3.2 min, is found in the early- to mid-afternoon sector, increasing to 9.5 ± 3.0 min in the mid-morning sector, and to 9.5 ± 3.1 min near to dusk. These times represent the delays in the appearance of perturbed flows in the “Polar” field-of-view following the arrival of IMF changes at the subsolar magnetopause. Overall, the results agree very well with those derived by Etemadi et al. (1988, Planet. Space Sci. 36, 471) from a general cross-correlation analysis of the IMF Bz and “Polar” beam-swinging vector flow data.  相似文献   

8.
Lyα Blobs (LAB), large, extended Lyα emitters, are enigmatic objects that calls on much attention in the field of galaxy formation and evolution. So far, discovery of more than twenty giant (>60 kpc) LABs have been reported and new searches are on-going. Many of the groups who study LABs attended the workshop in Heidelberg, “Understanding Lyα Emitters”, 2008 October 6–10. We had useful discussions on the various important aspects of LABs including the search or detection methods, origins of the extended Lyα emission, and their physical properties.  相似文献   

9.
It has been proposed that propagation of cosmic-rays at extreme-energy may be sensitive to Lorentz-violating metric fluctuations (“foam”). We investigate the changes in interaction thresholds for cosmic-rays and gamma-rays interacting on the CMB and IR backgrounds, for a class of stochastic models of space–time foam. The strength of the foam is characterized by the factor (E/MP)a, where a is a phenomenological suppression parameter. We find that there exists a critical value of a (dependent on the particular reaction: acrit3 for cosmic-rays, 1 for gamma-rays), below which the threshold energy can only be lowered, and above which the threshold energy may be raised, but at most by a factor of two. Thus, it does not appear possible in this class of models to extend cosmic-ray spectra significantly beyond their classical absorption energies. However, the lower thresholds resulting from foam may have signatures in the cosmic-ray spectrum. In the context of this foam model, we find that cosmic-ray energies cannot exceed the fundamental Planck scale, and so set a lower bound of 108 TeV for the scale of gravity. We also find that suppression of p→pπ0 and γ→e−e+ “decays” favors values aacrit. Finally, we comment on the apparent non-conservation of particle energy–momentum, and speculate on its re-emergence as dark energy in the foamy vacuum.  相似文献   

10.
We observed the molecular cloud NGC 2023 in the NH3 rotation-inversion lines NH3(1.1) and NH3(2.2). The hyperfine structures shown in these lines can be advantageously used to derive the physical parameters of the cloud. We re-confirm that the star HD 37903 is the unique exciting source of the cloud and heated dust is the exciting medium. Analysis leads to the conclusion that the star is located on the periphery of the cloud, as in Zuckerman's “blister” model. If HD 37003 originated in NGC 2023, then the star will have existed long enough to blow away the molecular cloud to a distance of 0.1 pc. There may exist clumps of relatively high density in the cloud.  相似文献   

11.
Galaxy clustering properties have been studied for decades to constrain cosmological parameters and have today, with large datasets of high-redshift sources piling up, become a powerful tool to discriminate and characterize primeval galaxies. In the last years, several Lyman-Alpha Emitter (LAE) galaxy samples have been gathered, which are big, uniform and compact enough to allow clustering analysis. Here we present a summary of the discussion session on the clustering properties of LAEs at the “Understanding Lyman-Alpha Emitters” conference.  相似文献   

12.
A three-component chemical evolution model of the Galaxy is presented, which we believe will cast a new light on the G-dwarf problem. The model is based on a scenario of the Galaxy consisting of three major evolutionary phases: halo, thick disk and thin disk, separated by two short interludes of rapid collapse. The evolution of different stellar populations are treated separately, the combination of which yields an overall metallicity distribution function for the solar neighbourhood. We tested three different models using the same set of basic equations: the “prompt initial enrichment” (PIE) model, the “proportional yield” (PPY) model and the “collapse” (CLP) model. Best-fit parameters are derived. The results show that the different populations have remarkably different IMFs, while mass exchange has only minimally affected the chemical evolution in the solar vicinity, so that the solar vicinity can be regarded as a closed system, at least in the late stage of the Galactic evolution.  相似文献   

13.
This paper presents our results for full 3-D simulations of very-high to ultra-high energy electromagnetic cascades – and the associated coherent Cherenkov radiation – as might be produced by high-energy neutrino interactions in dense media. Using “thinning” techniques, we develop an algorithm based on the existing “ZHS” code, and demonstrate that the new “ZHS-thinned” code can produce fast and accurate results for showers up to . Using ZHS-thinned, we develop new parameterisations for the radiation from showers in ice, salt, and the lunar regolith, with a separate treatment of the megaregolith (deep regolith). Our parameterisations include for the first time a method to simulate fluctuations in shower length induced by the LPM effect. Our results, which avoid the pit-falls of scaling simulations from lower energies, allow improved calculations of the detection probability for experiments searching for high-energy neutrinos using the radio technique.  相似文献   

14.
Growth patterns of the last ice age coral terraces at Huon Peninsula   总被引：1，自引：0，他引：1  
At Huon Peninsula, Papua New Guinea, prolific coral growth during the last-glacial was episodic and in response to a series of sea-level rises. The resultant step-like coral terraces are currently situated from 20 m up to 140 m above sea-level due to continuous tectonic uplift of the Peninsula. The sea-level rises were in response to periodic partial disintegration of Northern Hemisphere ice sheets associated with severe climate swings and occurred within decadal timescales. The relatively rapid 15 m to 35 m rise in sea-levels exposed new head-room for corals to colonize. The resulting terrace structures contain individual corals that do not appear to have grown sequentially in time and with elevation. Additionally, following the peak, sea level fell relatively slowly over several thousand years and corals grew and filled in the flanks of the terrace such that younger corals now occupy lower elevations. We have labeled these structures “pack-up” reefs. This is in contrast to coral terraces formed during major sea-level rises from glacial to interglacial or glacial to interstadial transitions where the rate of sea level rise is commensurate with coral growth rates and corals can keep up with sea-level rise by growing on top of each other in a time orderly sequence. Deriving sea-level information from pack-up terraces is difficult and is likely to be ambiguous. The periodic fluctuations in climate were associated with atmospheric radiocarbon swings that seem to have varied smoothly with time. The same corals that show a scatter in stratigraphic temporal ordering appear regularly distributed in time and with radiocarbon content attesting to the veracity of the age measurements and at the same time confirm the disordered distribution of corals in “pack-up” type reefs.  相似文献   

15.
We calculate the event rates induced by a 51Cr νe source and by a 90Sr---90Y source in BOREXINO through elastic scattering on electrons, assuming a nonzero neutrino magnetic moment μν. We consider a source activity of about 2 MCi and estimate the solar ν (“source-off”) background for various oscillation scenarios. It is shown that values of μν as low as 0.5 × 10−10μB ( 0.2 × 10−10μB) can be proved with the 51Cr source (90Sr source) in about 100 days of data taking.  相似文献   

16.
ASTRON has demonstrated the capabilities of a 4 m2, dense phased array antenna (Bij de Vaate et al., 2002) for radio astronomy, as part of the Thousand Element Array project (ThEA). Although it proved the principle, a definitive answer related to the viability of the dense phased array approach for the SKA could not be given, due to the limited collecting area of the array considered. A larger demonstrator has therefore been defined, known as “Electronic Multi-Beam Radio Astronomy Concept”, EMBRACE, which will have an area of 625 m2, operate in the band 0.4–1.550 GHz and have at least two independent and steerable beams. With this collecting area EMBRACE can function as a radio astronomy instrument whose sensitivity is comparable to that of a 25-m diameter dish. The collecting area also represents a significant percentage area (∼10%) of an individual SKA “station.” This paper presents the plans for the realisation of the EMBRACE demonstrator.  相似文献   

17.
We present an analysis of the very recent data from the CASA-MIA extensive air shower array (M.A.K. Glasmacher, Ph.D. Thesis, University of Michigan (1998) in terms of a search for the structure in the shower size spectrum claimed by us from an analysis of “the world's data”. Our earlier claim is found to be supported to the extent that there is strong evidence for the existence of structure in the spectrum which cannot obviously be explained by the conventional Galactic Modulation Model. There is modest evidence for the structure being of “our” form and strong support for “our” mass composition when “corrected” to the interaction model advocated by us. None of the results are inconsistent with there having been a recent, nearby, single supernova.  相似文献   

18.
In this paper we show the capabilities of the Large Volume Detector (INFN Gran Sasso National Laboratory) to identify a neutrino burst associated with a supernova explosion, in the absence of an “external trigger”, e.g., an optical observation. We describe how the detector trigger and event selection have been optimized for this purpose, and we detail the algorithm used for the on-line burst recognition. The on-line sensitivity of the detector is defined and discussed in terms of supernova distance and intensity at the source.  相似文献   

19.
A simple mathematical model for the evolution of a system of collisionally interacting bodies—such as the asteroid population—consists of two coupled, nonlinear, first-order differential equations for the abundances of “small” and “big” bodies. The model easily allows us to recover Dohnanyi's value for the exponent of the equilibrium mass distribution. Moreover, the model shows that any initial value for the ratio of “big” to “small” bodies rapidly relaxes to the equilibrium ratio, corresponding to the exponent, and that integrating the evolution equations backward in time—an attractive possibility to investigate the mass distribution of primordial planetesimals—leads to strong numerical instability.  相似文献   

20.
Considering the errors transmitted from all pre-treatments, I suggest a “division” method for removing the sky background including interference fringes in CCD images. Compared to the traditional method, which may be called a “subtraction” method, the “division” method is easier to carry out and gives more accurate brightness measurement especially for the fainter images.  相似文献   

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