共查询到20条相似文献，搜索用时 31 毫秒
1.
In this paper, we describe pulsar observations at decametric wavelengths using the Gauribidanur Radio Telescope made subsequent
to our earlier measurements (Deshpande & Radhakrishnan 1992). To improve the time-resolution in our measurements of pulse
profiles, we have used the ‘swept-frequency dedispersion’ method with some modifications to suit its application at such low
radio frequencies. We also present a new scheme that simplifies the calibration of the receiver gain characteristics. We present
average profiles on four pulsars from these improved measurements at 34.5 MHz. 相似文献
2.
We present a method for tracking and predicting the propagation and evolution of coronal mass ejections (CMEs) using the imagers on the STEREO and SOHO satellites. By empirically modeling the material between the inner core and leading edge of a CME as an expanding, outward propagating ellipsoid, we track its evolution in three-dimensional space. Though more complex empirical CME models have been developed, we examine the accuracy of this relatively simple geometric model, which incorporates relatively few physical assumptions, including i) a constant propagation angle and ii) an azimuthally symmetric structure. Testing our ellipsoid model developed herein on three separate CMEs, we find that it is an effective tool for predicting the arrival of density enhancements and the duration of each event near 1 AU. For each CME studied, the trends in the trajectory, as well as the radial and transverse expansion are studied from 0 to ～0.3 AU to create predictions at 1 AU with an average accuracy of 2.9 hours. 相似文献
3.
This paper presents an analysis of the phase‐calibration problem encountered in astronomy when mapping incoherent sources with aperture‐synthesis devices. More precisely, this analysis concerns the phase‐calibration operation involved in the self‐calibration procedures of phase‐closure imaging. The paper revisits and completes a previous analysis presented by Lannes in the Journal of the Optical Society of America A in 2005. It also benefits from some recent developments made for solving similar problems encountered in global navigation satellite systems. In radio‐astronomy, the related optimization problems have been stated and solved hitherto at the phasor level. We present here an analysis conducted at the phase level, from which we derive a method for diagnosing and solving the difficulties of the phasor approach. In the most general case, the techniques tobe implemented appeal to the algebraic graph theory and the algebraic number theory. The minima of the objective functionals to be minimized are identified by raising phase‐closure integer ambiguities. We also show that in some configurations, to benefit from all the available information, closure phases of order greater than three are to be introduced. In summary, this study leads to a better understanding of the difficulties related to the very principle of phase‐closure imaging. To circumvent these difficulties, we propose a strategy both simple and robust (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
4.
A.J. Horton D.F. Buscher C.A. Haniff 《Monthly notices of the Royal Astronomical Society》2001,327(1):217-226
We present a numerical analysis of free-space propagation of the beams inside a long-baseline optical/infrared interferometer. Unlike the models of beam propagation used in most previous studies, our analysis incorporates the effects of atmospheric seeing on the wavefronts entering the interferometer. We derive results for the changes in throughput, coherence loss and fringe-detection signal-to-noise ratio arising from diffraction along the propagation path. Our results for conditions of moderate seeing show that although the flux throughput decreases with propagation distance for a given beam diameter, the fringe contrast increases at the same time. In this case it becomes possible for diffraction to increase the signal-to-noise ratio of the fringe measurements. Previous studies have only considered an arrangement where all the apertures in the beam-propagation system have the same diameter. If the light at the end of the propagation path is collected with a fixed size aperture, we find that in many cases the signal-to-noise ratio for fringe detection is maximized when the initial beam diameter is approximately 30 per cent smaller than the final collector diameter. We discuss the implications of our results in the context of future interferometer designs. 相似文献
5.
6.
Modeling the impacts of grazing land management on land-use change for the Jordan River region 总被引：1，自引：0，他引：1
In this article, we describe a simulation method for investigating the impacts of different grazing land management strategies on the productivity of (semi-)natural vegetation and the resulting feedback on land-use change. In a first application, we analyze the effects of sustainable and intensive grazing land management in the Jordan River region. For this purpose, we adapt and use the regional version of the spatially explicit modeling framework LandSHIFT. Our simulation experiments indicate that the modeled feedback mechanism has a strong effect on the spatial extent of grazing land. Consequently, the results of our study underline that the inclusion of such feedback mechanisms in land-use models can help to represent and analyze the complex interactions between humans and the environment in a more differentiated and realistic way, but they also identify the demand for more detailed empirical data on grazing land degradation in order to further improve the explanatory power of the model. 相似文献
7.
Jinho Kim Hee Il Kim Hyung Mok Lee 《Monthly notices of the Royal Astronomical Society》2009,399(1):229-238
We obtain equilibrium solutions for rotating compact stars, including special relativistic effects. The gravity is assumed to be Newtonian, but we use the active mass density, which takes into account all energies such as the motion of the fluid, internal energy and pressure energy in addition to the rest-mass energy, in computing the gravitational potential using Poisson's equation. Such a treatment could be applicable to neutron stars with relativistic motions or a relativistic equation of state. We applied Hachisu's self-consistent field (SCF) method to find spheroidal as well as toroidal sequences of equilibrium solutions. Our solutions show better agreement with general relativistic solutions than the Newtonian relativistic hydrodynamic approach, which does not take into account the active mass. Physical quantities such as the peak density and equatorial radii in our solutions agree with the general relativistic ones to within 5 per cent. Therefore our approach can be used as a simple alternative to the fully relativistic one when a large number of model calculations is necessary, as it requires much fewer computational resources. 相似文献
8.
M. Focardi E. Pace M. Farina A. M. Di Giorgio J. Colomé Ferrer I. Ribas C. Sierra Roig L. Gesa Bote J. C. Morales J. Amiaux C. Cara J. L. Augurés E. Pascale G. Morgante V. Da Deppo M. Pancrazzi V. Noce S. Pezzuto M. Frericks F. Zwart G. Bishop K. Middleton P. Eccleston G. Micela G. Tinetti 《Experimental Astronomy》2018,45(1):1-20
In this paper we present the electromagnetic modeling and beam pattern measurements of a 16-elements ultra wideband sparse random test array for the low frequency instrument of the Square Kilometer Array telescope. We discuss the importance of a small array test platform for the development of technologies and techniques towards the final telescope, highlighting the most relevant aspects of its design. We also describe the electromagnetic simulations and modeling work as well as the embedded-element and array pattern measurements using an Unmanned Aerial Vehicle system. The latter are helpful both for the validation of the models and the design as well as for the future instrumental calibration of the telescope thanks to the stable, accurate and strong radio frequency signal transmitted by the UAV. At this stage of the design, these measurements have shown a general agreement between experimental results and numerical data and have revealed the localized effect of un-calibrated cable lengths in the inner side-lobes of the array pattern. 相似文献
9.
Fouad Sahraoui Melvyn L. Goldstein Alain Roux Patrick Canu Nicole Cornilleau-Wehrlin Thierry Dudok De Wit Khurom Kiyani 《Planetary and Space Science》2011,59(7):585-591
Investigating space plasma turbulence from single-point measurements is known to be characterized by unavoidable ambiguities in disentangling temporal and spatial variations. Solving this problem has been one of the major goals of the Cluster mission. For that purpose multipoint measurements techniques, such as the k-filtering, have been developed. Such techniques combine several time series recorded simultaneously at different points in space to estimate the corresponding energy density in the wavenumber space. Here we apply the technique to both simulated and Cluster magnetometer data in the solar wind (SW) and investigate the errors and limitations that arise due to the separation of the spacecraft and the quality of the tetrahedral configuration. Specifically, we provide an estimation of the minimum and maximum scales that can be accurately measured given a specific distance between the satellites and show the importance of the geometry of the tetrahedron and the relationship of that geometry to spatial aliasing. We also present recent results on characterizing small scale SW turbulence and provide scientific arguments supporting the need of new magnetometers having better sensitivity than the existing ones. Throughout the paper we emphasize technical challenges and their solutions that can be considered for a better preparation of the Cross-Scale mission. 相似文献
10.
Christian Wolf 《Monthly notices of the Royal Astronomical Society》2009,397(1):520-533
We combine in a single framework the two complementary benefits of χ2 template fits and empirical training sets used e.g. in neural nets: χ2 is more reliable when its probability density functions (PDFs) are inspected for multiple peaks, while empirical training is more accurate when calibration and priors of query data and training set match. We present a χ2 empirical method that derives PDFs from empirical models as a subclass of kernel regression methods, and apply it to the Sloan Digital Sky Survey Data Release 5 sample of >75 000 quasi-stellar objects, which is full of ambiguities. Objects with single-peak PDFs show <1 per cent outliers, rms redshift errors <0.05 and vanishing redshift bias. At z > 2.5 , these figures are two times better. Outliers result purely from the discrete nature and limited size of the model, and rms errors are dominated by the intrinsic variety of object colours. PDFs classed as ambiguous provide accurate probabilities for alternative solutions and thus weights for using both solutions and avoiding needless outliers. E.g. the PDFs predict 78.0 per cent of the stronger peaks to be correct, which is true for 77.9 per cent of them. Redshift incompleteness is common in faint spectroscopic surveys and turns into a massive undetectable outlier risk above other performance limitations, but we can quantify residual outlier risks stemming from size and completeness of the model. We propose a matched χ2 error scale for noisy data and show that it produces correct error estimates and redshift distributions accurate within Poisson errors. Our method can easily be applied to future large galaxy surveys, which will benefit from the reliability in ambiguity detection and residual risk quantification. 相似文献
11.
J.M. Alcal B. Stelzer E. Covino G. Cupani A. Natta S. Randich E. Rigliaco L. Spezzi L. Testi F. Bacciotti R. Bonito S. Covino E. Flaccomio A. Frasca D. Gandolfi F. Leone G. Micela B. Nisini E. Whelan 《Astronomische Nachrichten》2011,332(3):242-248
We present preliminary results of our X‐shooter survey in star forming regions. In this contribution we focus on subsamples of young stellar and sub‐stellar objects (YSOs) in the Lupus star forming region and in the TW Hya association. We show that the X‐shooter spectra are suitable for conducting several parallel studies such as YSO + disk fundamental parameters, accretion and outflow activity in the very low‐mass (VLM) and sub‐stellar regimes, as well as magnetic activity in young VLM YSOs, and Li abundance determinations. The capabilities of X‐shooter in terms of wide spectral coverage, resolution and limiting magnitudes, allow us to assess simultaneously the accretion/outflow, magnetic activity, and disk diagnostics, from the UV and optical to the near‐IR, avoiding ambiguities due to possible YSO variability (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
12.
We describe a forward modeling method developed to study the coronal mass ejections observed with STEREO/SECCHI. We present a survey of 26 CMEs modeled with this method. We selected most of the bright events observed since November 2007 to August 2008, after when the separation was greater than 40° degrees, thus showing noticeable differences between the two views. From these stereoscopic observations and using a geometric model of a flux rope, we are able to determine the three-dimensional direction of propagation, the three-dimensional velocity and acceleration of the CME front, and in most of the cases the flux rope orientation and length. We define a merit function that allows us to partially automate the fit, as well as perform a sensitivity analysis on the model parameters. We find a precision on the longitude and latitude to be of a maximum of ±17° and ±4°, respectively, for a 10% decrease of the merit function but a precision on the flux rope orientation and length to be almost one order of magnitude larger, showing that these parameters are more difficult to estimate using only coronagraph data. Finally, comparison with independent measurements shows a good agreement with the direction and speed we estimated. 相似文献
13.
The common practice in luminosity calibration of sample truncation according to relative parallax error λ can lead to bias with indirect methods such as reduced parallaxes as well as with direct methods. This bias is not cancelled by the Lutz–Kelker corrections and in fact can be either negative or positive. Making the selection stricter can actually lead to a larger absolute amount of bias and lower accuracy in certain cases.
The degree to which this bias is present depends upon whether the sample is more nearly specified by the relative parallax error or by the limiting apparent magnitude when both limits formally apply; when the latter limit dominates it is absent. The difference between the means for the two extreme cases is what is customarily termed the Malmquist bias. However, it is not truly bias, but rather what we call here an offset .
For a sample to be effectively magnitude-limited, there is a lower bound imposed on the mean absolute magnitude which depends on the limiting magnitude. If a wide-ranging luminosity relation such as the Wilson–Bappu relation is to be calibrated, some portion of the relation may be magnitude-limited and the rest not. In that case there will be offsets between the different parts of the relation, including the transition region between the two extremes, as well as bias outside the magnitude-limited part.
Another, less common, practice is truncation according to weight, specifically with the reduced parallax method. Such truncation can also bias the calibration with one variant of the method. Indeed, the weighting scheme used with that variant introduces bias even without truncation.
For calibration it is probably best to use a general maximum likelihood method such as the grid method with a magnitude-limited sample and no limit on relative parallax error. The Malmquist shift could then be applied to obtain an estimate of the volume-limited mean. 相似文献
The degree to which this bias is present depends upon whether the sample is more nearly specified by the relative parallax error or by the limiting apparent magnitude when both limits formally apply; when the latter limit dominates it is absent. The difference between the means for the two extreme cases is what is customarily termed the Malmquist bias. However, it is not truly bias, but rather what we call here an offset .
For a sample to be effectively magnitude-limited, there is a lower bound imposed on the mean absolute magnitude which depends on the limiting magnitude. If a wide-ranging luminosity relation such as the Wilson–Bappu relation is to be calibrated, some portion of the relation may be magnitude-limited and the rest not. In that case there will be offsets between the different parts of the relation, including the transition region between the two extremes, as well as bias outside the magnitude-limited part.
Another, less common, practice is truncation according to weight, specifically with the reduced parallax method. Such truncation can also bias the calibration with one variant of the method. Indeed, the weighting scheme used with that variant introduces bias even without truncation.
For calibration it is probably best to use a general maximum likelihood method such as the grid method with a magnitude-limited sample and no limit on relative parallax error. The Malmquist shift could then be applied to obtain an estimate of the volume-limited mean. 相似文献
14.
Andrzej A. Zdziarski 《Monthly notices of the Royal Astronomical Society》2005,360(3):816-824
We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependences of the level of local variability (as expected from the propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As an example of local emission, we consider blackbody, Wien and thermal Comptonization spectra. In addition to numerical results, we present a number of analytical formulae for the resulting rms. We also find an analytical formula for the disc Wien spectrum, which we find to be a very good approximation to the disc blackbody. We compare our results to the rms spectrum observed in an ultrasoft state of GRS 1915+105. 相似文献
15.
B. Loupias E. Falize M. Koenig S. Bouquet N. Ozaki A. Benuzzi-Mounaix C. Michaut M. Rabec le Goahec W. Nazarov C. Courtois Y. Aglitskiy A. YA. Faenov T. Pikuz 《Astrophysics and Space Science》2007,307(1-3):103-107
We present experiments performed with the LULI2000 nanosecond laser facility. We generated plasma jets by using specific designed
target. The main measured quantities related to the jet such as its propagation velocity, temperature and emissive radius
evolution are presented. We also performed analytical work, which explains the jet evolution in some cases. 相似文献
16.
The space mission Solar TErrestrial RElations Observatory (STEREO) will provide images from two viewpoints. An important aim
of the STEREO mission is to get a 3D view of the solar corona. We develop a program for the stereoscopic reconstruction of
3D coronal loops from images taken with the two STEREO spacecraft. A pure geometric triangulation of coronal features leads
to ambiguities because the dilute plasma emissions complicates the association of features in image 1 with features in image
2. As a consequence of these problems, the stereoscopic reconstruction is not unique and multiple solutions occur. We demonstrate
how these ambiguities can be resolved with the help of different coronal magnetic field models (potential, linear, and non-linear
force-free fields). The idea is that, due to the high conductivity in the coronal plasma, the emitting plasma outlines the
magnetic field lines. Consequently, the 3D coronal magnetic field provides a proxy for the stereoscopy, which allows to eliminate
inconsistent configurations. The combination of stereoscopy and magnetic modelling is more powerful than one of these tools
alone. We test our method with the help of a model active region and plan to apply it to the solar case as soon as STEREO
data become available. 相似文献
17.
In this paper, we consider the process of alignment of a spinning black hole and a surrounding misaligned accretion disc. We use a simplified set of equations, that describe the evolution of the system in the case where the propagation of warping disturbances in the accretion disc occurs diffusively, a situation likely to be common in the thin discs in active galactic nuclei (AGN). We also allow the direction of the hole spin to move under the action of the disc torques. In such a way, the evolution of the hole–disc system is computed self-consistently. We consider a number of different situations and we explore the relevant parameter range, by varying the location of the warp radius R w and the propagation speed of the warp. We find that the dissipation associated with the twisting of the disc results in a large increase in the accretion rate through the disc, so that AGN accreting from a misaligned disc are likely to be significantly more luminous than those accreting from a flat disc. We compute explicitly the time-scales for the warping of the disc and for the alignment process and compare our results with earlier estimates based on simplified steady-state solutions. We also confirm earlier predictions that, under appropriate circumstances, accretion can proceed in a counter-aligned fashion, so that the accreted material will spin-down the hole, rather than spinning it up. Our results have implication in a number of different observational features of AGN such as the orientation and shape of jets, the shape of X-ray iron lines and the possibility of obscuration and absorption of X-ray by the outer disc as well as the general issue of the spin history of black holes during their growth. 相似文献
18.
Defining the solar brightness temperature accurately at millimeter wavelengths has always been challenging. One of the main reasons has been the lack of a proper calibration source. New Moon was used earlier as a calibration source. We carried out a new extensive set of observations at 8 mm using the New Moon for calibration. The solar and Moon observations were made using the 14-meter radiotelescope operated by the Aalto University Metsähovi Radio Observatory in Finland. In this article, we present our method for defining the brightness temperature of the quiet-Sun level (QSL). Based on these observations, we found \(8100~\mbox{K} \pm 300~\mbox{K}\) to be the mean value for the QSL temperature. This value is between the values that were reported in earlier studies. 相似文献
19.
《New Astronomy》2017
We investigate the stability of stationary integral solutions of an ideal irrotational fluid in a general static and spherically symmetric background, by studying the profile of the perturbation of the mass accretion rate. We consider low angular momentum axisymmetric accretion flows for three different accretion disk models and consider time dependent and radial linear perturbation of the mass accretion rate. First we show that the propagation of such perturbation can be determined by an effective 2 × 2 matrix, which has qualitatively similar acoustic causal properties as one obtains via the perturbation of the velocity potential. Next, using this matrix we analytically address the stability issues, for both standing and travelling wave configurations generated by the perturbation. Finally, based on this general formalism we briefly discuss the explicit example of the Schwarzschild spacetime and compare our results of stability with the existing literature, which instead address this problem via the perturbation of the velocity potential. 相似文献