首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 255 毫秒
1.
We propose a next generation space instrument: the Fresnel imager, a large aperture and lightweight focusing device for UV astrophysics. This paper presents the laboratory setup used to validate the Fresnel imager at UV at wavelengths around 260 nm, and the results obtained. The validation of this optical concept in the visible domain has been previously published, with the first results on sky objects. In this paper we present new optical tests in the UV, of diffractive focusing and chromatic correction at wavelengths around 260 nm. The results show images free from chromatic aberration, thanks to a chromatic corrector scheme similar to the one used in the visible. To complete these tests and reach real astrophysical UV sources, we propose a short space mission featuring a Fresnel imager prototype placed on the international space station: during the mission this small aperture instrument would be aimed at UV sources such as bright stars and solar system objects, to assess at relatively low cost the limits in contrast and resolution of diffractive focusing in space conditions, on real UV astrophysical objects. At wavelengths from 100 to 300 nm, covering Lyman-α, we expect some scientific return from this mission, but the main goal is to increase the TRL, improving the chances of success for a later proposal featuring a full fledged Fresnel imager 10 meters in aperture or more, that would explore new domains of UV astrophysics at very high angular resolution and very high contrast.  相似文献   

2.
This paper provides new detailed specifics and characteristics of the close double white dwarfs (CDWDs) LISA foreground noise floor using the newest information on the astrophysics with binary population algorithms, which were judged to satisfy the observations. We use the binary population synthesis approach of CDWDs in combination with the latest findings of synthesis models from Han (Mon. Not. R. Astron. Soc. 296:1019, 1998), here, specifically formation by the three evolutionary channels of (a) stable Roche lobe overflow plus common envelope (RLOF?+?CE), (b) CE plus CE, (c) exposed core plus CE. We collect resulting CDWDs in the frequency range of 10?8 Hz to 10?2 Hz and compare strength and number statistics per resolvable frequency bin to derive the properties of the noise floor and the nature of possible outliers that may become individually resolvable. We find resolvable outliers to be solely produced by CE?+?CE evolution; the other two channels contribute only to the unresolvable foreground noise. Within the CE?+?CE evolution a detailed look into the chemical buildup of the stars reveals dominantly double helium (He?+?He) WDs or carbon-oxygen CO?+?He WDs.  相似文献   

3.
One of the key goals of NASA’s astrophysics program is to answer the question: How did galaxies evolve into the spirals and elliptical galaxies that we see today? We describe a space mission concept called Galaxy Evolution Spectroscopic Explorer (GESE) to address this question by making a large spectroscopic survey of galaxies at a redshift, z～1 (look-back time of ～8 billion years). GESE is a 1.5-m space telescope with an ultraviolet (UV) multi-object slit spectrograph that can obtain spectra of hundreds of galaxies per exposure. The spectrograph covers the spectral range, 0.2–0.4 μm at a spectral resolving power, R～500. This observed spectral range corresponds to 0.1–0.2 μm as emitted by a galaxy at a redshift, z=1. The mission concept takes advantage of two new technological advances: (1) light-weighted, wide-field telescope mirrors, and (2) the Next-Generation MicroShutter Array (NG-MSA) to be used as a slit generator in the multi-object slit spectrograph.  相似文献   

4.
The Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) filtergrams, taken at six wavelengths around the Fe i 6173.3 Å line, contain information about the line-of-sight velocity over a range of heights in the solar atmosphere. Multi-height velocity inferences from these observations can be exploited to study wave motions and energy transport in the atmosphere. Using realistic convection-simulation datasets provided by the STAGGER and MURaM codes, we generate synthetic filtergrams and explore several methods for estimating Dopplergrams. We investigate at which height each synthetic Dopplergram correlates most strongly with the vertical velocity in the model atmospheres. On the basis of the investigation, we propose two Dopplergrams other than the standard HMI-algorithm Dopplergram produced from HMI filtergrams: a line-center Dopplergram and an average-wing Dopplergram. These two Dopplergrams correlate most strongly with vertical velocities at the heights of 30?–?40 km above (line center) and 30?–?40 km below (average wing) the effective height of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity information from two layers separated by about a half of a scale height in the atmosphere, at best. The phase shifts between these multi-height Dopplergrams from observational data as well as those from the simulated data are also consistent with the height-difference estimates in the frequency range above the photospheric acoustic-cutoff frequency.  相似文献   

5.
The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a European Space Agency hard X-ray/γ-ray observatory for astrophysics, covering photon energies from 15 keV to 10 MeV. It was launched in 2002, and since then the Bismuth Germanate (BGO) detectors of the Anti-Coincidence Shield (ACS) of the Spectrometer on INTEGRAL (SPI) have detected many hard X-ray (HXR) bursts from the Sun, producing light curves at photon energies above ≈?100 keV. The spacecraft has a highly elliptical orbit, providing long uninterrupted observing (about 90 % of the orbital period) with nearly constant background due to the shorter time needed to cross Earth’s radiation belts. However, because of technical constraints, INTEGRAL cannot be pointed at the Sun, and high-energy solar photons are always detected in nonstandard observation conditions. To make the data useable for solar studies, we have undertaken a major effort to specify the observing conditions through Monte Carlo simulations of the response of ACS for several selected flares. We checked the performance of the model employed for the Monte Carlo simulations using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations for the same sample of solar flares. We conclude that although INTEGRAL was not designed to perform solar observations, ACS is a useful instrument for solar-flare research. In particular, its relatively large effective area allows determining good-quality HXR/γ-ray light curves for X- and M-class solar flares and, in some cases, probably also for C-class flares.  相似文献   

6.
We employ ring-diagram analysis to study the sub-surface thermal structure of active regions. We present results using a large number of active regions over the course of Solar Cycle 23. We present both traditional inversions of ring-diagram frequency differences, with a total sample size of 264, and a statistical study using Principal Component Analysis. We confirm earlier results on smaller samples that sound speed and adiabatic index are changed below regions of strong magnetic field. We find that sound speed is decreased in the region between approximately r=0.99?R ⊙ and r=0.995?R ⊙ (depths of 3 Mm to 7 Mm) and increased in the region between r=0.97?R ⊙ and r=0.985?R ⊙ (depths of 11 Mm to 21 Mm). The adiabatic index [Γ1] is enhanced in the same deeper layers where sound-speed enhancement is seen. A weak decrease in adiabatic index is seen in the shallower layers in many active regions. We find that the magnitudes of these perturbations depend on the strength of the surface magnetic field, but we find a great deal of scatter in this relation, implying that other factors may be relevant.  相似文献   

7.
Following several recommendations presented by the Astrophysics Decadal Survey 2010 centered around the need to define “a future ultraviolet-optical space capability”, on 2012 May 25, NASA issued a Request for Information (RFI) seeking persuasive ultraviolet (UV) and visible wavelength astrophysics science investigations. The goal was to develop a cohesive and compelling set of science objectives that motivate and support the development of the next generation of ultraviolet/visible space astrophysics missions. Responses were due on 10 August 2012 when 34 submissions were received addressing a number of potential science drivers. A UV/visible Mission RFI Workshop was held on 2012 September 20 where each of these submissions was summarized and discussed in the context of each other. We present a scientific analysis of these submissions and presentations and the pursuant measurement capability needs, which could influence ultraviolet/visible technology development plans for the rest of this decade. We also describe the process and requirements leading to the inception of this community RFI, subsequent workshop and the expected evolution of these ideas and concepts for the remainder of this decade.  相似文献   

8.
Wesson obtained a limit on quantum and gravitational mass in the universe by combining the cosmological constant Λ, Planck’s constant ?, the speed of light c, and also the gravitational constant G. The corresponding masses are 2.0×10?62 kg and 2.3×1054 kg respectively, and in general can be obtained with the help of a generic dimensional analysis, or from an analysis where the cosmological constant appears in a four dimensional space-time and as a result of a higher dimensional reduction. In this paper our goal is to establish a relation for both quantum and gravitational mass as function of the information number bit N. For this reason, we first derive an expression for the cosmological constant as a function of information bit, since both masses depend on it, and then various resulting relations are explored, in relation to information number of bits N. Fractional information bits imply no information extraction is possible. We see, that the order of magnitude of the various parameters as well as their ratios involve the large number 10122, that is produced naturally from the fundamental parameters of modern cosmology. Finally, we propose that in a complete quantum gravity theory the idea of information the might have to be included, with the quantum bits of information (q-bits) as one of its fundamental parameters, resulting thus to a more complete understanding of the universe, its laws, and its evolution.  相似文献   

9.
Non extensive statistical physics has been applied to various problems in physics including astrophysics. In this paper we explore the possibility of using non-extensive approach to explain the recently observed pulsed γ-ray from Crab pulsar above 100 GeV observed by VERITAS γ-ray telescope.  相似文献   

10.
We report on ab initio coupled-cluster calculations of the interaction potential energy surface for the HCNH+–He complex. The aug-cc-pVTZ Gaussian basis, to which is added a set of bond functions placed at mid-distance between HCNH+ center of mass and He atom is used. The HCNH+ bonds length are set to their values at the equilibrium geometry, i.e., r e [HC]=1.0780 Å, r e [CN]=1.1339 Å and r e [NH]=1.0126 Å. The interaction energy presents a global minimum located $266.9~\mathrm{cm^{-1}}$ below the HCNH+–He dissociation limit. Using the interaction potential obtained, we have computed rotational excitation cross sections in the close-coupling approach and downward rate coefficients at low temperature (T≤120 K). It is expected that the data worked out in this study may be beneficial for further astrophysical investigations as well as laboratory experiments.  相似文献   

11.
The recently released Planck data have constrained 4-dimensional inflationary parameters even more accurately than ever. We consider an extension of the braneworld model with induced gravity and a non-minimally coupled scalar field on the brane. We constraint the inflation parameters in this setup, by adopting six types of potential, in confrontation with the joint Planck + WMAP9 + BAO data. We show that a potential of the type V(φ)=V 0exp(?βφ) has the best fit with newly released observational data.  相似文献   

12.
Due to its sensitivity and speed, the detector still widely used in Cerenkov astrophysics experiments remains the PhotoMultiplier Tube (PMT). For instance, recent pathbreaking experiments in Very High Energy astrophysics (VHE), such as MAGIC and HESS, have used mainstream PMT technology [Aharonian, F. et al Astron. Astrophys. 492(1):L25–L28 (2008)]. Moreover the Cerenkov Telescope Array (CTA) which is now in its design phase, is also planed to be based on PMT’s. However, there are some disadvantages to the PMT technology: the rather poor quantum efficiency, the use of high voltages, the high cost when used in large number in a matrix arrangement and the large weight. Hence, we have investigated the possibility to design future Cerenkov telescopes based on solid state technology, specifically Geiger avalanche photodiodes. In a preliminary development test, we placed HAMAMATSU avalanche photodiodes at the focal plane of a 60 cm diameter telescope at the Pic du Midi in the French Pyrénées, in order to record incident cosmic rays. In this paper, we describe not only the experimental setup but we also put special emphasis to the reduction of the semi-conductor noise. We also show first data that were recorded during two runs in the fall of 2006, and conclude by the presentation of the design of an “integrated, low-cost solid state photodiode arrangement” which might be an alternative to PMT’s for future VHE telescopes.  相似文献   

13.
We present a seismological method for probing the solar atmosphere above sunspot umbrae with three-minute oscillations. Our technique allows us to estimate both the vertical distance between atmospheric layers and the wave-propagation speed, without specifying any additional parameters, in particular, the phase speed of the wave or the emission formation heights. Our method uses the projected wave paths of slow magnetohydrodynamic waves that propagate through the atmospheric layers of different heights and are guided by the magnetic field. The length of the projected wave path depends upon the distance between the layers and the inclination angle of the magnetic field with respect to the line of sight, allowing us to estimate the distance between the layers from measured projected wave paths and the local magnetic-field vector. In turn, the wave-propagation delay registered at different heights allows for the calculation of the phase speed. We estimated the vertical distance between the emission layers at the temperature minimum (1600 Å) and transition region (304 Å), as well as the average phase speed above the sunspot umbrae, for three active regions. We found that the distance between the 1600 Å emission layer and the transition region above the sunspot umbrae lies in the range of 500?–?800 km. The average phase speed between these layers was found to be about 30 km?s?1, giving a sound speed of 6 km?s?1. The temperature between the layers has been roughly estimated as 3000 K and corresponds to the region of the temperature minimum. The results obtained are consistent with the semiempirical model of the sunspot-umbrae atmosphere by Fontenla et al. (Astrophys. J.707, 482, 2009).  相似文献   

14.
We consider the planar restricted three-body problem and the collinear equilibrium point L 3, as an example of a center × saddle equilibrium point in a Hamiltonian with two degrees of freedom. We explore numerically the existence of symmetric and non-symmetric homoclinic orbits to L 3, when varying the mass parameter μ. Concerning the symmetric homoclinic orbits (SHO), we study the multi-round, m-round, SHO for m ≥ 2. More precisely, given a transversal value of μ for which there is a 1-round SHO, say μ 1, we show that for any m ≥ 2, there are countable sets of values of μ, tending to μ 1, corresponding to m-round SHO. Some comments on related analytical results are also made.  相似文献   

15.
In this work, we present a classification of laboratory astrophysics experiments. We introduce different invariance concepts in order to build scaling laws and to determine the astrophysical relevant of laboratory experiments. Finally we present an analysis of the two-temperature radiating fluid scalability.  相似文献   

16.
Degeneracy effects for bosons are more important for smaller particle mass, smaller temperature and higher number density. Bose condensation requires that particles be in the same lowest energy quantum state. We propose a cosmic background Bose condensation, present everywhere, with its particles having the lowest quantum energy state, ?c/λ, with λ about the size of the visible universe, and therefore unlocalized. This we identify with the quantum of the self gravitational potential energy of any particle, and with the bit of information of minimum energy. The entropy of the universe (～10122 bits) has the highest number density (～1036 bits/cm3) of particles inside the visible universe, the smallest mass, ～10?66 g, and the smallest temperature, ～10?29 K. Therefore it is the best candidate for a Cosmic Background Bose Condensation (CBBC), a completely calmed fluid, with no viscosity, in a superfluidity state, and possibly responsible for the expansion of the universe.  相似文献   

17.
Studying the evolution of magnetic clouds entrained in coronal mass ejections using in-situ data is a difficult task, since only a limited number of observational points is available at large heliocentric distances. Remote sensing observations can, however, provide important information for events close to the Sun. In this work we estimate the flux rope orientation first in the close vicinity of the Sun (2?–?20 R ⊙) using forward modeling of STEREO/SECCHI and SOHO/LASCO coronagraph images of coronal mass ejections and then in situ using Grad–Shafranov reconstruction of the magnetic cloud. Thus, we are able to measure changes in the orientation of the erupted flux ropes as they propagate from the Sun to 1 AU. We present both techniques and use them to study 15 magnetic clouds observed during the minimum following Solar Cycle 23 and the rise of Solar Cycle 24. This is the first multievent study to compare the three-dimensional parameters of CMEs from imaging and in-situ reconstructions. The results of our analysis confirm earlier studies showing that the flux ropes tend to deflect towards the solar equatorial plane. We also find evidence of rotation on their travel from the Sun to 1 AU. In contrast to past studies, our method allows one to deduce the evolution of the three-dimensional orientation of individual flux ropes rather than on a statistical basis.  相似文献   

18.
Roche-lobe overflow and common envelope evolution are very important in binary evolution, which is believed to be the main evolutionary channel to hot subdwarf stars. The details of these processes are difficult to model, but adiabatic expansion provides an excellent approximation to the structure of a donor star undergoing dynamical time scale mass transfer. We can use this model to study the responses of stars of various masses and evolutionary stages as potential donor stars, with the urgent goal of obtaining more accurate stability criteria for dynamical mass transfer in binary population synthesis studies. As examples, we describe here several models with the initial masses equal to 1 M ⊙ and 10 M ⊙, and identify potential limitations to the use of our results for giant-branch stars.  相似文献   

19.
We analyze the 26 November 2005 solar radio event observed interferometrically at frequencies of 244 and 611 MHz by the Giant Metrewave Radio Telescope (GMRT) in Pune, India. These observations are used to make interferometric maps of the event at both frequencies with the time cadence of 1 s from 06:50 to 07:12 UT. These maps reveal several radio sources. The light curves of these sources show that only two sources at 244 MHz and 611 MHz are well correlated in time. The EUV flare is more localized with flare loops located rather away from the radio sources. Using SoHO/MDI observations and potential magnetic field extrapolation we demonstrate that both the correlated sources are located in the fan structure of magnetic field lines starting from a coronal magnetic null point. Wavelet analysis of the light curves of the radio sources detects tadpoles with periods in the range P=10?–?83 s. These wavelet tadpoles indicate the presence of fast magnetoacoustic waves that propagate in the fan structure of the coronal magnetic null point. We estimate the plasma parameters in the studied radio sources and find them consistent with the presented scenario involving the coronal magnetic null point.  相似文献   

20.
We study the impact of ionospheric disturbances on the Earth’s environment caused by the solar events that occurred from 20 April to 31 May 2010, using observations from the Mexican Array Radio Telescope (MEXART). During this period of time, several astronomical sources presented fluctuations in their radio signals. Wavelet analysis, together with complementary information such as the vertical total electron content (vTEC) and the Dst index, were used to identify and understand when the interplanetary scintillation (IPS) could be contaminated by ionospheric disturbances (IOND). We find that radio signal perturbations were sometimes associated with IOND and/or IPS fluctuations; however, in some cases, it was not possible to clearly identify their origin. Our Fourier and wavelet analyses showed that these fluctuations had frequencies in the range ≈?0.01 Hz?–?≈?1.0 Hz (periodicities of 100 s to 1 s).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号