Detection of iron lines in the persistent emission of some X-ray burst sources observed by EXOSAT

We have observed the X-ray burst sources 4U1728-33 and 4U1813-14 with the ME detectors aboard EXOSAT, and present here results of a spectral analysis of their persistent emissions. For both sources the data can be well fitted by a double blackbody continuum and a Gaussian emission line. The two spectral components can be interpreted in terms of a blackbody radiation from the neutron star, and emission from the inner part of an accretion disc. The line feature is consistent with the 6.7 keV iron emission line expected in the presence of a hot extended object (X-ray corona) around the neutron star.     

Solar microwave bursts — A review

We review the observational and theoretical results on the physics of microwave bursts that occur in the solar atmosphere. We particularly emphasize the advances made in burst physics over the last few years with the great improvement in spatial and time resolution especially with instruments like the NRAO three element interferometer, Westerbork Synthesis Radio Telescope and more recently the Very Large Array (VLA).We review the observations on pre-flare build-up of an active region at centimeter wavelengths. In particular we discuss the observations that in addition to the active region undergoing brightness and polarization changes on time scales of the order of an hour before a flare, there can be a change of the sense of polarization of a component of the relevant active region situated at the same location as the flare, implying the emergence of a flux of reverse polarity at coronal levels. The intensity distribution of cm- bursts is similar to that of soft X-ray and hard X-ray bursts. Indeed, it appears that the flaring behavior of the Sun at cm wavelengths is similar to that of some other cosmic transients such as flare stars and X-ray bursters.We discuss three distinct phases in the evolution of cm bursts, namely, impulsive phase, post-burst phase, and gradual rise and fall. The radiation mechanism for the impulsive phase of the microwave burst is gyrosynchrotron emission from mildly relativistic electrons that are accelerated near the energy release site and spiral in the strong magnetic field in the low corona. The details of the velocity distribution function of the energetic electrons and its time evolution are not known. We review the spectral characteristics for two kinds of velocity distribution, e.g., Maxwellian and Maxwellian with a power law tail for the energetic electrons. In the post-burst phase the energetic electrons are gradually thermalized. The thermal plasma released in the energy release region as well as the expanded parts of the overheated upper chromosphere may alter the emission mechanism. Thus, in the post-burst phase, depending on the average density and temperature of the thermal plasma, the emission mechanism may change from gyrosynchrotron to collisional bremsstrahlung from a thermal plasma. The gradual rise and fall (GFR) burst represents the heating of a flare plasma to temperatures of the order of 10⁶ K, in association with a flare or an X-ray transient following a filament disruption.We discuss the flux density spectra of centimeter bursts. The great majority of the bursts have a single spectral maximum, commonly around 6 cm- The U-shaped signature sometimes found in cm-dcm burst spectrum of large bursts is believed to a be a reflection of only the fact that there are two different sources of burst radiation, one for cm- and the other for dcm-, with different electron energy distributions and different magnetic fields.Observations of fine structures with temporal resolutionof 10–100 ms in the intensity profiles of cm- bursts are described. The existence of such fine time structures imply brightness temperatures in burst sources of order 10¹⁵ K; their interpretation in terms of gyrosynchrotron measuring or the coherent interaction of upper hybrid waves excited by percipitating electron beams in a flaring loop is discussed.High spatial resolution observations (a few seconds of arc to 1 arc) are discussed, with special reference to the one- and two-dimensional maps of cm burst sources. The dominance of one sense of circular polarization in some weak 6 cm bursts and its interpretation in terms of energetic electrons confined in an asymmetric magnetic loop is discussed. Two-dimensional snapshot maps obtained with the VLA show that multi-peak impulsive 6 cm burst phase radiation originates from several arcades of loops and that the burst source often occupies a substantial portion of the flaring loop, and is not confined strictly to the top of the loop. This phenomenon is interpreted in terms of the trapping of energetic electrons due to anomalous doppler resonance instability and the characteristic scale length of the magnetic field variation along the loop. The VLA observations also indicate that the onset of the impulsive phase of a 6 cm burst can be associated with the appearance of a new system of loops. The presence of two loop systems with opposite polarities or a quadrupole field configuration is reminiscent of flare models in which a current sheet develops in the interface between two closed loops.We provide an extensive review of the emission and absorption processes in thermal and non-thermal velocity distributions. Unlike the thermal plasma where absorption and emission are inter-related through Kirchoff's law, the radiation emitted from a small population of non-thermal electrons can be reabsorbed from the same electrons (self-absorption) or from the background (thermal) electrons through gyro-resonance absorption, and free-free absorption. We also suggest that the non-thermal electrons can be unstable and these instabilities can be the source of very high brightness temperature, fine structure ( 10 ms) pulsations.Finally in the last part of this review we present several microwave burst models-the magnetic trap model, the two-component model, thermal model and the flaring loop model and give a critical discussion of the strength and weakness of these models.     

Tenma observations of bright binary X-ray sources

Spectroscopic study of bright binary X-ray sources, performed with the gas scintillation proportional counters on board Tenma, is reviewed. Properties of an iron emission line from two classes of bright binary X-ray sources: X-ray pulsars and low-mass binary sources, are first presented. It is shown that a most likely candidate for the line emitting region is an Alfven shell in case of X-ray pulsars, whereas that of low mass binary sources is an outer accretion disk. Next, nature of the continuum emission from low-mass binary sources is consistently interpreted by a picture that an optically thick accretion disk extends down to very near the surface of a weakly magnetized neutron star. Origin of ultrasoft spectra of black hole candidate sources is also discussed.     

On maximum luminosities of X-ray bursts

In this paper we address problems associated with the observation of super-Eddington luminosities in X-ray burst sources and discuss some possible solutions.     

Space power systems requirements and issues: the next decade

Some of the more important space power technology issues, requirements, and challenges of the 1990s are described, and the impact of new component technology on the overall performance of space power systems is assessed. Advanced component, subsystem and system technologies that will significantly affect the performance, reliability, and survivability of next-generation baseload and burst mode space power systems are emphasized. Technology disciplines related to power sources (solar/nuclear and chemical), power conversion, energy storage, power conditioning/distribution and control, and waste-heat acquisition, transport, and rejection are primarily addressed. For some of them, performance trends that can be used as the basis for projecting future advanced power-system performance are developed. Performance capabilities for several different types of space power system for both baseload and burst mode applications are postulated on the basis of evolving technology and point designs that incorporate projections of advanced component capabilities     

Effect of Radar Amplitude and Phase Weighting Errors on Clutter Rejection by Burst Waveforms

This correspondence derives simple expressions for the ambiguity function X and the Q function of uniformly spaced burst waveforms that are subject to independent random errors in the transmitter and receiver pulse phase and amplitude weightings. The expressions are similar to those in [1], but have been put in more simplified form, and are valid for more general sources of error.     

A high-resolution Ge spectrometer for Gamma-Ray burst astronomy

The Transient Gamma-Ray Spectrometer (TGRS) to be flown aboard the WIND spacecraft is primarily designed to perform high resolution spectroscopy of transient -ray events, such as cosmic -ray bursts and solar flares over the energy range 25 keV to 8.2 MeV with an expected spectroscopic resolution of 3 keV at 1 MeV. The detector itself consists of a 215 cm³ high purityn-type Ge crystal kept at cryogenic temperatures by a passive radiative cooler. The geometric field of view defined by the cooler is 1.8 steradian. To avoid continuous triggers by soft solar events, a thin BeCu Sun-shield around the sides of the cooler has been provided. A passive Mo/Pb occulter, which modulates signals from within ±5° of the ecliptic plane at the spacecraft spin frequency, is used to identify and study solar flares, as well as emission from the galactic plane and center. Thus, in addition to transient event measurements, the instrument will allow the search for possible diffuse background lines and monitor the 511 keV positron annihilation radiation from the galactic center. In order to handle the typically large burst count rates, which can be in excess of 100 kHz, burst data are stored directly in an onboard 2.75 Mbit burst memory with an absolute timing accuracy of ±1.5 ms after ground processing. The memory is capable of storing the entire spectral data set of all but the largest bursts. WIND is scheduled to be launched on a Delta II launch vehicle from Cape Canaveral on November 1, 1994. After injection into a phasing orbit, the spacecraft will execute a double lunar swing-by before being moved into a controlled halo orbit about theL1 Lagrangian point (250R _e towards the Sun). This will provide a 5 light-second light travel time with which to triangulate gamma-ray burst sources with Earth-orbiting systems, such as those on-board the Gamma-Ray Observatory (GRO). The response of instrument to transient -ray events such as GRB's and solar flares will be presented as well as the expected response to steady state point sources and galactic center line emission.     

Evidence for a common source of fragmented decimeter emission and meter wave type II bursts in some solar type IV bursts

The paper presents facts of relevance for flare particle acceleration and shock wave excitation in the solar corona from a detailed analysis of meter wave (40–800 MHz OSRA Tremsdorf spectrograms) and microwave data (3.1–50 GHz polarimeter records Bern University) of several type IV bursts. We argue for a slowly uprising (about 0.1 of the local Alfvén speed) quasi-periodically acting (1...3 min period) accelerator in the 10¹¹...10⁹ cm^–3 density region. It produces particles emitting patches of fragmented decimetric/metric radio emission. One of the quasi-periodically appearing patches is associated with the microwave burst emission, the same (or a later, lower frequency one) immediately preceeds the meter wave type II burst lanes. Therefore the onset of the patch source seems to induce also the MHD-like disturbance which can steepen to the type II burst emitting shock. In some events the fragmented patches in the frequency-time plane show a linear spreading toward lower and higher frequencies. This can be a signature of an accelerated movement. Our interpretation of the data allows to understand some of the timing and location problems between the type II shockfront and other CME or flare components.     

The polarization of celestial X-rays

The processes by which X-radiation may be emitted by celestial sources are investigated, and the net polarization such radiation would possess is predicted. The amount of polarization observed at the Earth would depend both on the mechanism producing the radiation and on the fractional extent of the source over which it is coherently polarized. Highly polarized X-radiation would suggest synchrotron emission as the source mechanism. Radiation whose polarization is low but nonzero would be produced by synchrotron emission from a source whose magnetic fields are inhomogeneous on a large scale, or by bremsstrahlung of electrons whose velocity vectors lie in one predominant direction. Nonobservable polarization would result from sources of synchrotron radiation whose magnetic fields show small-scale irregularities, from the bremsstrahlung of electrons whose velocity vectors are either random or spherically symmetric in direction over the entire source, or from sources whose mechanism of producing the radiation imparts no net polarization, e.g., thermal radiation from neutron stars, line radiation from electronic shell transitions within atoms, and the inverse Compton effect. Measurement of the net polarization of the X-rays from the several known celestial sources could thus lead to the specification of the mechanisms capable of producing the radiation from such sources.     

Neutrinos from Supernovae

Neutrinos are fundamental particles in the collapse of massive stars. Because of their weakly interacting nature, neutrinos can travel undisturbed through the stellar core and be direct probes of the still uncertain and fascinating supernova mechanism. Intriguing recent developments on the role of neutrinos during the stellar collapse are reviewed, as well as our current understanding of the flavor conversions in the stellar envelope. The detection perspectives of the next burst and of the diffuse supernova background will be also outlined. High-energy neutrinos in the GeV-PeV range can follow the MeV neutrino emission. Various scenarios concerning the production of high-energy neutrinos are discussed.     

Synchrotron Radiation from Galactic Sources: What We Can Learn About Particle Acceleration

I review the observations of galactic synchrotron sources, focusing on shell supernova remnants (SNRs), with particular attention to attributes that constrain the properties of electron acceleration. Radio observations provide information on source fluxes, spectral index, morphology, and polarization. Recent observations give us strong reason to believe that several young SNRs show synchrotron X-ray emission. Even if X-rays are thermal, however, limits can be set on the maximum energy to which electrons can be accelerated without a spectral break, since no galactic SNR is observed to have X-ray emission (due to any source) as bright as the extrapolation from radio frequencies of radio synchrotron emission. If synchrotron X-rays are detected or inferred, their morphology and spectrum provide important information on mechanisms governing acceleration to the highest energies. I describe models of synchrotron emission from SNRs and their comparison with observations. Finally, I describe the tasks ahead for both observers and theoreticians, to make better use of what SNR synchrotron emission tells us about particle acceleration.     

非均匀温度场下声发射源定位方法

针对非均匀温度场下声发射信号传播速度不一致导致时差定位法定位精度下降的问题,研究基于波束形成法的声发射源定位效果.首先,基于板波理论研究声发射信号的传播特性,分析频散现象等对声发射信号速度测试的影响;然后通过数值仿真研究声发射波束形成法定位精度对不同重建速度的定位效果,进而发现在声发射波束形成法在阵列方向具有速度不敏感性.在加热前后的薄钢板上进行断铅定位实验,其结果表明声发射波束形成法可以在非均匀温度场中保持较高的定位精度,在机匣温度不均的航空发动机碰摩定位领域有良好的应用前景.      

A cosmochemical view of cosmic rays and solar particles

The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above 15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z > 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.Miller Institute Professor, 1972–73.     

星载二次电源的电磁兼容预设计

针对星载二次电源电磁兼容试验中辐射发射的超标问题,提出了电磁兼容预设计方法.分析二次电源电磁干扰产生的机理,利用SPICE仿真器建立传导干扰的电路等效模型,优化电路设计得到传导干扰源;应用修正的单线模型近似计算辐射干扰,根据计算结果,利用遗传算法优化等效偶极子的数目和分布位置建立场等效模型;基于场路结合的方法,以传导干...     

Solar Ultraviolet Bursts

The term “ultraviolet (UV) burst” is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016–2017.     

基于多物理场耦合的双脉冲发动机点火过程数值模拟

为研究双脉冲固体火箭发动机Ⅱ脉冲点火瞬态过程,发展一套多物理场耦合求解器。流体控制方程基于有限体积法求解,时间推进采用双时间步LU-SGS(Lower Upper Symmetric Guass Seidel)方法;固体推进剂表面温度基于耦合传热方法计算;结构动力学运动方程基于有限元方法离散,采用经典的Newmark格式进行时间推进,流固耦合采用松耦合算法,并通过算例验证求解器的可靠性。计算结果表明:该求解器能够数值模拟Ⅱ脉冲启动过程中的点火药气体冲击、燃气非定常流动及金属膜片机械响应过程,获得金属膜片的破裂时间和压强;且随着点火质量流率增加,推进剂装药首次点燃时间和金属膜片破裂时间变短,膜片破裂压强降低;金属膜片破裂时间和压强不仅与作用在其表面的压强载荷大小相关,而且与压强载荷加载的过程相关;金属膜片厚度越薄,膜片破裂时间越短,膜片轴向位移越大,膜片破裂压强越低。     

基于改进的传声器阵列数据分析技术的飞机机体噪声实验研究

 发展了一种平面传声器阵列数据处理新方法——洁净信号处理算法,通过对传声器阵列混淆和旁瓣信号的有效抑制,改进了对声源的分辨。基于改进的数据分析方法,研究了真实飞机结构和真实飞行条件下飞机起落架、襟翼等声源的频谱特性、指向特性。研究结果表明,新的信号处理算法可以更加清晰地分离出飞机表面的气动噪声源。起落架噪声的频谱是由宽频随机噪声与一些较为明显的单音噪声源组成,在不同辐射角度时,起落架噪声的声级在不同频率范围有一定的差异。襟翼侧缘噪声频谱是一个包含有个别明显单音噪声的宽频噪声谱,襟翼内侧缘噪声谱和外侧缘噪声谱有明显的差异,在低频范围,襟翼侧缘噪声的指向性比较明显,但在高频范围,襟翼侧缘噪声没有明显的指向性。     

Diffuse galactic gamma-ray continuum emission

Some of the most important questions about the diffuse gamma-ray continuum emission from the Galaxy are reviewed, based on Compton Observatory (CGRO) results, especially COMPTEL and EGRET, and also earlier COS-B analyses. The key issues include the rôle of emission from cosmic-ray interactions with molecular hydrogen and its energy dependence, emissivity gradients and their interpretation, the cosmic-ray electron spectrum and the effect of discrete sources. The relative contribution of the various emission processes at low and high latitudes is estimated and a plausible synthesis of the observed spectrum over 5 decades of energy is presented. In the energy range above 30 MeV, models based either on explicit cosmic-ray gradients or cosmic-ray/gas coupling can give acceptable fits to the data, and a clear distinction has yet to be made. The quality of the EGRET data may make this possible in the future. The value of the CO-to-H₂ conversion factor from -rays is still uncertain and there is considerable evidence for cloud-to-cloud variations. The existence of a small emissivity gradient is well established, but is difficult to explain in a diffusive cosmic-ray propagation picture with sources distributed like SNR or pulsars unless there is a larger halo than suggested by cosmic-ray composition studies. In the energy range 1–30 MeV covered by COMPTEL the spectrum of the diffuse emission has been measured and is consistent with a combination of bremsstrahlung and inverse-Compton emission; spatial analysis shows strong evidence for a component with a wide latitude extent which is plausibly identified with the inverse-Compton component. The molecular hydrogen appears to be only a weak -ray emitter at low energies, which can be interpreted in terms of reduced MeV cosmic-ray electron density in molecular clouds. New data on the hard X-ray diffuse galactic emission is becoming available and indicates the need for a low-energy upturn in the electron spectrum or some other additional component. The contribution of unresolved sources to the diffuse emission is unknown but-probably lies in the range 10–20%. At high latitudes the galactic emission is intense enough to significantly complicate the identification of the extragalactic component; in particular the inverse-Compton emission from a halo a few kpc in extent can account for much of the high-latitude galactic emission. The detection of the Large Magellanic Cloud and the non-detection of the Small Magellanic Cloud provide constraints on extragalactic cosmic-rays, and provide an interesting comparison with the properties of the galactic system. On account of the large amount of data from CGRO now available, this is a subject in rapid development, and this paper provides a snapshot of the situation around mid-1995.     

The Use of Extraterrestrial Radio Sources in the Measurement of Antenna Parameters

Extraterrestrial radio sources, whose emission characteristics (flux density, spectrum, angular size) and coordinates have been firmly established by careful observations, have application in the measurement of the effective area (aperture efficiency and gain) of an antenna and its radiation pattern. The radio-emission characteristics of the strong discrete (celestial) radio sources, of the sun, and of the moon are presented. Problems encountered when the sensitivity is insufficient for complete radiation pattern determination , when the width of the radio source is comparable to the beamwidth of the antenna, when the illumination of the antenna aperture varies with pointing direction, or when the techniques are extended (after determining the gain of the ground-based antenna) to the measurement of the effective radiated power from a satellite are discussed.