RKRV Potential Energy Curves, Dissociation Energies, γ-Centroids And Franck-Condon Factors Of YO, CrO, BN, ScO, SiO And AlO Molecules

The potential energy curves for the electronic ground states of astrophysically important YO, CrO, BN, ScO, SiO and AlO molecules are constructed techniques using the five-parameter Hulburt-Hirschfelder function. The estimated dissociation energies are 7.235±0.15, 4.337 ± 0.09, 3.917 ± 0.09, 6.899 ± 0.14, 8.181 ± 0.17 and 5.202 ± 0.11 eV for YO, CrO, BN, ScO, SiO and AlO, respectively. The estimated D₀ values are in reasonably good agreement with literature values. The r-Centroids and Franck-Condon factors for the bands of B^{2 +} – X^{2 +} of YO, B⁵ –X⁵ > of CrO, A³ – X³ of BN, B^{2 +} –X^{2 +} of ScO, E^{1 +} – X^{1 +} of SiO and D^{2 +} – X^{2 +} and B^{2 +} –X^{2 +} of AlO molecules have been determined. The Franck-Condon factors are evaluated by the approximate analytical method of Jarmain and Fraser. The absence of the bands in these systems is explained.     

Three-dimensional,periodic, ‘halo’ orbits

A largely numerical study was made of families of three-dimensional, periodic, halo orbits near the collinear libration points in the restricted three-body problem. Families extend from each of the libration points to the nearest primary. They appear to exist for all values of the mass ratio , from 0 to 1. More importantly, most of the families contain a range of stable orbits. Only near L₁, the libration point between the two primaries, are there no stable orbits for certain values of . In that case the stable range decreases with increasing , until it disappears at =0.0573. Near the other libration points, stable orbits exist for all mass ratios investigated between 0 and 1. In addition, the orbits increase in size with increasing .     

Secular brightness increases of Titan,Uranus, and Neptune, 1972–1976

The brightnesses of Titan, Uranus, and Neptune in b (4718 ÅA) and y (5508 ÅA) have increased linearly since 1972 at rates ranging from 0.005 to 0.025 mag yr^?1. The observations were made differentially on a number of nights each season with respect to a network of comparison stars whose relative magnitudes were determined by independent measurements. Solar phase coefficients were derived for each object, and all observations have been normalized to zero solar phase angle and mean heliocentric distances. No explanation for the changes has been found, but a possible influence of solar activity upon planetary albedo is suggested by the fact that all of the objects observed have brightened during the declining half of the solar cycle.     

宇宙学,新进程

大约100年以前,埃德温·哈勃的两个发现改变了人们对宇宙的认识。首先,他第一次测定了仙女星系的距离,这一距离远远超过了银河系恒星分布的边界,人们这才发现拥有上千亿颗恒星的银河系也不过是宇宙众多“星系岛”中的一个。     

再见,机遇号

2002年,美国“双胞胎”火星探测器出生了,它们被分别赋予了"勇气号"与"机遇号"的名字。为了寻找火星上可能出现的水,2003年6月10日与7月7日,勇气号与机遇号两姐妹从家乡地球岀发,并分别于2004年1月4日、1月25日安全着陆于火星的两面,承担着人类的使命,开始对这颗红色的星球进行探测。     

High-Resolution 0.33-0.92 μm Spectra of Iapetus, Hyperion, Phoebe, Rhea, Dione, and D-Type Asteroids: How Are They Related?

New high-resolution spectra in the 0.33 to 0.92 μm range of Iapetus, Hyperion, Phoebe, Dione, Rhea, and three D-type asteroids were obtained on the Palomar 200-inch telescope and the double spectrograph. The spectra of Hyperion and the low-albedo hemisphere of Iapetus can both be closely matched by a simple model that is the linear admixture of the spectrum of a medium-sized, high-albedo icy saturnian satellite and D-type material. Our results support an exogenous origin to the dark material on Iapetus; furthermore, this material may share a common origin and a similar means of transport with material on the surface of Hyperion. The recently discovered retrograde satellites of Saturn (Gladman et al., Nature412, 163-166) may be the source of this material. The leading sides of Callisto and the Uranian satellites may be subjected to a similar alteration mechanism as that of Iapetus: accretion of low-albedo dust originating from outer retrograde satellites. Phoebe does not appear to be related to either Iapetus or Hyperion. Separate spectra of the two hemispheres of Phoebe show no identifiable global compositional differences.     

Поле излучения в полубесконечной атмосфере, содержащей источники энергии

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Оптимальное демпфирование нутационного движения спутников, стаБилизируемых вращением

, . , , . , , -, This article investigates the dynamics of a system for damping the nutating motion of a spin-stabilized satellite. The equations of motion of the satellite-damper system are derived omitting consideration of the influence of external torques. The conditions of stability of the stationary spinning are obtained and the optimal parameters of the satellite and the damper ensuring a maximal rate of damping of the nutation motion are determined.     

Слектры релятивистских эиектронов и изиучения, формируемьЫе комптоновским рассеянием лри отличном от нуля потоке

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Polarization evolution accompanying the very early sharp decline of gamma-ray burst X-ray afterglows

In the synchrotron radiation model, the polarization property depends on both the configuration of the magnetic field and the geometry of the visible emitting region. Some peculiar behaviours in the X-ray afterglows of gamma-ray bursts (GRBs) observed with Swift , such as energetic flares and a plateau followed by a sharp drop, might be highly linearly polarized because the outflows powering these behaviours may be dominated by Poynting flux. The breakdown of the symmetry of the visible emitting region may also be well hidden in the peculiar X-ray data and may give rise to interesting polarization signatures. In this paper, we focus on the polarization accompanying the very early sharp decline of GRB X-ray afterglows. We show that strong polarization evolution is possible in both the high latitude emission model and the dying central engine model, which are used to interpret this sharp X-ray decline. It is thus not easy to efficiently probe the physical origin of the very early X-ray sharp decline with future polarimetry. Strong polarization evolution is also possible in the decline phase of X-ray flares and in the shallow decline phase of X-ray light curves characterized by chromatic X-ray versus optical breaks. A detector such as the X-ray Telescope (XRT), but with polarization capability, on board a satellite like Swift would be suitable for testing our predictions.     

X-ray flares, plateaus and chromatic breaks of GRB afterglows from up-scattered forward-shock emission

Scattering of the forward-shock synchrotron emission by a relativistic outflow located behind the leading blast wave may produce an X-ray emission brighter than that coming directly from the forward shock and may explain four features displayed by Swift X-ray afterglows: flares, plateaus (slow decays), chromatic light-curve breaks and fast post-plateau decays. For a cold scattering outflow, the reflected flux overshines the primary one if the scattering outflow is nearly baryon-free and highly relativistic. These two requirements can be relaxed if the scattering outflow is energized by weak internal shocks, so that the incident forward-shock photons are also inverse-Compton scattered, in addition to bulk scattering. Sweeping-up of the photons left behind by the forward shock naturally yields short X-ray flares. Owing to the boost in photon energy produced by bulk scattering, the reflected emission is more likely to overshine that coming directly from the forward shock at higher photon energies, yielding light-curve plateaus and breaks that appear only in the X-ray. The brightness, shape and decay of the X-ray light-curve plateau depend on the radial distribution of the scatterer's Lorentz factor and mass flux. Chromatic X-ray light-curve breaks and sharp post-plateau decays cannot be accommodated by the direct forward-shock emission and argue in favour of the scattering-outflow model proposed here. On the other hand, the X-ray afterglows without plateaus, those with achromatic breaks and those with very long lived power-law decays are more naturally accommodated by the standard forward-shock model. Thus, the diversity of X-ray light curves arises from the interplay of the scattered and direct forward-shock emissions.     

The dust scattering model cannot explain the shallow X-ray decay in GRB afterglows

A dust scattering model was recently proposed to explain the shallow X-ray decay (plateau) observed prevalently in Gamma-Ray Burst (GRB) early afterglows. In this model, the plateau is the scattered prompt X-ray emission by the dust located close (about 10 to a few hundred pc) to the GRB site. In this paper, we carefully investigate the model and find that the scattered emission undergoes strong spectral softening with time, due to the model's essential ingredient that harder X-ray photons have smaller scattering angle thus arrive earlier, while softer photons suffer larger angle scattering and arrive later. The model predicts a significant change, that is  Δβ∼ 2–3  , in the X-ray spectral index from the beginning of the plateau towards the end of the plateau, while the observed data show close to zero softening during the plateau and the plateau-to-normal transition phase. The scattering model predicts a big difference between the harder X-ray light curve and the softer X-ray light curve, i.e. the plateau in harder X-rays ends much earlier than in softer X-rays. This feature is not seen in the data. The large scattering optical depths of the dust required by the model imply strong extinction in optical,   A _V ≳ 10  , which contradicts current findings of   A _V = 0.1–0.7  from optical and X-ray afterglow observations. We conclude that the dust scattering model cannot explain the X-ray plateaus.     

Jet breaks in the X-ray light-curves of Swift gamma-ray burst afterglows

In the set of 236 gamma-ray burst (GRB) afterglows observed by Swift between 2005 January and 2007 March, we identify 30 X-ray light-curves that have power-law fall-offs that exhibit a steepening ('break') at 0.1–10 d after they are triggered, to a decay steeper than t ^−1.5. For most of these afterglows, the X-ray spectral slope and the decay indices before and after the break can be accommodated by the standard jet model although a different origin of the breaks cannot be ruled out. In addition, there are 27 other afterglows which have X-ray light-curves that may also exhibit a late break to a steep decay, but the evidence is not that compelling. The X-ray emissions of 38 afterglows decay slower than t ^−1.5 until after 3 d, half of them exhibiting such a slow decay until after 10 d. Therefore, the fraction of well-monitored Swift afterglows with potential jet breaks is around 60 per cent, whether we count only the strongest cases for each type or all of them. This fraction is comparable to the 75 per cent of pre-Swift afterglows which have optical light-curves that displayed similar breaks at ∼1 d. The peak energy of the GRB spectrum of Swift afterglows with light-curve breaks shows the same correlations with the burst isotropic output (Amati relation) and with the burst collimated output (Ghirlanda relation) as previously found for pre- Swift optical afterglows with light-curve breaks. However, we find that the Ghirlanda relation is largely a consequence of Amati's and that the use of the jet-break time leads to a stronger Ghirlanda correlation only when the few objects that do not satisfy the Amati relation are included.     

Fitting X-ray Afterglow Light curves of Gamma-ray Bursts by Using the Magnetar Energy Injection Model

The central compact object for some gamma-ray bursts (GRBs) may be a strongly magnetized millisecond pulsar. It can inject energy to the outer shock of the GRB by through the magnetic dipole radiation, and therefore causes the shallow decay of the early afterglow. Recently, from a large number of GRB X-ray afterglows observed by Swift/XRT(X-ray telescope), it is revealed that many of them exhibit the shallow decay about 10²∼10⁴ s after the burst prompt emission. We have fitted the X-ray afterglow light curves of 11 GRBs by using the energy injection model of a magnetar with the rotation period in the millisecond order of magnitude. The obtained result shows the validity and universality of the magnetar energy injection model in explaining the shallow decay of afterglows, and simultaneously provides some constraints on the magnetic field strength and rotation period of the central magnetar.     

Taxonomy of gamma-ray burst optical light curves: identification of a salient class of early afterglows

The temporal behaviour of the early optical emission from gamma-ray burst afterglows can be divided into four classes: fast-rising with an early peak, slow-rising with a late peak, flat plateaus and rapid decays since first measurement. The fast-rising optical afterglows display correlations among peak flux, peak epoch and post-peak power-law decay index that can be explained with a structured outflow seen off-axis, but the shock origin (reverse or forward) of the optical emission cannot be determined. The afterglows with plateaus and slow rises may be accommodated by the same model, if observer location offsets are larger than for the fast-rising afterglows, or could be due to a long-lived injection of energy and/or ejecta in the blast wave. If better calibrated with more afterglows, the peak flux–peak epoch relation exhibited by the fast- and slow-rising optical light curves could provide a way to use this type of afterglows as standard candles.     

磁星能量注入模型拟合伽玛射线暴X射线余辉光变曲线

某些伽玛射线暴(简称伽玛暴)的中心致密天体可能是一颗具有强磁场的毫秒脉冲星,它通过磁偶极辐射可对伽玛暴外激波注入能量,从而导致早期余辉光变曲线的变平.近年来,从Swift卫星观测到的大量伽玛暴X射线余辉中发现,很多X射线余辉光变曲线在暴后10~2～10~4s期间的确存在明显的变平现象.利用周期为毫秒量级的磁星能量注入模型对11个加玛暴的X射线余辉光变曲线进行了拟合,显示该模型在解释余辉变平现象上的有效性和广泛性,通过对余辉光变曲线的拟合,同时也给出了相关中心磁星的磁场强度和旋转周期.     

伽玛暴及其早期X射线余辉的观测特征

Swift卫星的X射线望远镜观测揭示部分伽玛暴的早期余辉光变曲线有一个缓慢衰减的成分,而相当一部分却没有这样的成分.研究比较这两种暴的观测性质发现两类暴的持续时间、伽玛辐射总流量、谱指数、谱硬度比峰值能量等物理量均没有显著差异.然而有该成分的那些伽玛暴谱比较软、早期X射线余辉比较弱、伽玛射线辐射效率显著高于没有这个成分的那些暴.结果表明两类暴的前身星和中心机制一致,是否呈现这个缓慢衰减成分可能取决于外部介质.     

Detecting radio afterglows of gamma-ray bursts with FAST

Using the generic hydrodynamic model of gamma-ray burst(GRB) afterglows, we calculate the radio afterglow light curves of low luminosity, high luminosity,failed and standard GRBs in different observational bands of FAST’s energy window.The GRBs are assumed to be located at different distances from us. Our results rank the detectability of GRBs in descending order as high luminosity, standard, failed and low luminosity GRBs. We predict that almost all types of radio afterglows except those of low luminosity GRBs could be observed by a large radio telescope as long as the domains of time and frequency are appropriate. It is important to note that FAST can detect relatively weak radio afterglows at a higher frequency of 2.5 GHz for very high redshift up to z = 15 or even more. Radio afterglows of low luminosity GRBs can only be detected after the completion of the second phase of FAST. FAST is expected to significantly expand the sample of GRB radio afterglows in the near future.     

A possible explanation for the radio afterglow of GRB 980519: the dense medium effect

GRB 980519 is characterized by its rapidly declining optical and X-ray afterglows. Explanations of this behaviour include models invoking a dense medium environment, which makes the shock wave evolve quickly into the subrelativistic phase, a jet-like outflow, and a wind-shaped circumburst medium environment. Recently, Frail et al. found that the latter two cases are consistent with the radio afterglow of this burst. Here, by considering the transrelativistic shock hydrodynamics, we show that the dense medium model can also account for the radio light curve quite well. The potential virtue of the dense medium model for GRB 980519 is that it implies a smaller angular size of the afterglow, which is essential for interpreting the strong modulation of the radio light curve. Optical extinction arising from the dense medium is not important if the prompt optical–UV flash accompanying the γ -ray emission can destroy dust by sublimation out to an appreciable distance. Comparisons with some other radio afterglows are also discussed.