The Master Mobile Astronomical System. Optical Observations of Gamma-Ray Bursts

The operating principles of the telescope-robot system MASTER (Mobile Astronomical System of Telescopes-Robots, http://observ.pereplet.ru), designed to search for fast transient phenomena in the optical range, are described. The robot-telescope includes the following: a Richter-Slefogt telescope with D=355 mm, F/D=2.4, a Richter-Slefogt telescope with D=200 mm, F/D=2.4, a Flugge telescope with D=280 mm, F/D=2.5, a TV camera with a field of 20x40 degrees, and three CCD cameras. A German mount with a slew rate of 8 deg/s is used. MASTER obtains images down to 19^m in a field of 6 square degrees in a 1.5 minute exposure. We present some observations of the optical afterglow of cosmic gamma-ray bursts. MASTER was the first system in Europe to record optical emission from GRB030329.__________Translated from Astrofizika, Vol. 48, No. 3, pp. 463–475 (August 2005).     

Polarization in Gamma-Ray Bursts Produced by Pinch Discharge

Large-voltage, high-temperature plasma columns produced by pinch discharge can generate γ-ray flashes with energy spectra and spectral evolution consistent with what are observed in γ-ray bursts (GRBs), and the inverse Compton scattering (ICS) during the discharge process can produce high linear polarization. Our calculation indicates that the observed polarization depends on the angle between the line-of-sight todischarge, but only weakly depends onthe GRB and the direction of the pinch discharge, but only weakly depends on observed γray energy.     

Gamma-Ray Bursts and magnetar-forming Supernovae

The connection between long Gamma Ray Bursts (GRBs) and Supernovae (SNe) have been established through the well observed cases. These events can be explained as the prompt collapse to a black hole (BH) of the core of a massive star (M≳40M _⊙). The energies of these GRB-SNe were much larger than those of typical SNe, thus these SNe are called Hypernovae (HNe). The case of SN 2006aj/GRB060218 appears different: the GRB was weak and soft, being called an X-Ray Flash (XRF); the SN is dimmer and has very weak oxygen lines. The explosion energy of SN 2006aj was smaller, as was the ejected mass. In our model, the progenitor star had a smaller mass than other GRB-SNe (M∼20M _⊙), suggesting that a neutron star (NS) rather than a BH was formed. If the nascent NS was strongly magnetized as a magnetar and rapidly spinning, it may launch a weak GRB or an XRF. The peculiar light curve of Type Ib SN 2005bf may also be powered by a magnetar. The blue-shifted nebular emission lines of 2005bf indicate the unipolar explosion possibly related to standing accretion shock instability (SASI) associated with a newly born NS.     

伽玛射线暴及其余辉研究进展

伽玛射线暴是一种来自宇宙空间随机方向的短时间内伽玛射线突然增亮的现象。伽玛射线暴虽然早在1967年就由Vela卫星观测到,但直到1997年人们才通过余辉观测确定其寄主星系,并通过寄主星系的红移最终确定了伽玛射线暴的宇宙学起源。对伽玛射线暴研究概况进行了评述:详细介绍了伽玛射线暴及其余辉的观测进展,特别是近期Swift卫星和Fermi卫星带来的新发现;系统描述了伽玛射线暴标准火球模型、伽玛射线暴余辉物理(相对论性外流与暴周环境介质的相互作用过程、辐射产生机制等)及伽玛射线暴的前身星等。也对伽玛射线暴的未来研究进行了展望。     

γ射线暴的时变分析

γ射线暴是天空中突然的硬X射线/γ射线爆发现象,有着非常复杂的光变曲线。由于光变现象和辐射过程直接相关,因此,研究γ射线暴的时变规律是非常重要的。对γ射线暴的一些时变现象以及通过时变研究得出的分类、脉冲形状、功率谱、时间演化、光度等性质进行了总结,并对一些结果进行了讨论。     

再探伽玛射线暴中能量与脉冲宽度的依赖关系

探索来源于BATSE(http://cossc:gsfc:nasa:gov/batse)的GRB(Gamma-Ray Burst,伽玛射线暴)观测数据的半峰宽度(FWHM)与能量之间的关系,基于64个用KRL函数模型(即,文[1]中的(22)式)能很好地拟合的FRDE(Fast Rise and Exponential Decay,快速上升指数下降)型脉冲样本。发现64个样本中有63个的半峰宽度(FWHM)与能量之间的关系属于平台—幂率—平台型结构或峰型结构。64个样本的半峰宽度(FWHM)与能量之间的关系在观测中可详细分为5种类型:a)有34个样本为幂率形式关系;b)18个样本为低能段平台关系;c)有7个样本为高能段平台关系;d)4个样本为峰型结构关系;e)另外有1个样本为其他结构。结果表明:GRB观测数据的半峰宽度(FWHM)与能量之间的确存在有幂率的关系。此结果进一步确认了文[2](Qin etal 2005)的观点,即半峰宽度(FWHM)与能量之间的关系是由于火球模型的多普勒效应(Doppler effect)导致的。     

Hypernovae and Gamma-Ray Bursts

The nature of very energetic supernovae (hypernovae) is discussed. They are the explosive death of stars more massive than ~20–25M _⊙, probably linked to the enigmatic Gamma-Ray Bursts. The optical properties of hypernovae indicate that they are significantly aspherical. Synthetic light curves and late-phase spectra of aspherical supernova/hypernova models are presented. These models can account for the optical observations of SNe 1998bw and 2002ap. The abundance patterns of hypernovae are characterized by large ratios (Zn, Co)/Fe and small ratios (Mn, Cr)/Fe, indicating a significant contribution of hypernovae to the early Galactic chemical evolution.     

伽马射线暴的光度-光变复杂度关系

伽马射线暴的光变复杂度是描述其光变曲线复杂程度的物理量.由已知红移的伽马射线暴,Reichart等人发现其光变复杂度与各向同性光度之间有正相关性(LαVα,α在1.77～3.5之间),即光变越复杂,光度越高.此相关性类似于造父变星的周光关系,可用来估计伽马暴的距离和红移.调研、分析了各种光变复杂度的定义、算法和光变复杂度-各向同性光度关系的拟合结果,最后对光变复杂度和光度之间的关系做了总结和展望.     

Core-Collapse Supernovae and Gamma-Ray Bursts in TMT Era

Study of energetic cosmic explosions as a part of time domain astronomy is one of the key areas that could be pursued with upcoming Giant segmented optical-IR telescopes with a very large photon collecting area applying cutting edge technology. Existing 8–10 m class telescopes have been helpful to improve our knowledge about core-collapse supernovae, gamma-ray bursts and nature of their progenitors and explosion mechanisms. However, many aspects about these energetic cosmic explosions are still not well-understood and require much bigger telescopes and back-end instruments with high precision to address the evolution of massive stars and high-redshift Universe in more detail. In this presentation, possible thrust research areas towards core-collapse supernovae and gamma-ray bursts with the Thirty-Meter Telescope and back-end instruments are presented.     

Ring-Shaped Jets in Gamma-Ray Bursts

When the axis of a gamma-ray burst (GRB) does not coincide with the spin axis of its source, there may result a ring-shaped jet. Using some refined jet dynamics, we calculate multi-wavelength afterglow light curves for such ring-shaped jets. In the R-band we find an obvious break in the afterglow light curve due to the beaming effect and the break is affected by many parameters, such as the electron energy fraction ξe, the magnetic energy fraction ξ2B, the width of ring A0 and the medium number density n. The overall light curve can be divided into three power-law stages, I.e., an ultra-relativistic stage, an after-break stage and a deep Newtonian stage. For each stage the power-law index is larger in the ring-shaped jet than in the corresponding conical jet.     

Coherent Synchrotron Radiation of Gamma-Ray Bursts

By now there is no doubt that the gamma-ray bursts (GRB) have a cosmological origin. This allows to regard GRB as the most powerful known energy sources, ε∼ 10⁵⁴ erg (with a total number of gamma quanta N_γ∼ 10⁶⁰). A plausible mechanism of coherent synchrotron radiation (CSR) of relativistic electrons driven by a local magnetic field is studied in this paper. We consider relativistic electrons arising in the Compton scattering of a GRB in directions close to that of the ray from the source to a ground-based observer. The synchrotron pulses from Compton electrons located at different points on the line between the GRB source and the observer arrive at the observation point simultaneously. This simultaneity ensures the coherence of the detected radiation. Both molecular clouds in the host galaxy of the GRB and our own Galaxy, as well as the Earth atmosphere are assumed to be scatterers of the GRB radiation. Signals of each scatterer reach the Earth surface, and can be detected at radio wavelengths. We estimate the characteristics of this radiation. The comparison of GRB data with the corresponding information on CSR pulses offers a way to determine some global characteristics of the medium between the Earth and the GRB source.     

Oscillation structure of gamma-ray bursts and their possible origin

It is hypothesized that thermonuclear burning of the matter from the envelope of a massive compact star accreting onto a hot neutron star produced by spherically symmetric collapse of a stellar iron core can proceed in oscillation mode (much as is the case during thermal explosions of carbon-oxygen cores in lower mass stars). Local density oscillations near the neutron-star surface can generate shock waves; in these shocks, the electron-positron plasma is stratified from the remaining matter, and shells of an expanding relativistic fireball with an oscillation time scale in cosmological gamma-ray bursts (GRBs) of ～10^?2 s are formed. It is pointed out that the GRB progenitors can be nonrotating massive Wolf-Rayet (WR) stars whose collapse, according to observational data, can proceed without any substantial envelope ejection.     

The Early X-ray Afterglows of Optically Bright and Dark Gamma-Ray Bursts

A systematic study on the early X-ray afterglows of both optically bright and dark gamma-ray bursts (B-GRBs and D-GRBs) observed by Swift is presented. Our sample includes 25 GRBs of which 13 are B-GRBs and 12 are D-GRBs. Our results show that the distributions of the X-ray afterglow fluxes (Fx), the gamma-ray fluxes (5r), and the ratio (Rr,x.) are similar for the two kinds of GRBs, that any observed differences should be simply statistical fluctuation. These results indicate that the progenitors of the two kinds of GRBs are of the same population with comparable total energies of explosion. The suppression of optical emission in the D-GRBs should result from circumburst but not from their central engine.     

两类γ暴的谱形和光变曲线的统计差异

使用currentBATSE catakog中的一部分数据，定义了2个分别反映γ射线暴的能谱形和光变曲线的物理量FR和TR，同时对它们的分布作了统计分析，发现对于两类不同的γ射线暴，它们的分布存在统计上的较为明显的差异。这意味着两类暴可能产生于不同的辐射区域，两类暴的暴源可能有本质的差异，这些结果支持了把γ射线暴分为长暴和短暴的分类方法。     

A Restriction on the Duration and Peak Energy of Gamma-Ray Bursts

Two dimensional distributions of T90 versus Epeak(or Ebreak)for three bright GRB samples have been investigated.The result shows that although both T90 and Epeak(or Ebreak) each span over a wide range,they are restricted to the region log(T90)≤－log(Epeak) 5.24.This cannot be explained by the current fireball model.It may represent a constraint on the fireball model.     

Analysis of the spatial distribution of gamma-ray bursts in their host galaxies

We compare the radial distributions of known localized gamma-ray bursts (GRBs) relative to the centers of their host galaxies with the distributions of known objects in nearby galaxies (supernovae of various types, X-ray binaries), the hypothetical dark-matter profiles, and the distribution of luminous matter in galaxies in the model of an exponential disk. By comparing the moments of empirical distributions, we show that the radial distribution of GRBs in galaxies differs significantly from that of other sources. We suggest a new statistical method for comparing empirical samples that is based on estimating the number of objects within a given radius. The exponential disk profile was found to be in best agreement with the radial distribution of GRBs. The distribution of GRBs relative to the centers of their host galaxies also agrees with the dark matter profile at certain model parameters.     

Gamma—Ray Bursts：Afterglows and Central Engines

Gamma-ray bursts (GRBs) are the most intense transient gamma-ray events in the sky; this, together with the strong evidence (the isotropic and inhomogeneous distribution of GRBs detected by BASTE) that they are located at cosmological distances, makes them the most energetic events ever known. For example, the observed radiation energies of some GRBs are equivalent to the total convertion into radiation of the mass energy of more than one solar mass. This is thousand times stronger than the energy of a supernova explosion. Some unconventional energy mechanism and extremely high conversion efficiency for these mysterious events are required. The discovery of host galaxies and association with supernovae at cosmoligical distances by the recently launched satellite of BeppoSAX and ground based radio and optical telescopes in GRB afterglow provides further support to the cosmological origin of GRBs and put strong constraints on their central engine. It is the aim of this article to review the possible central engines, energy mechanisms, dynamical and spectral evolution of GRBs, especially focusing on the afterglows in multi-wavebands.     

γ射线暴谱硬度比的时间演化特征

采用小波分析方法对康普顿γ射线天文台(CGRO)上的BATSE仪器观测到的γ暴光变曲线数据进行去噪处理,并提出时间分辨的谱硬度比定义,用包含有599个暴的样本研究谱硬度比随时间的演化规律,结果表明:(1)样本中77%暴的谱硬度比随时间按"硬到软"规律演化,而23%暴谱硬度比随时间按"软到硬"规律演化,尤其是3%暴在整个暴中呈现单调的"软到硬"趋势。(2)平均谱硬度比随时间单调地以"硬到软"规律演化。(3)持续时间不同的暴,其谱硬度比演化曲线有较大的差异,其中"硬到软"演化趋势最显著的是持续时间在23～35s之间的那些暴;持续时间在1～10s的暴的演化特征跟其它组差异最显著,呈现比较明显的"软到硬"演化规律;持续时间在35～53s之间的暴平均谱硬度比几乎没有随时间演化。(4)不同辐射强度的暴平均谱硬度比的演化特征不同,随着峰值流量的增大,谱硬度比演化由"硬-软-硬"逐渐转变为"软-硬-软":峰值流量越大,"软-硬-软"的趋势越显著,而峰值流量越小,其峰值流量"硬-软-硬"的趋势越显著。我们对这些结果进行了简单的讨论。     

Swift时代伽马暴的观测及研究进展

Swift卫星从2004年11月20日升空开始运转到现在已有2年多时间.到目前为止,它-共观测到了200多个伽马暴及其余辉现象.由于Swift观测到了早期X射线余辉、短暴余辉和高红移伽马暴等新的重要现象,伽马暴研究进入了新的时代.该文首先对伽马暴的研究历史做简短回顾,然后简要介绍伽马暴的物理图像和Swift卫星的构成及特点,最后全面评述Swift的观测成就及由此引起的理论挑战.     

γ爆与不同富性的Abell团的交叉相关分析

我们分析了强γ爆和弱γ爆与不同富性的Ａｂｅｌｌ团之间的交叉相关特性。发现弱γ爆与所有富性的Ａｂｅｌｌ团都没有交叉相关，而强γ爆除了与Ｒ≥２的Ａｂｅｌｌ团没有交叉相关外，与Ｒ＝０，Ｒ＝１的Ａｂｅｌｌ团都有１σ以上的交叉相关。