ALMA探秘伽玛暴

宇宙中最剧烈的爆炸当属伽玛暴（Gamma—Ray Burst,简称为GRB）。我们知道光本质上就是电磁波。而电磁波具有一个宽泛的波长范围,不同的波长对应不同类型的电磁辐射。不同波长的辐射对应的能量也大不相同（图1）。波长越短的电磁波携带的能量越强。在实际的研究中,天文学家也主要是依靠遥远的天体发出的各种波长的电磁波来研究它们。图1中最右端、波长最短的电磁波即为伽玛射线。     

伽玛暴与宇宙中的生命灭绝

有几个亘古不变的问题始终贯穿在整个人类发展的文明史中。从我们诞生开始这些问题就被深深地烙入我们的基因：我们是谁？从哪里来？又将去向何方？为了回答这些问题,我们不断地寻找人类的起源,乃至地球上一切生命的起源。同时我们也在努力尝试寻找地外文明,想尽办法与他们建立联系,试图寻求和理解我们存在于这个宇宙的意义。     

伽玛暴宇宙学的研究

伽玛射线暴(简称伽玛暴,gamma-ray burst (GRB))是一种来自宇宙空间中的伽玛射线波段流量突然增亮的现象,最早由Vela卫星在1967年发现.1997年人们通过余辉测得了伽玛暴的红移,从而确定了其宇宙学的起源.伽玛暴宇宙学包括用长暴的标准烛光关系限制暗能量和宇宙学参数,用长暴研究高红移的恒星形成率,研究金属丰度的演化、尘埃及量子引力等.     

伽玛射线暴的层层迷团

几十年来,天文学家一直感到困惑的是,宇宙中不知道什么地方,突然像闪光灯一样亮了一下,又恢复了平静。当然,天文学家绝不会认为这是外星人在偷偷地向地球拍照,这是一次高能量的伽玛射线爆炸（见图1）。     

伽玛射线暴还是超新星？

“超新星”SN 2008D,第一个人们实时观测到的超新星爆发（见《天文爱好者》2008．7“宇宙信息”）,又给人们带来了新的惊喜。最新研究表明,它并不是一次纯粹的超新星爆发事件,也不是能量更高的伽玛射线暴,而是两者的混合体。     

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。     

尘埃消光对伽玛射线暴余辉的影响

为了研究尘埃消光对伽玛射线暴余辉的影响,基于严格的Mie理论和最新的星际尘埃光学性质,进行了高精度的数值计算,并分析具有不同物理参数的尘埃所产生的消光曲线.结果表明,介质密度和金属丰度是决定消光总量的主要物理参数,而尘埃颗粒大小的分布则是产生不同消光曲线轮廓的重要物理参数.如果尘埃颗粒相互聚集形成导致尺度增大,将产生较平或者较灰的消光曲线,同时绝对总量将减少;相反,如果尘埃颗粒由于某种原因发生离解导致尺度变小,将产生较陡的消光曲线,同时消光总量将增加.这些结果将对理解光学暗暴的形成机制提供重要的启示.     

致密介质中柱状喷流的伽玛射线暴余辉

很多伽玛射线暴应当是产生于极端相对论性的喷流.关于喷流,绝大多数的讨论都是围绕锥状喷流展开.然而有观测发现,一些天体中的喷流在很长距离上始终保持着几乎不变的截面积,即表现为柱状喷流.研究致密介质环境中有侧向膨胀的柱状喷流的余辉,描述其动力学演化和辐射过程,分别得到解析解和数值解,并对两者进行了对比.研究的暴周星际介质是光学厚的,在初始的主暴阶段,喷流辐射出高度准直的高能射线,升华了暴周介质,形成一个在光学波段光学薄的柱状通道.余辉阶段,由于喷流是有侧向膨胀的,观测者只能收集到视觉面积占比例越来越小的光学辐射,理论上可得到衰减极为快速的光变曲线,流量随时间的衰减约为Svα t-p-1(p为电子幂律分布的谱指数).如此迅速的衰减使得光学余辉将难以被观测到,提供了一种对暗伽玛射线暴的解释.     

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.     

伽玛射线暴内光度和峰值能量关系的再研究

使用了185个伽玛射线暴(简称伽玛暴)的5 218个时间分辨谱数据,重新研究了伽玛暴内光度和峰值能量的关系及该关系对火球模型的限制二研究结果表明:(1)不管是在伽玛暴内还是在伽玛暴间各向同性等值光度Liso和静止系中vF,谱的峰值能量E'p之间关系式Liso∝E'2p都存在;(2)不管是动能主导的内激波模型还是磁耗散主导的外激波模型都能很好地解释关系式Liso∝ E'2p及ω的值.这些结论与Liang等人的结论是一致的.     

The Role of T_(50) in the Classification of Gamma-ray Bursts

The role of T50 in classifying gamma-ray bursts (GRBs) is investigated. We take T50=0.7s as the line of division and find that some bursts belonging to the class of long bursts defined by T90≥2s now become short bursts (sample 1), while some belonging to the class of short bursts defined by T90 < 2 s now become long bursts (sample 2). We study how these sources are affected by the two methods of classification and find the change of classes of sample 1 is due to some peculiar properties of the light curves. Based on their characters, most of the bursts of sample 1 should be taken as short bursts.     

两类γ暴的谱形和光变曲线的统计差异的K-S检验

最近的研究表明：短γ射线暴（γ暴）的辐射特性和长γ暴的前两秒的辐射特征相似，这引发了对以前争论的问题，即：这两类暴是否是本质上相同的暴？本文主要对和γ暴谱形和光变曲线有关的两个量的分布进行了K—S检验。分析表明：这两类暴的分布是不同的，这和广为接受的两类γ暴事件产生于不同机制的观点是一致的。     

Sequences in the Hardness Ratio-Peak Energy Plane of Gamma-Ray Bursts

The narrowness of the distribution of the peak energy of the vFv spectrum of gamma-ray bursts (GRBs) and the unification of GRB populations are great puzzles yet to be solved. We investigate the two puzzles based on the global spectral behaviors of different GRB populations, the long GRBs, the short GRBs, and the X-ray flashes (XRFs), in the HR - Ep plane (HR the spectral hardness ratio) with BATSE and HETE-2 observations. It is found that the long GRBs and the XRFs observed by HETE-2 seem to follow the same sequence in the HR-Ep plane, with the XRFs at the low end of this sequence. We fit the sequence by a universal Band function, and find that this sequence is mainly defined by the low energy index α, and is insensitive to the high energy index,β. With fixed β=-5, a best fit is given by α=-1.00 with X^2min/dof=2.2. The long and short GRBs observed by BATSE follow significantly different sequences in the HR-Ep plane, with most of the short GRBs having a larger hardness ratio than the long GRBs at a given Ep. For the long GRBs a best-fit yields α=-0.30 and β=-2.05. For the short GRBs, a best fit gives α=-0.60 with x^2min=1.1 (withβfixed at-2.0 because it is numerically unstable). The α value for the short GRBs is significantly greater than that for the long GRBs. These results indicate that the global spectral behaviors of the long GRB sample and the XRF sample are similar, while that of the short GRBs is different. The short GRBs seem to be a unique subclass of GRBs, and they are not the higher energy extension of the long GRBs.     

长γ暴时域功率密度谱与光度的强相关

提供一个基于光变曲线的长γ暴光度的估计量．对BASTE记录到的12个已知红移的γ暴，利用时域上的时变分析方法计算了各暴的功率密度谱，用功率密度的峰值P表征光变曲线变化的剧烈程度．通过拟合发现在共动坐标系P与γ暴的各向同性峰值光度L之间存在着相关关系．这是继Norris等和Reichart等发现时间延迟与光度、变化率与光度的相关性之后又一个γ暴时变特征量与其光度之间的相关关系．     

The correlation between the local density of galaxies and the presence of Gamma‐Ray Bursts

In order to explore the characteristics of the environment surrounding Gamma‐Ray Bursts (GRBs), we have studied the correlation between the local density of galaxies and the presence of bursts. Using the Main Galaxy sample of the Sloan Digital Sky Survey Data Release 3 (SDSS DR3) and the Current BATSE GRB Catalog, we calculate the local surface density of galaxies within a radius of <2° surrounding each burst. It turns out that there is no correlation between the local density of galaxies and the presence of GRBs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Room temperature solid-state detectors for Gamma ray burst spectroscopy

We have evaluated several solid state detectors which offer excellent energy resolution at room temperature for soft X-rays. For soft X-rays (< 1 keV to 20 keV), silicon P-intrinsic-N (PIN) and avalanche-mode photodiodes (APD's) have been studied. Using commercially available charge sensitive pre-amplifiers, these photodiodes provide 1 keV resolution without cooling. Their detection efficiencies are limited to about 20 keV and 15 keV, respectively. To overcome this constraint, we have studied thick (1.5 mm) PIN detectors made by Micron Semiconductor Ltd., U.K., as well as compound semiconducting materials with high Z constituents such as CZT and PbI₂. PbI₂ allows high detection efficiencies of photons up to 100 keV with detectors 100–300 microns thick. These new detectors offer the capability to study the low-energy spectral evolution of Gamma ray bursts (GRBs). A matrix of these detectors could be used as an image plane detector with moderate spatial resolution for imaging.     

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。     

Effects of Magnetic Fields on Neutrino-dominated Accretion Model for Gamma-ray Bursts

Many models of gamma-ray bursts suggest a common central engine:a black hole of several solar masses accreting matter from a disk at an accretion rate from 0.01 to 10 M_⊙s~(-1),the inner region of the disk is cooled by neutrino emission and large amounts of its binding energy are liberated,which could trigger the fireball.We improve the neutrino- dominated accreting flows by including the effects of magnetic fields.We find that more than half of the liberated energy can be extracted directly by the large-scale magnetic fields in the disk,and it turns out that the temperature of the disk is a bit lower than the neutrino-dominated accreting flows without magnetic field.Therefore,the outflows are magnetically-dominated rather than neutrino dominated.In our model,the neutrino mechanism can fuel some GRBs (not the brightest ones),but cannot fuel X-ray flares.The magnetic processes(both BZ and electromagnetic luminosity from a disk)are viable mechanisms for most of GRBs and their following X-ray flares.     

A Close Correlation between the Spectral Lags and Redshifts of Gamma-Ray Bursts

Based on nine BATSE GRBs with known redshifts, we found that the maximum spectral lag of all the pulses in a gamma-ray burst （GRB） appears to be anti-correlated with the redshift of the burst. In order to confirm this finding, we analyzed 10 GRBs detected by HETE-2 with known redshifts and found a similar relation. Using the relation, we estimated the redshifts of 878 long GRBs in the BATSE catalog, then we investigated the distributions of the redshifts and 869 Eiso of these GRBs. The distribution of the estimated redshifts is concentrated at z = 1.4 and the distribution of Eiso peaks at 10^52.5 erg. The underlying physics of the correlation is unclear at present.