YNAO OpenIR  > 星系研究组
伽玛暴脉冲光变曲线与能谱的关系
Alternative TitleRelation between light curves and spectra of gamma-ray bursts
贾兰伟
Subtype博士
Thesis Advisor白金明 ; 覃一平
2008-03-24
Degree Grantor中国科学院研究生院(云南天文台)
Place of Conferral北京
Degree Discipline天体物理
Keyword伽玛暴 多普勒效应 光变曲线 脉冲宽度 能谱 谱硬度
Abstract伽玛暴是一种来自宇宙深处的伽玛射线在短时间内突然增强的现象,是目前观测到的宇宙间最剧烈的爆发现象。伽玛暴在1967年首次被探测,由于它的极端高能,极短时标,巨大的辐射能等观测特征,使得现有天体物理学模型面对新的挑战,对伽玛暴的研究因此具有巨大的诱惑力。本文首先对伽玛暴领域的主要研究成果(包括观测的与理论的成果)进行了综述,然后详细介绍了本人在攻读博士学位期间从两个不同的角度来探讨伽玛暴光变曲线与伽玛暴能谱之间的关系所做的两个研究工作。 第一个工作是探讨局域脉冲(local pulse)的半峰全宽FWHM与幂率指数alpha之间的关系。统计研究发现伽玛暴脉冲的半峰宽度与能量存在指数约为alpha=0.4的幂率关系。这表明,至少在脉冲形态中,光变曲线与能谱之间是存在着联系的。以往的研究涉及到的脉冲数据为直接的观测值,而由于火球曲率效应的作用,该观测值是火球不同地方产生的局域脉冲的叠加。要了解光变曲线和能谱之间更本质的联系,我们应以局域脉冲为研究对象。我们想知道局域脉冲到底与什么物理量存在联系。我们将研究对象锁定在典型伽玛暴单峰源中,这些源的光变曲线呈现快速上升指数下降的形态(通常称为FRED形态)。我们选择七个在其它文献中已被详细研究的单峰源,其中六个单峰源的局域脉冲已通过考虑曲率效应的辐射流量公式对观测数据的拟合而获得,而余下的一个我们采用同样的方法求得其局域脉冲。此外我们求出这些源的幂率指数alpha并根据所给的局域脉冲求得其半峰宽度FWHM。我们发现局域脉冲的FWHM与幂率指数alpha之间存在反相关关系。这一关系未曾被火球曲率效应所预言,亦没有其它模型做出过预言。该关系来源于对七个源的研究需要更大的样本对之进行验证。不管怎样,探讨局域脉冲的特性与观测脉冲宽度对能量的依赖程度之间的关系为我们提供了一个研究伽玛暴光变曲线与伽玛暴能谱关系的新的视角。本工作的创新之处是首次探讨由拟合得到的火球局域脉冲与由观测值得到的幂率指数之间的关系,所得二者之间存在的相关关系即为新发现的一种关系。 第二个工作是利用由Kocevski et al.(2003)给出的著名的伽玛暴脉冲样本检验Qin et al.(2006)做出的有关伽玛暴脉冲硬度系数演化规律的若干预言。这是从另一个角度来研究伽玛暴光变曲线与伽玛暴能谱之间关系的一个工作。Qin et al.(2006)研究了火球曲率效应模型中谱硬度在一个脉冲中随时间变化的规律,据此做了若干预言。为了能够讨论谱硬度在一个脉冲中的演化细节,他们定义了一个称为RHR(raw hardness ratio)的物理量,发现RHR曲线随相应的源其谱的硬软程度而呈现不同形态,其中非常硬的源其RHR曲线表现为一个无凹陷的完整的脉冲,而硬谱源其RHR则表现为一个在其下降段有凹陷的脉冲,而软谱源其RHR曲线仅有凹陷而无脉冲。在他们的工作中,仅有少数几个观测例子得到讨论。他们所预言的现象是否能够在一个较大样本中得到证实还不得而知。为了对Qin et al.(2006)的预言在统计上做出检验,我们选择了由Kocevski et al.(2003)给出的著名的伽玛暴脉冲样本,此样本中的脉冲光变曲线已在前人的工作中被证实是与火球曲率效应所预言的观测特征相吻合。从该样本我们得到66个伽玛暴脉冲。这些脉冲的已减除背景的光变曲线数据由BATSE网站提供。根据Qin et al.(2006),我们对去背景的光变曲线数据加入一个给定的常量背景数据,利用这些数据求得RHR(这样可以保证硬度比的定义中不会有分母为零的现象出现)。在得到的66个源的RHR曲线中,除了两个可能因为谱硬度信号过低无法辨认外,其余64个源均呈现Qin et al.(2006)所预言的形态。仿照Qin et al.(2006),我们将这64个源分为三类:第一类源的RHR呈现出无凹陷的完整的脉冲,第二类源的RHR为一个在其下降段有凹陷的脉冲,而 第三类源的RHR曲线仅有凹陷而无脉冲。在Qin et al.(2006)的预言中,这三类源的谱硬度是不同的,其中第一类最硬,第二类次之,第三类最软。为了检验这一预言,我们计算它们的硬度比HR。与RHR不同的是,HR是个时间积分,能够较粗略地判断谱的软硬程度。(注意,在我们的分类中,我们仅仅根据RHR曲线的形态而不是RHR的数值,如RHR的峰值,进行判断。)我们做了这三类源的HR分布图并对这些分布做了K-S检验,发现第二类源确实比第三类源硬,第一类源数目太少(3个源)未能做统计检验,但从它们的HR值还是看得出来它们是相对较硬的。此外,我们的研究还证实了Qin et al.(2006)中的另一个基于火球曲率效应的预言,即第一和第二类源的RHR峰值先于光变曲线的峰值出现。我们的一个意外发现是RHR曲线的最小值比传统采用的HR对分辨脉冲的软硬程度更为敏感。我们建议根据RHR的曲线的形态或(和)RHR的最小值f_{m2}将伽玛暴脉冲分为三类。普遍的观点认为,伽玛暴脉冲产生于内激波或外激波。激波通过同步辐射将部分运动能转化为辐射能。脉冲的硬度很可能与激波强弱有关与磁场强度有关。因此将脉冲如此分类将在一定程度上把光变曲线和物理过程联系了起来。这一工作的创新点是从统计上检验由曲率效应预言的伽玛暴脉冲硬度比演化曲线的形态及其分类,所得到的RHR曲线的最小值比传统采用的HR对分辨脉冲的软硬程度更为敏感的结论为首次发现。
Other AbstractGamma-ray bursts (GRBs) are the events of gamma-ray flash from cosmological distances, which are the most violent explosion ever detected in space. The events were first detected in 1967. Modern astrophysical models have been challenged due to its extreme high energy photons, short time scales, and large amount of radiative energy. In this thesis, I give a brief review on both theoretical and observational progress of GRBs, and then present two investigations on the relation between GRB light curves and spectra from different aspects. Statistical analysis revealed that the observed FWHM of gamma-ray burst pulses is related to energy by a power-law with its index being alpha =0.4. The first investigation is the analysis of the relationship between the local pulse width and the power-law index alpha. What investigated in the past are pulses obtained from direct observation. However, as an influence of the curvature effect, what one observes is in fact a combination of various local pulses that are emission from different parts of the fireball surface. To reveal the relation between light curves and spectra, we should pay our attention to local pulses. For example, one might wish to know what quantities are related with local pulses. We focus our attention to single pulse sources for which the pulses are seen to comprise a fast rise and an exponential decay (FRED). We choose seven single pulse GRB sources which have been carefully studied elsewhere to perform the analysis. Local pulses for six of the seven sources have been presented vias the fit, of the flux density formulas which take into account the curvature effect, to the light curve data. For the other one, we fit its light curve data with the same method and obtain its local pulse. For these sources, the power-law index alpha is calculated according to their light curve data. We find that, for this sample, the local pulse FWHM width is obviously anti-correlated with the index alpha. This phenomenon has not been predicted by the curvature effect, nor has ever been predicted by other models. Since the number of the sample is small, one needs a sizable sample to check this new finding. Nevertheless, investigating the relation between the local pulse width and the power-law index provides us a new view on the relation between light curves and spectra of GRB pulses. What is new in this investigation is that we pay our attention to local pulses and investigate their relation with spectra. The anti-correlation between the local pulse width and the power-law index alpha is a new finding which has not been revealed before. The second investigation is an statistical analysis which is using the Kocevski et al. (2003) sample to check the predictions on the hardness ratio curve of GRB pulses proposed recently by Qin et al.(2006). Compared with what discussed above, the relation between GRB light curves and spectra is investigated from another aspect. Qin et al. (2006) analyzed the evolutionary law of the hardness of spectra based on the scenario of the curvature effect, and they presented several predictions based on their research. They introduced a so-called raw hardness ratio (RHR) in order to reveal the details of the evolution of the hardness of spectra during the period of pulses. They found that the RHR curve shows different types of profile, with one having a perfect pulse shape without any sinkage, another possessing a pulse-like shape with a sinkage in its decaying phase, and the other showing no pulse-like shape but having only a sinkage shape. In their paper, they studied only few sources, and hence it is unclear whether their predictions are true if a sizable sample is employed. To check their predictions, we adopt the Kocevski et al. (2003) sample in which the light curves of GRBs are in agreement with what the curvature effect predicts. We get 66 GRB pulses from the sample. The light curve data, for which the background counts have been subtracted, are available in the BATSE website. According to the paper of Qin et al. (2006), we add to the signal data a certain background count (in this way, RHR can be well defined through out a pulse), and with these new data we calculate RHR. For two of the 66 sources their hardness ratio signals are too faint to be recognized, and for the rest their RHR curves show exactly what predicted in Qin et al. (2006). Following Qin et al. (2006), we classify the 64 sources into three types merely according to the profiles of their RHR curves: the first type contains sources with a perfect pulse-like profile, sources of the second type possess a pulse-like profile with a sinkage in its decaying phase, and for the third type the sources show no pulse-like profile but have only a sinkage profile. Qin et al. (2006) pointed out that among the three types, sources of the first type should be the hardest, those of the second type should be less hard, and sources of the third type should be the softest. To check this prediction, we calculate the conventional hardness ratio of sources of the three types. Different from RHR, the conventional hardness ratio HR is calculated with the time integral flux (the so-called fluence), which can roughly represent the hardness of sources. (Note that, in our classification, we relay only on the profiles of the RHR curves; no any RHR values such as the peak of RHR are used.) We analyze the HR distributions of the three types and perform K-S tests for these distributions. The analysis reveals that sources of the second type are indeed harder than those of the third type. The number of the first type is small (only three sources) and hence we cannot perform any statistical analysis. However, one can also notice from their HR values that they are relatively harder. Another prediction proposed in Qin et al. (2006) is also confirmed in our analysis: the peak of the RHR curves appears in advance of the peak of the corresponding light curves. A finding beyond our expectation is that in distinguishing sources of different hardness the minimum of the RHR curve is more sensitive than the conventional HR. We therefore suggest that GRB pulses be divided into three types according to their RHR profile and/or the minimum value of RHR(f_{m2}). It is general believed that GRB pulses arise from inner (or outer) shocks. It is the shocks that convey parts of the kinetic energy into radiation. The hardness of a GRB pulse might probably be connected with the strength of the shock or/and the strength of the magnetic field. Such a classification might in some extent connect light curves of GRB pulses and the real physical process. The main contribution of this investigation is checking, in terms of statistics, the predicted profile of the hardness ratio curve of GRB pulses proposed previously based on the curvature effect. The fact that the minimum of the RHR curve is more sensitive than the conventional HR in distinguishing GRB pulses of different hardness is a new finding.
Subject Area天文学
Pages177
Language中文
Document Type学位论文
Identifierhttp://ir.ynao.ac.cn/handle/114a53/7166
Collection星系研究组
Recommended Citation
GB/T 7714
贾兰伟. 伽玛暴脉冲光变曲线与能谱的关系[D]. 北京. 中国科学院研究生院(云南天文台),2008.
Files in This Item:
File Name/Size DocType Version Access License
伽玛暴脉冲光变曲线与能谱的关系.pdf(8533KB)学位论文 开放获取CC BY-NC-SAView Application Full Text
Related Services
Recommend this item
Bookmark
Usage statistics
Export to Endnote
Google Scholar
Similar articles in Google Scholar
[贾兰伟]'s Articles
Baidu academic
Similar articles in Baidu academic
[贾兰伟]'s Articles
Bing Scholar
Similar articles in Bing Scholar
[贾兰伟]'s Articles
Terms of Use
No data!
Social Bookmark/Share
File name: 伽玛暴脉冲光变曲线与能谱的关系.pdf
Format: Adobe PDF
All comments (0)
No comment.
 

Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.