论文题目：伽玛射线暴及其余辉的研究-环境效应、辐射过程和能源机制
作者简介:
王祥玉，男，1974年01月出生，1998年09月师从于南京大学陆埮教授，于2001年09月获博士学位。
摘
要
γ射线暴 (简称γ暴)是来自宇宙空间的一种突然的γ光子爆发现象。它的探测率大约为每天一次，从能量释放功率看，人们知道它是宇宙中最猛烈的爆发现象。过去几年里γ暴的研究取得了重大突破，包括发现了变暗的X射线、光学和射电余辉，探测到位于宇宙学距离的寄主星系，发现γ暴与恒星形成区成协、甚至可能与超新星关联的证据等等。理论上，γ暴及其余辉辐射的相对论火球激波模型也取得了进展。实际上，在余辉发现后不久，一个关于γ暴及其余辉的标准火球激波模型就已建立了起来。然而随着观测数据的积累，出现了一些偏离标准模型的情况，它们对γ暴暴周环境、γ暴及其余辉的辐射机制等提供了重要信息。另外，γ暴的一些问题还未被认识清楚，比如γ暴极高能光子的起源、γ暴---超新星关联的能源机制等等。本论文正是在这些方面作了研究，取得了一些新的结果。
在第二章，我们以1998年5月19日发现的一个γ暴---GRB980519为例，研究了γ暴暴周环境对余辉随时间演化关系的影响。如果暴周环境是密度较高的致密介质，极端相对论的火球激波会很快地减速到非相对论速度。致密介质模型被发现与许多暴的光学和X射线余辉观测一致。γ暴的观测也发现了射电余辉，因射电余辉随时间的演化与光学和X射线余辉有显著不同，研究射电余辉的行为便具有特别重要的意义。我们首次研究了致密介质情形下射电余辉在跨相对论阶段的行为，发现考虑到激波的跨相对论动力学和相对论电子的同步自吸收效应对射电辐射的影响，致密介质模型既可以解释GRB980519的快速衰减的X射线和光学余辉，也可以很好地解释其射电余辉的特点。由于致密介质很可能就是巨分子云，位于恒星形成区，所以这一工作有力地支持了γ暴起源于大质量恒星塌缩的观点，此观点现已被广泛接受。
通过对某一个γ暴物理参数的限制，人们可以知道关于γ暴的许多重要信息。GRB990123是目前唯一发现有瞬时光学闪的γ暴。在第三章，我们利用这个暴独特的瞬时光学闪结合余辉两方面的观测数据对GRB990123的几个重要物理参数进行了限制，比如火球的初始运动洛仑兹因子、火球激波中磁场能量和电子能量占激波内能的比例等。火球的初始运动洛仑兹因子是一个直接表征其相对论运动的重要物理量,确定它的大小本身就是对火球模型的检验；另一方面，因为它直接反映火球中重子物质的多少，它的大小可以对暴源的能源模型提供有价值的信息。然而从γ暴的光谱人们只知道它的下限。利用描述反向激波的一套方程，我们较精确地定出了火球的初始运动洛仑兹因子以及反向激波的运动洛仑兹因子。计算结果表明GRB990123的火球初始运动洛仑兹因子在1000左右，是极端相对论的；结果也表明反向激波在穿过火球物质时是相对论运动的，这个结论与Sari 和 Piran提出的理论判据一致；计算结果还支持 Wijers 和 Galama 提出的激波中磁场能量和电子能量占激波内能比例为固定常数的观点。
当γ暴相对论火球受到外部介质减速时形成两个激波，正向激波传入外部介质中而反向激波传回到火球物质中。过去人们只对余辉阶段正向激波区域的逆康普顿散射过程作了较详细的研究，在第四章我们第一次仔细研究了极早期正、反向激波区域当中以及正反向激波区域相互之间的相对论电子与光子的逆康普顿散射过程。我们的分析和数值计算结果都表明，在几十MeV到几十GeV能段，反向激波区域中电子的同步自康普顿辐射比其它的逆康普顿散射和同步辐射更重要。我们发现这个过程能够很好地解释Compton卫星上EGRET探测器探测到的一些γ暴高能光子的起源问题。相比于其他需要假定特别的暴周环境或特别的辐射机制的模型，本文模型完全建立在被广泛认可的火球模型基础之上，而且本模型预言了γ暴高能光子辐射与瞬时光学闪相关联，可望在不久的将来被观测所检验。研究还发现在TeV能段，当激波中电子分布为中等陡的情形时，正向激波和反向激波区域之间电子与光子的联合康普顿散射以及正向激波区域中电子的同步自康普顿辐射变得更加重要。
越来越多的观测证据显示γ暴与超新星相关联。基于中子星相变过程产生γ暴的模型（Cheng & Dai 1996），我们在第五章提出了一种解释此关联的新机制。我们假定超新星爆发后留下一个大质量、快旋转的中子星（大质量可以由从超新星回落的吸积物质所致），这个中子星在数天内由于通常的磁偶极辐射阻尼或者热的、年轻中子星r---模式不稳定性导致的引力辐射阻尼（Andersson 1998; Lindblom et al. 1998）而快速自转变慢，核心密度不断增加。当核心密度达到核子物质的解禁闭密度，中子星发生相变，由此产生的极端相对论火球沿重子污染低的通道方向产生观测上与超新星关联的γ暴。
总之，在本篇论文中，我首先对γ暴研究概况作了一个评述 (第一章)，然后主要阐述了自己的研究工作 (第二章到第五章)。研究工作主要包括：γ暴周围环境对余辉演化的影响、γ暴的物理参数的限制、极高能γ射线的起源、极早期外激波区域的逆康普顿散射光谱与观测效应以及γ暴与超新星关联的理论解释。本文也指出了这个领域的一些存在问题和发展前景。
关键词：伽玛射线暴；余辉；中子星；辐射机制
Abstract
Gamma-ray Bursts (GRBs) are sudden, intense flashes of gamma-rays. They are detected at the rate of about once a day. From the power emitted in an event, we now know that GRBs are the biggest explosions in nature. Major advances have been made in the last a few years, including the discovery of fading X-ray, optical and radio afterglows of GRBs, the identification of host galaxies at cosmological distances, and evidence showing that many GRBs are associated with star-forming regions of the host galaxies and possibly a peculiar type of supernova. Progress has been made in understanding how the GRB and afterglow radiations are produced in the frame of a relativistic fireball shock model. Shortly after the discovery of afterglows, a standard fireball shock model has been established. As the observation data accumulate, some data show deviations from the standard model, which may provide information about the circumburst environment，the radiation mechanisms of GRBs and their afterglows, etc.. In addition, some issues about GRBs have not been settled, such as the origin of the energetic gamma-ray emissions, the energy source of the GRB---supernova connection, etc. In this thesis, we will study these questions.
In chapter 2, we study how the circumburst environment influences the temporal evolution of the afterglow. If the burst is surrounded by a dense medium with a high density, the relativistic fireball shock will be quickly decelerated into the sub-relativistic phase. This dense medium model is shown to be consistent with the optical and X-ray afterglow observations of many bursts. Due to that the radio afterglows of GRBs show significantly different behavior from the optical and X-ray afterglows, studies on them become very important and should be fruitful. For the first time, we study the radio afterglow behavior during the trans-relativistic phase of the fireball shock wave. Considering the trans-relativistic shock dynamic and the synchrotron self-absorption effect of the relativistic electrons on the radio emission, we find that the dense medium model can not only explain the fast decline of the X-ray and optical afterglows of GRB980519, but also account for its radio afterglow quite well. Because the dense medium is very likely associated with molecular cloud in star-forming regions, our work strongly supports that GRBs originate from the collapse of massive stars, an idea now wildly accepted by peoples.
Constraints of the physical parameters of GRBs can give us much important information about the burst. GRB990123 is the only burst that exhibits a prompt optical flash up to now. In chapter 3, in terms of the prompt optical flash and afterglow data of GRB990123, we constraint the physical parameters of this burst, such as the initial Lorentz factor of the fireball, the energy fractions of the shock transferred to the magnetic field and the relativistic electrons, the external medium density, etc. The initial Lorentz factor is a crucial ingredient for constraining models of the source itself, since it specifies how `clean' the fireball is. The GRB spectrum can provide only a lower limit to this Lorentz factor. In our method, it has been inferred more precisely from the full set of equations describing the reverse shock. Our results shows: (1) the initial Lorentz factor of the fireball of GRB990123 is about 1000; (2) the reverse external shock may have become relativistic before it passed through the fireball ejecta, which is in agreement with the criterion presented by Sari and Piran; (3) the values of the two energy fractions are consistent with the suggestion by Wijers and Galama that they may be universal parameters.
When the relativistic ejecta is significantly decelerated by the external medium, a relativistic forward shock expands into the external medium and a reverse shock moves into and heats the fireball ejecta. While inverse Compton (IC) emissions from the forward shock during the afterglow phase have attracted a lot of investigation, the studies for the IC processes in the reverse shocks and combined-IC scattering processes between the reverse and forward shocks are still quite preliminary. In chapter 3, for the first time, we carefully study the inverse Compton scattering processes in the reverse and forward shocks as well as the combined-IC processes between the forward and reverse shocks during early external shock phase. We find that the synchrotron self-Compton (SSC) emission from the reverse shock dominates over other emission processes in energy bands from tens MeV to tens of GeV, for a wide range of shock parameters. We then suggest that this mechanism may be responsible for the prompt high energy gamma-rays detected by EGRET on the Compton observatory. This model is entirely based on the fireball model and does not need a particular circumburst environment or a particular radiation mechanism as other models involve. At TeV energy bands, however, the combined-IC emissions and/or the SSC emission from the forward shocks become increasingly dominant for a moderately steep distribution of shocked electrons.
There is increasing evidence showing that some GRBs may be connected with supernova. In chapter 4, we propose a new explanation for this connection, based on the model that GRBs are produced by the phase transition of neutron stars (Cheng & Dai 1996). A supernova explosion leaves a massive and rapidly rotating neutron star, which then spins down in a few days by emitting the usual magnetic dipole radiation or the gravitational radiation due to r-mode instabilities of hot, young neutron stars (Andersson 1998; Lindblom et al. 1998). Its central density may reach the deconfinement density of the nuclear matter and therefore the phase transition of the neutron star takes place. The accompanied fireball, accelerated to ultra-relativistic velocity, flows out along the baryon-free funnel and subsequently produces the GRB associated with supernova.
In summary, in this Ph. D. thesis, after reviewing the progresses in GRBs in chapter 1, I mainly present my own researches in this field in chapter 3---6. The researches include the dense medium effect on the evolution of afterglows, constraints on the physical parameters of GRBs, the origin of the energetic gamma-ray emission from GRBs, the inverse Compton spectra in the very early afterglows, and energy source mechanism of the supernova-GRB connection. Finally, we also point out some unresolved questions and prospects in this field.
Key Words: gamma-rays: bursts---neutron stars---radiation mechanisms
