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Blazars高能伽马射线的起源
其他题名The Origin of High-Energy $\gamma$-ray From Blazars
曹刚
学位类型硕士
导师王建成
2014-05
学位授予单位中国科学院研究生院(云南天文台)
学位授予地点北京
学位专业天体物理
关键词Bl Lac天体 伽马射线 辐射机制 粒子加速
摘要blazars是一类特殊的活动星系核(AGNs),它们的辐射被认为起源于与我们视线方向夹角很小的相对论性喷流。它们通常表现出快速的光变、高的偏振以及强的非热辐射。blazars的光谱能量分布(SEDs)通常由两个比较宽的成分组成:一个是低能成分,从射电到X射线波段,另一个是高能成分,从X射线到伽马射线波段。普遍认为它们SEDs的低能成分由相对论电子的同步辐射产生。然而高能成分的起源目前仍然存在争议。两类模型被用于解释blazarsSEDs的高能成分:轻子模型和强子模型。轻子模型认为blazars的高能辐射由电子与背景光子的逆康普顿散射过程产生。强子模型则认为blazars的高能辐射起源于质子的同步辐射或者光子强子相互作用。我们从轻子模型和强子模型两个方面研究了blazars高能伽马辐射的起源,主要的结果是: 1.我们研究了21个具有准同步SEDs的平谱射电类星体(FSRQs)伽马射线辐射区的位置。我们提出了一种限定伽马射线辐射区的方法。如果伽马射线辐射区在宽线区之内,逆康普顿(IC)散射发生在KN区域,伽马射线谱应该比光学近红外谱更陡。如果伽马射线辐射区在宽线区之外,IC散射发生在Thomson区域,伽马射线谱的谱指数应该与光学近红外谱的谱指数相同。为了测试我们的理论,我们用一区的轻子模型包括同步自康普顿(SSC model)和外康普顿模型(EC model)拟合了21个FSRQs准同步的SEDs,我们认为X射线辐射由SSC过程,GeV辐射来自于EC过程,其中EC辐射可能起源于相对论电子IC散射吸积盘和BLR的辐射或者是尘埃环的辐射。我们根据谱型和SEDs的拟合结果猜测5个FSRQs的GeV辐射区在宽线区之内,其它16个在宽线区之外。我们的SEDs拟合表明在21个FSRQs喷流中磁场和电场的能量密度接近均分。 2.我们用具有电子加速的SSC模型研究了Mrk421在2008年6月68日期间X射线和伽马射线的爆发活动。观测发现X射线和伽马射线明显相关,然而光学和X射线波段并不明显相关。我们认为Mrk421的辐射可能起源于两个不同的成分。一个是在外部区域的稳定成分,它主要产生光学辐射,在这个区域电子被一阶Fermi加速过程加速,我们用稳态的电子谱来产生这一区域的SSC辐射。另一个是在内部区域的变化成分,在这个区域电子被随机加速过程加速,我们用一个含时的SSC模型来产生这一区域的辐射。我们认为爆发是由随机加速过程导致电子谱变硬引起的,电子谱的变硬将会引起X射线和伽马射线谱的变硬。此外我们发现在两个区域中磁场和电子的能量密度都接近均分。 3.来自于遥远blazars非常硬的伽马射线谱挑战了标准的SSC模型。硬的伽马射线谱可能表明了存在一个高于SSC辐射的谱成分。我们研究了1ES1101232非常硬的伽马射线谱的可能起源。H.E.S.S.的观测表明VHE流量在任一时标内都不存在明显的光变。因此我们认为blazar1ES1101232硬的伽马射线谱可能起源于高能质子的辐射。我们提出了一个解释blazar1ES1101232硬的伽马射线谱的模型。在这个模型中,光学和X射线的辐射来自于原初电子和次级电子的同步辐射,GeV辐射由SSC过程产生,然而非常硬的伽马射线起源于中性π0的衰变,其中中性π0由高能质子与喷流内同步辐射光子发生pγ相互作用产生。我们的模型能够很好地解释1ES1101232的多波段能谱,尤其是1ES1101232非常硬的伽马射线谱。为了通过pγ相互作用有效地产生伽马射线,我们的模型要求相当极端的喷流功率。
其他摘要Blazars are a special subclass of active galaxy nucleis (AGNs). Radiation from blazars is thought to originate in a relativistic jet oriented at a small angle with respect to the line of sight. They usually show rapid variability, high polarization and strong non-thermal emission. Spectral energy distributions (SEDs) of blazars are characterized by two broad components: a low-energy component from radio to X-ray frequency, and a high-energy component from X-ray to $\gamma$-ray. It is generally accepted that the low-energy component of blazars SEDs is produced by synchrotron radiation from relativistic electrons in the jet. However, the origin of high-energy component is still a matter of debate. There are two classes of models that are used to explain the high-energy emission of blazars: the leptonic model and the hadronic model. In the leptonic model, the high-energy emission is produced by inverse Compton scattering of electrons on background photons. In the hadronic model, the high-energy emission originates from proton synchrotron or photon-hadronic interactions. We study the origin of high-energy emission from blazars based on leptonic model and the hadronic model. The main results are as following: 1. We study the location of the GeV emission region of 21 flat spectrum radio quasars (FSRQs) with quasi-simultaneous spectral energy distributions. We propose a method to constrain the location of the GeV emission region based on the spectral shapes. If the $\gamma$-ray emission region is located inside the BLR, the IC scattering could occur at the Klein-Nishina (KN) regime and the $\gamma$-ray spectrum should be steeper than the optical-infrared spectrum. If the $\gamma$-ray emission region is located far beyond the the broad-line region (BLR), the IC scattering could take place at the Thomson regime and the $\gamma$-ray spectrum should have the same spectral index as the optical-infrared spectrum. In order to test our scenario, We reproduced the simultaneous SEDs of 21 FSRQs using one-zone leptonic model with the synchrotron-self Compton (SSC) and external Compton (EC) processes. We suggest that the X-ray emission is produced by SSC process, the GeV emission comes from the external Compton (EC) process, in which the EC emission may originate from the inverse Compton (IC) scattering of photons from BLR and accretion disc or dust torus by the same electron population. We infer from the spectral shapes and SED modeling that the location of the GeV emission region is inside the BLR for 5 FSRQs and beyond the BLR for 16 FSRQs. Our results show that the ratio of the magnetic field and electron energy density is close to equipartition condition for 21 FSRQs. 2. We investigate the X-ray and $\gamma$-ray flares of Mrk 421 on 2008 June 6-15 using the SSC model with electron acceleration, in which an evident correlation between the X-ray and $\gamma$-ray bands appears, while no significant correlation between the optical and X-ray band is observed. We argue that the emission from Mrk 421 may originate from two different components. One is the steady component from the outer region that is mainly attributed to the optical radiation, in which the electrons are accelerated by first-order Fermi acceleration mechanism. We use a steady electron spectrum to produce the SSC emission from the steady component. The other is the variable component from the inner region, in which the electrons are accelerated by the stochastic acceleration process. We use the time-dependent SSC model to produce the emission from the variable component. We suggest that the flares are due to the hardening of the electron spectrum under the process of the stochastic acceleration, which leads to the hardening of the observed spectrum in the X-ray and $\gamma$-ray bands. Furthermore, we find that the energy densities of electrons and magnetic fields are near equipartition in both jet regions. 3. The very hard $\gamma$-ray spectrum from distant blazars chall
学科领域天文学
页数108
语种中文
文献类型学位论文
条目标识符http://ir.ynao.ac.cn/handle/114a53/5100
专题高能天体物理研究组
推荐引用方式
GB/T 7714
曹刚. Blazars高能伽马射线的起源[D]. 北京. 中国科学院研究生院(云南天文台),2014.
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