1 射电天文基础姜碧沩北京师范大学天文系 2009/08/24-28 日，贵州大学. 2009/08/24-28 日射电天文暑期学校 2 Emission Mechanisms of Continuous Radiation

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射电天文基础射电天文基础姜碧沩姜碧沩

北京师范大学天文系北京师范大学天文系

2009/08/24-282009/08/24-28 日，贵州大学日，贵州大学

2009/08/24-282009/08/24-28 日日射电天文暑期学校射电天文暑期学校 22

Emission Mechanisms of Continuous Emission Mechanisms of Continuous RadiationRadiation

• The Nature of Radio Sources• Radiation from an Accelerated Electron• The Frequency Distribution of Bremsstrahlung for an I

ndividual Encounter• The Radiation of an Ionized Gas Cloud• Nonthermal Radiation Mechanisms• Review of the Lorentz Transformation• The Synchrotron Radiation of a Single Electron• The Spectrum and Polarization of Synchrotron Radiati

on• The Spectral Distribution of Synchrotron Radiation• Energy Requirements of Synchrotron Sources• Low-Energy Cutoffs in Nonthermal Sources• Inverse Compton Scattering


The Nature of Radio SourcesThe Nature of Radio Sources

• Two large families– Locations: galactic and extragalactic– SED: The nature of discrete sources was invest

igated by measurements at different frequencies to determine the spectral characteristics

• Roughly constant flux density with increasing frequency

• More intense at lower frequency

– Emission mechanisms• Thermal • Nonthermal



Blackbody Radiation from Blackbody Radiation from Astronomical ObjectsAstronomical Objects

• Solar system objects– Solid bodies, τ=∞

• Dust in molecular clouds

• 2.7K cosmic microwave background

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Radiation from an Accelerated ElectronRadiation from an Accelerated Electron

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The Frequency Distribution of BremsstrahlunThe Frequency Distribution of Bremsstrahlung for an Individual Encounterg for an Individual Encounter

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Spectral Energy DistributionSpectral Energy Distribution

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The Radiation of an Ionized Gas CloudThe Radiation of an Ionized Gas Cloud

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Emission and Absorption CoefficientsEmission and Absorption Coefficients dpvdNpvPd ),(),(4

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Emission Measure and Optical DepthEmission Measure and Optical Depth

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Nonthermal Radiation MechanismsNonthermal Radiation Mechanisms

• Relativistic electrons moving in intricately “tangled” magnetic fields of extended coronas believed to surround certain kinds of stars

• Radiation from relativistic cosmic ray electrons that move in the general interstellar magnetic field


Review of the Lorentz TransformationReview of the Lorentz Transformation

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The Synchrotron Radiation of a The Synchrotron Radiation of a Single ElectronSingle Electron

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The Total Power RadiatedThe Total Power Radiated

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The Angular Distribution of RadiationThe Angular Distribution of Radiation

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The Frequency Distribution of the EmissionThe Frequency Distribution of the Emission

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The Spectrum and Polarization of The Spectrum and Polarization of Synchrotron RadiationSynchrotron Radiation

• The instantaneous radiation is in general elliptically polarized, but since the position angle of the polarization ellipse is rotating with the electron, the time averaged polarization is linear. This is true also for the radiation emitted by an ensemble of monoenergetic electrons moving in parallel orbits.

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The Spectral Distribution of The Spectral Distribution of Synchrotron Radiation from an Synchrotron Radiation from an

Ensemble of ElectronsEnsemble of Electrons

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Homogeneous Magnetic FieldHomogeneous Magnetic Field

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Random Magnetic FieldRandom Magnetic Field

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Energy Requirements of Synchrotron SourcesEnergy Requirements of Synchrotron Sources

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Low-Energy Cut-offs in Nonthermal SourcesLow-Energy Cut-offs in Nonthermal Sources

• Synchrotron radiation at frequencies below the low-frequency cutoff ν1 should have a spectral index of n=1/3

• In synchrotron radiation fields spontaneous photon emission will be accompanies by absorption and stimulated emission as in any other radiation fields. This absorption can become important in compact, high-intensity radio sources at low frequencies when the optical depth becomes large.

• The Razin effect• Foreground thermal plasma may absorb may syn

chrotron emission at lower frequencies


Inverse Compton ScatteringInverse Compton Scattering

• Compton Scattering– An X-ray or gamma-ray photon collides with a

particle, usually an electron. Some of the photon’s energy is transferred to the particle and the photon is reradiated at a longer wavelength

• Inverse Compton Scattering– A low-energy photon collides with a fast-moving

electron. The electron passes on a small proportion of its energy to the photon, the photon’s wavelength decreases. The electron has to suffer a large number of collisions before it loses an appreciable fraction of its energy


The Sunyaev-Zeldovich EffectThe Sunyaev-Zeldovich Effect

• Photons from a cold source, the 2.7K background, interact with a hot foreground source, a cluster of galaxies. Such clusters have free electrons with Tk>107K, so the bremsstrahlung radiation peaks in the X-ray range. The net effect of an interaction of the photons and electrons is to shift longer wavelength photons to shorter wavelength

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Energy Loss from High-Brightness SourcesEnergy Loss from High-Brightness Sources

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ExerciseExercise• The Orion hot core is a molecular source with an average

temperature of 160K, angular size 10", located 500pc from the Sun. The average local density of H2 is 107cm-3.– Calculate the line-of-sight depth of this region in pc, if this is taken

to be the diameter– Calculate the column density N(H2) which is the integral of density

along the line-of-sight. Assume that the region is uniform– Obtain the flux density at 1.3mm using Tdust=160K, the parameter

b=1.9 and solar metallicity in equation (9.7)– Use the Rayleigh-Jeans relation to obtain the dust continuum main

beam brightness temperature from this flux density in a 10" beam. Show that this is much smaller than Tdust.

– At long millimeter wavelengths, a number of observations have shown that the optical depth of such radiation is small. Then the observed temperature is T=Tdustτdust, where the quantities on the right hand side of this equation are the dust temperature and dust optical depth. From this relation determine τdust.

– At what wavelength is τdust=1 if τdust~λ-4?


ExerciseExercise• From Fig. 9.1, determine the ‘turnover’ frequency of the Orio

n A HII region, that is the frequency at which the flux density stops rising and starts to decrease. This can be obtained by noting the frequency at which the linear extrapolation of the high and low frequency parts of the plot of flux density versus frequency meet. At this point, the optical depth τff of free-free emission through the center of Orion A is unity, that is τff =1, call this frequency ν0.

• From equation (9.36) in ‘Tools’, the relation of turnover frequency, electron temperature Te and emission measure EM=Ne2 is ν0=0.3045(Te )-0.643(EM)0.476. This relation applies to a uniform density, uniform temperature region, actual HII regions have gradients in both quantities, so this relation is at best only a first approximation. Determine EM for an electron temperature Te=8300K

• The FWHP size of Orion A is 2.5’, and Orion A is 500pc from the Sun. What is the linear diameter for the FWHP size? Combine the FWHP size and emission measure to obtain the RMS electron density.


ExerciseExercise

• The source Cas A is a cloud of ionized gas associated with the remnant of a star which exploded about 330 years ago. The radio emission has the relation of flux density as a function of frequency shown in Fig. 9.1 in ‘Tools’. For the sake of simplicity, assume that the source has a constant temperature and density, in the shape of a ring, which thickness 1’ and outer radius of angular size 5.5’. What is the actual brightness temperature at 100MHz, 1GHz, 10GHz, 100GHz?


热和非热射电源的一些例子热和非热射电源的一些例子 • 宁静太阳• HII 区的射电辐射• 超新星和超新星遗迹• 超新星遗迹的流体动力学演化• 较老的超新星遗迹的射电演化• 脉冲星• 河外源


宁静太阳宁静太阳• 太阳射电辐射的检测

– 射电天文史前• 19 世纪末：探测器的低灵敏度• 20 世纪初：观测的停滞• Jansky ：太阳活动极小年• 1942 年：宁静太阳和活动太阳的射电辐射

• 辐射源– 日冕– 热辐射

• 等离子体对低频端的影响– 非直线的传播

• 逆转的温度结构– 中频段的临边增亮现象


HIIHII 区的射电辐射区的射电辐射• HII 区 Orion A 的热辐射

– 轫致辐射– 距离： 450pc– 两个波段的比较

• 分辨率• 核的亮温度• 辐射量度的计算• 大小

– 简单模型的改进• 电离星风的射电辐射

– 热辐射– 非热辐射


超新星和超新星遗迹超新星和超新星遗迹• 超新星

– 分类• 大质量红巨星的爆发： II 型• 白矮星和的双星系统： I 型

– 银河系中发生的频率• 预计： 50 年一个• 已知最近的观测： 1606 年， Kepler 超新星； 1667 ， Cas A

• 遗迹的证认– 形状：展源

• 距离：银河系内天体• 能谱：与 HII 区的区别

– 膨胀的壳层– 与脉冲星成协


较老的超新星遗迹的射电演化较老的超新星遗迹的射电演化• 同步辐射的强度• 参数的变化

– 磁场强度– 电子能量– 谱指数

• 辐射流量的变化• Cas A 的情况


超新星遗迹的流体动力学演化超新星遗迹的流体动力学演化• 自由膨胀阶段

– 被膨胀壳层扫过的气体质量小于初始质量– Rt– 几十年

• 绝热阶段– 遗迹以被扫荡的物质为主– 辐射损耗比超新星产生的总能量小得多– Rt2/5

• 辐射阶段– 辐射损耗– Rt1/4

• 耗散阶段– 激波速度降低到声速以下，与星际介质混合


脉冲星脉冲星• 探测和源的本质• 距离估算和在银河系的分布• 强度谱和脉冲形状• 脉冲星定时• 旋转变慢和磁矩• 双星脉冲星和毫秒脉冲星• 射电辐射机制


河外源河外源• 类型

– AGN ：类星体， Seyfert 星系，射电星系• 辐射机制：同步辐射

– 射电星系• 苏尼阿耶夫－泽尔多维奇效应• 相对论效应和时变

Documents

1 射电天文基础 姜碧沩北京师范大学天文系 2009/08/24-28 日，贵州大学. 2009/08/24-28 日射电天文暑期学校 2 Emission Mechanisms of Continuous Radiation

1 射电天文基础姜碧沩北京师范大学天文系 2009/08/24-28 日，贵州大学. 2009/08/24-28 日射电天文暑期学校 2 Emission Mechanisms of Continuous Radiation