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1、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,The Nature of Radio SourcesRadiation from an Accelerated ElectronThe Frequency Distribution of Bremsstrahlung for an Individual EncounterThe Radiation of an Ionized Gas CloudNonthermal Radi
2、ation MechanismsReview of the Lorentz TransformationThe Synchrotron Radiation of a Single ElectronThe Spectrum and Polarization of Synchrotron RadiationThe Spectral Distribution of Synchrotron RadiationEnergy Requirements of Synchrotron SourcesLow-Energy Cutoffs in Nonthermal SourcesInverse Compton
3、Scattering,2009/08/24-28日,射电天文暑期学校,3,The Nature of Radio Sources,Two large familiesLocations:galactic and extragalacticSED:The nature of discrete sources was investigated by measurements at different frequencies to determine the spectral characteristics Roughly constant flux density with increasing
4、frequencyMore intense at lower frequencyEmission mechanismsThermal Nonthermal,2009/08/24-28日,射电天文暑期学校,4,2009/08/24-28日,射电天文暑期学校,5,Blackbody Radiation from Astronomical Objects,Solar system objectsSolid bodies,=Dust in molecular clouds2.7K cosmic microwave background,2009/08/24-28日,射电天文暑期学校,6,Radiati
5、on from an Accelerated Electron,=/2 points in the direction of v,2009/08/24-28日,射电天文暑期学校,7,The Frequency Distribution of Bremsstrahlung for an Individual Encounter,An electron moving past an ion of charge Ze,2009/08/24-28日,射电天文暑期学校,8,Spectral Energy Distribution,2009/08/24-28日,射电天文暑期学校,9,The Radiati
6、on of an Ionized Gas Cloud,2009/08/24-28日,射电天文暑期学校,10,Emission and Absorption Coefficients,2009/08/24-28日,射电天文暑期学校,11,Emission Measure and Optical Depth,2009/08/24-28日,射电天文暑期学校,12,SED,2009/08/24-28日,射电天文暑期学校,13,Nonthermal Radiation Mechanisms,Relativistic electrons moving in intricately“tangled”magn
7、etic fields of extended coronas believed to surround certain kinds of starsRadiation from relativistic cosmic ray electrons that move in the general interstellar magnetic field,2009/08/24-28日,射电天文暑期学校,14,Review of the Lorentz Transformation,2009/08/24-28日,射电天文暑期学校,15,Velocity,2009/08/24-28日,射电天文暑期学校
8、,16,Acceleration,2009/08/24-28日,射电天文暑期学校,17,Time,2009/08/24-28日,射电天文暑期学校,18,The Synchrotron Radiation of a Single Electron,2009/08/24-28日,射电天文暑期学校,19,The Total Power Radiated,2009/08/24-28日,射电天文暑期学校,20,The Angular Distribution of Radiation,2009/08/24-28日,射电天文暑期学校,21,The Frequency Distribution of the
9、 Emission,2009/08/24-28日,射电天文暑期学校,22,The Spectrum and Polarization of Synchrotron 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 al
10、so for the radiation emitted by an ensemble of monoenergetic electrons moving in parallel orbits.,2009/08/24-28日,射电天文暑期学校,23,2009/08/24-28日,射电天文暑期学校,24,The Spectral Distribution of Synchrotron Radiation from an Ensemble of Electrons,2009/08/24-28日,射电天文暑期学校,25,Homogeneous Magnetic Field,2009/08/24-28
11、日,射电天文暑期学校,26,Random Magnetic Field,2009/08/24-28日,射电天文暑期学校,27,Energy Requirements of Synchrotron Sources,2009/08/24-28日,射电天文暑期学校,28,Low-Energy Cut-offs in Nonthermal Sources,Synchrotron radiation at frequencies below the low-frequency cutoff 1 should have a spectral index of n=1/3In synchrotron rad
12、iation 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 effectForeground thermal
13、 plasma may absorb may synchrotron emission at lower frequencies,2009/08/24-28日,射电天文暑期学校,29,Inverse Compton Scattering,Compton ScatteringAn X-ray or gamma-ray photon collides with a particle,usually an electron.Some of the photons energy is transferred to the particle and the photon is reradiated at
14、 a longer wavelengthInverse Compton ScatteringA low-energy photon collides with a fast-moving electron.The electron passes on a small proportion of its energy to the photon,the photons wavelength decreases.The electron has to suffer a large number of collisions before it loses an appreciable fractio
15、n of its energy,2009/08/24-28日,射电天文暑期学校,30,The 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 Tk107K,so the bremsstrahlung radiation peaks in the X-ray range.The net effect of
16、an interaction of the photons and electrons is to shift longer wavelength photons to shorter wavelength,2009/08/24-28日,射电天文暑期学校,31,Energy Loss from High-Brightness Sources,2009/08/24-28日,射电天文暑期学校,32,Exercise,The Orion hot core is a molecular source with an average temperature of 160K,angular size 10
17、,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 diameterCalculate the column density N(H2)which is the integral of density along the line-of-sight.Assume that the region is uniformObtain the flux
18、 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 nu
19、mber of observations have shown that the optical depth of such radiation is small.Then the observed temperature is T=Tdustdust,where the quantities on the right hand side of this equation are the dust temperature and dust optical depth.From this relation determine dust.At what wavelength is dust=1 i
20、f dust-4?,2009/08/24-28日,射电天文暑期学校,33,Exercise,From Fig.9.1,determine the turnover frequency of the Orion 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
21、 frequency parts of the plot of flux density versus frequency meet.At this point,the optical depth ff of free-free emission through the center of Orion A is unity,that is ff=1,call this frequency 0.From equation(9.36)in Tools,the relation of turnover frequency,electron temperature Te and emission me
22、asure EM=Ne2 is 0=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=8300KThe FWHP size of Orion A is 2
23、.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.,2009/08/24-28日,射电天文暑期学校,34,Exercise,The source Cas A is a cloud of ionized gas associated with the remnant of a star which exploded about
24、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 ac
25、tual brightness temperature at 100MHz,1GHz,10GHz,100GHz?,2009/08/24-28日,射电天文暑期学校,35,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,2009/08/24-28日,射电天文暑期学校,36,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
26、,2009/08/24-28日,射电天文暑期学校,37,HII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,2009/08/24-28日,射电天文暑期学校,38,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,2009/08/24-28日,射电天文暑期学校,39,
27、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,2009/08/24-28日,射电天文暑期学校,40,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,2009/08/24-28日,射电天文暑期学校,41,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,2009/08/24-28日,射电天文暑期学校,42,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,
