射电天文基础课件.ppt

1、 The Nature of Radio Sources Radiation from an Accelerated Electron The Frequency Distribution of Bremsstrahlung for an Individual Encounter The Radiation of an Ionized Gas Cloud Nonthermal Radiation Mechanisms Review of the Lorentz Transformation The Synchrotron Radiation of a Single Electron The S

2、pectrum and Polarization of Synchrotron Radiation The Spectral Distribution of Synchrotron Radiation Energy Requirements of Synchrotron Sources Low-Energy Cutoffs in Nonthermal Sources Inverse Compton Scattering Two large families Locations:galactic and extragalactic SED:The nature of discrete sourc

3、es was investigated 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 Solar system objects Solid bodies,=Dust in molecular clouds 2.7K cosmi

4、c microwave backgrounddust11/exp1)(dust00beTTTT2H21dust107bNZZrtirctveE2expsin)(2=/2 points in the direction of v22322sin41rcveS322)(32)(ctvetPdtvcedttPW23232)(An electron moving past an ion of charge Zelv32lZemcospl vpmceZW1432362vpmZeC2)0(pvpv851.11632g for 0323for 138),(gg222362pvvpmceZvpPkTmvkTm

5、vvf2exp24)(22/32dvdpvvpfNNpvdN)(2),(eidpvdNpvPd),(),(4122ei362ln234ppkTmmNNceZ)(TB1232ei62ln)(2134ppmkTNNceZpc/023-e6pccmcm pcEMssdN),(cm pcEMGHzK10235.861.235.12TTe476.06643.0e0cm pcEM),(K3045.0GHzTT Relativistic electrons moving in intricately“tangled”magnetic fields of extended coronas believed t

6、o surround certain kinds of stars Radiation from relativistic cosmic ray electrons that move in the general interstellar magnetic field)()(cxttzzyyvtxxcv2/12)1(cucudtdtdtdtvudtdtdtdxdtdxuxxxx11)(cuvuu|1cuuu|1aaaa2|3|aaaa2|3|33xxxaua32xyyyyacuaua tt1tcvtr()()dmedtcvvB0)(2mcdtd0|dtdv)(BvvmedtdBeBmcGBe

7、Bmc|tanvvGauss6.17MHzGBvaB2B2T223222432vucmcEcmBveP4/2BBu2252e2220Tcm106.65r38mce38 PowerIncident Power ScatteredIP222222 32)cos1(cossin1)cos1(141),(aceddP1tan|vcuusin11B3tc/3/5c23)(sin3)(dKmcBePsin23sin23B3G2c|tanvv The instantaneous radiation is in general elliptically polarized,but since the posi

8、tion 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.)()(sin2323xGxFmcBeP)()(sin2323|xGxFmcBeP)()(|xFxGPPPPpEdEENEP)(),()(21)(EEEdE

9、KEdEEN对于n)()1(21nnnnBcmemceKna15323)sin(4383)()(291radJy GHz/1026.6)(933.0)(nnKLBnaI3/51nnpnnnBcmemceKnb1532343)()(291radJy GHz/1026.6)(5.13)(nnKLBnbI)(magpmagparttotuuVWWWmaxmin1pEEdEEKu2mag81Bu22/1tot81BBGKVWnnnBRVKHS127/37/87/47/3tot)6(47VRSHGWn Synchrotron radiation at frequencies below the low-

10、frequency cutoff 1 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 frequenc

11、ies when the optical depth becomes large.The Razin effect Foreground thermal plasma may absorb may synchrotron emission at lower frequencies Compton Scattering An 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

12、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 photons wavelength decreases.The electron has to suffer a large number of collisions before it los

13、es an appreciable fraction of its energy 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 an interaction of the photons a

14、nd electrons is to shift longer wavelength photons to shorter wavelength LNTLNcmkTTTee34eT2e1024.2321ph8104.21udtdEEBph6nsynchrotroCompton10ELELuu 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 i

15、s 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

16、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 s

17、uch 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 if dust-4?From Fig.9.1,determine the turnover frequency of t

18、he 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 frequency parts of the plot of flux density versus frequency meet.At this point,the optical dep

19、th 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 measure EM=Ne2 is 0=0.3045(Te)-0.643(EM)0.476.This relation applies to a uniform density,uniform t

20、emperature 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

21、FWHP size and emission measure to obtain the RMS electron density.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

22、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?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 太阳射电辐射的检测 射电天文史

23、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 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 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

24、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 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 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 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 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