射電天文基礎課件

射電天文基礎姜碧溈北京師范大學天文系2009/08/24-28日，貴州大學1射電天文基礎姜碧溈1EmissionMechanismsofContinuousRadiation

TheNatureofRadioSourcesRadiationfromanAcceleratedElectronTheFrequencyDistributionofBremsstrahlungforanIndividualEncounterTheRadiationofanIonizedGasCloudNonthermalRadiationMechanismsReviewoftheLorentzTransformationTheSynchrotronRadiationofaSingleElectronTheSpectrumandPolarizationofSynchrotronRadiationTheSpectralDistributionofSynchrotronRadiationEnergyRequirementsofSynchrotronSourcesLow-EnergyCutoffsinNonthermalSourcesInverseComptonScattering2009/08/24-28日2射電天文暑期學校EmissionMechanismsofContinuTheNatureofRadioSourcesTwolargefamiliesLocations:galacticandextragalacticSED:ThenatureofdiscretesourceswasinvestigatedbymeasurementsatdifferentfrequenciestodeterminethespectralcharacteristicsRoughlyconstantfluxdensitywithincreasingfrequencyMoreintenseatlowerfrequencyEmissionmechanismsThermalNonthermal2009/08/24-28日3射電天文暑期學校TheNatureofRadioSourcesTwo2009/08/24-28日4射電天文暑期學校2009/08/24-28日4射電天文暑期學校BlackbodyRadiationfromAstronomicalObjectsSolarsystemobjectsSolidbodies,τ=∞Dustinmolecularclouds2.7Kcosmicmicrowavebackground2009/08/24-28日5射電天文暑期學校BlackbodyRadiationfromAstroRadiationfromanAcceleratedElectronθ=π/2pointsinthedirectionofv2009/08/24-28日6射電天文暑期學校RadiationfromanAcceleratedTheFrequencyDistributionofBremsstrahlungforanIndividualEncounterAnelectronmovingpastanionofchargeZe2009/08/24-28日7射電天文暑期學校TheFrequencyDistributionofSpectralEnergyDistribution2009/08/24-28日8射電天文暑期學校SpectralEnergyDistribution20TheRadiationofanIonizedGasCloud2009/08/24-28日9射電天文暑期學校TheRadiationofanIonizedGaEmissionandAbsorptionCoefficients2009/08/24-28日10射電天文暑期學校EmissionandAbsorptionCoeffiEmissionMeasureandOpticalDepth2009/08/24-28日11射電天文暑期學校EmissionMeasureandOpticalDSED2009/08/24-28日12射電天文暑期學校SED2009/08/24-28日12射電天文暑期學校NonthermalRadiationMechanismsRelativisticelectronsmovinginintricately“tangled”magneticfieldsofextendedcoronasbelievedtosurroundcertainkindsofstarsRadiationfromrelativisticcosmicrayelectronsthatmoveinthegeneralinterstellarmagneticfield2009/08/24-28日13射電天文暑期學校NonthermalRadiationMechanismReviewoftheLorentzTransformation2009/08/24-28日14射電天文暑期學校ReviewoftheLorentzTransforVelocity2009/08/24-28日15射電天文暑期學校Velocity2009/08/24-28日15射電天文暑期Acceleration2009/08/24-28日16射電天文暑期學校Acceleration2009/08/24-28日16射電Time2009/08/24-28日17射電天文暑期學校Time2009/08/24-28日17射電天文暑期學校TheSynchrotronRadiationofaSingleElectron2009/08/24-28日18射電天文暑期學校TheSynchrotronRadiationofaTheTotalPowerRadiated2009/08/24-28日19射電天文暑期學校TheTotalPowerRadiated2009/0TheAngularDistributionofRadiation2009/08/24-28日20射電天文暑期學校TheAngularDistributionofRaTheFrequencyDistributionoftheEmission2009/08/24-28日21射電天文暑期學校TheFrequencyDistributionofTheSpectrumandPolarizationofSynchrotronRadiationTheinstantaneousradiationisingeneralellipticallypolarized,butsincethepositionangleofthepolarizationellipseisrotatingwiththeelectron,thetimeaveragedpolarizationislinear.Thisistruealsofortheradiationemittedbyanensembleofmonoenergeticelectronsmovinginparallelorbits.2009/08/24-28日22射電天文暑期學校TheSpectrumandPolarization2009/08/24-28日23射電天文暑期學校2009/08/24-28日23射電天文暑期學校TheSpectralDistributionofSynchrotronRadiationfromanEnsembleofElectrons2009/08/24-28日24射電天文暑期學校TheSpectralDistributionofSHomogeneousMagneticField2009/08/24-28日25射電天文暑期學校HomogeneousMagneticField2009RandomMagneticField2009/08/24-28日26射電天文暑期學校RandomMagneticField2009/08/2EnergyRequirementsofSynchrotronSources2009/08/24-28日27射電天文暑期學校EnergyRequirementsofSynchroLow-EnergyCut-offsinNonthermalSourcesSynchrotronradiationatfrequenciesbelowthelow-frequencycutoffν1shouldhaveaspectralindexofn=1/3Insynchrotronradiationfieldsspontaneousphotonemissionwillbeaccompaniesbyabsorptionandstimulatedemissionasinanyotherradiationfields.Thisabsorptioncanbecomeimportantincompact,high-intensityradiosourcesatlowfrequencieswhentheopticaldepthbecomeslarge.TheRazineffectForegroundthermalplasmamayabsorbmaysynchrotronemissionatlowerfrequencies2009/08/24-28日28射電天文暑期學校Low-EnergyCut-offsinNontherInverseComptonScatteringComptonScatteringAnX-rayorgamma-rayphotoncollideswithaparticle,usuallyanelectron.Someofthephoton’senergyistransferredtotheparticleandthephotonisreradiatedatalongerwavelengthInverseComptonScatteringAlow-energyphotoncollideswithafast-movingelectron.Theelectronpassesonasmallproportionofitsenergytothephoton,thephoton’swavelengthdecreases.Theelectronhastosufferalargenumberofcollisionsbeforeitlosesanappreciablefractionofitsenergy2009/08/24-28日29射電天文暑期學校InverseComptonScatteringCompTheSunyaev-ZeldovichEffectPhotonsfromacoldsource,the2.7Kbackground,interactwithahotforegroundsource,aclusterofgalaxies.SuchclustershavefreeelectronswithTk>107K,sothebremsstrahlungradiationpeaksintheX-rayrange.Theneteffectofaninteractionofthephotonsandelectronsistoshiftlongerwavelengthphotonstoshorterwavelength2009/08/24-28日30射電天文暑期學校TheSunyaev-ZeldovichEffectPhEnergyLossfromHigh-BrightnessSources2009/08/24-28日31射電天文暑期學校EnergyLossfromHigh-BrightneExerciseTheOrionhotcoreisamolecularsourcewithanaveragetemperatureof160K,angularsize10",located500pcfromtheSun.TheaveragelocaldensityofH2is107cm-3.Calculatetheline-of-sightdepthofthisregioninpc,ifthisistakentobethediameterCalculatethecolumndensityN(H2)whichistheintegralofdensityalongtheline-of-sight.AssumethattheregionisuniformObtainthefluxdensityat1.3mmusingTdust=160K,theparameterb=1.9andsolarmetallicityinequation(9.7)UsetheRayleigh-Jeansrelationtoobtainthedustcontinuummainbeambrightnesstemperaturefromthisfluxdensityina10"beam.ShowthatthisismuchsmallerthanTdust.Atlongmillimeterwavelengths,anumberofobservationshaveshownthattheopticaldepthofsuchradiationissmall.ThentheobservedtemperatureisT=Tdustτdust,wherethequantitiesontherighthandsideofthisequationarethedusttemperatureanddustopticaldepth.Fromthisrelationdetermineτdust.Atwhatwavelengthisτdust=1ifτdust~λ-4?2009/08/24-28日32射電天文暑期學校ExerciseTheOrionhotcoreisExerciseFromFig.9.1,determinethe‘turnover’frequencyoftheOrionAHIIregion,thatisthefrequencyatwhichthefluxdensitystopsrisingandstartstodecrease.Thiscanbeobtainedbynotingthefrequencyatwhichthelinearextrapolationofthehighandlowfrequencypartsoftheplotoffluxdensityversusfrequencymeet.Atthispoint,theopticaldepthτffoffree-freeemissionthroughthecenterofOrionAisunity,thatisτff=1,callthisfrequencyν0.Fromequation(9.36)in‘Tools’,therelationofturnoverfrequency,electrontemperatureTeandemissionmeasureEM=Ne2isν0=0.3045(Te)-0.643(EM)0.476.Thisrelationappliestoauniformdensity,uniformtemperatureregion,actualHIIregionshavegradientsinbothquantities,sothisrelationisatbestonlyafirstapproximation.DetermineEMforanelectrontemperatureTe=8300KTheFWHPsizeofOrionAis2.5’,andOrionAis500pcfromtheSun.WhatisthelineardiameterfortheFWHPsize?CombinetheFWHPsizeandemissionmeasuretoobtaintheRMSelectrondensity.2009/08/24-28日33射電天文暑期學校ExerciseFromFig.9.1,determiExerciseThesourceCasAisacloudofionizedgasassociatedwiththeremnantofastarwhichexplodedabout330yearsago.TheradioemissionhastherelationoffluxdensityasafunctionoffrequencyshowninFig.9.1in‘Tools’.Forthesakeofsimplicity,assumethatthesourcehasaconstanttemperatureanddensity,intheshapeofaring,whichthickness1’andouterradiusofangularsize5.5’.Whatistheactualbrightnesstemperatureat100MHz,1GHz,10GHz,100GHz?2009/08/24-28日34射電天文暑期學校ExerciseThesourceCasAisa熱和非熱射電源的一些例子寧靜太陽HII區(qū)的射電輻射超新星和超新星遺跡超新星遺跡的流體動力學演化較老的超新星遺跡的射電演化脈沖星河外源2009/08/24-28日35射電天文暑期學校熱和非熱射電源的一些例子寧靜太陽2009/08/24-28寧靜太陽太陽射電輻射的檢測射電天文史前19世紀末：探測器的低靈敏度20世紀初：觀測的停滯Jansky：太陽活動極小年1942年：寧靜太陽和活動太陽的射電輻射輻射源日冕熱輻射等離子體對低頻端的影響非直線的傳播逆轉的溫度結構中頻段的臨邊增亮現(xiàn)象20