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Radio AstronomyEmission Mechanisms in Radio AstronomyB0.209 - April 11th, 2013Lecturer: Jason Hessels ([email protected])Lecture 4Master Astronomy and Astrophysics - 5214RAAS6Y
Radio Astronomy -5214RAAS6YEmission Mechanisms inRadio Astronomy
Radio Astronomy -5214RAAS6YLecture outlineThermal emission processesBlackbody radiation, free-free emissionSpectral line emissionRadio-recombination lines, 21-cm line, molecular lines, MASERsNon-thermal emission processesCyclotron emission, synchrotron emission, inverse Compton, synchrotron self-Compton, pulsar emissionPropagation effectsBringing it all together
Radio Astronomy -5214RAAS6Y“Boson Astronomy”Understanding the emission/absorption ofelectromagnetic radiation is key to astronomy...but it’s not our only window on the Universe.Boson Astronomy:Photon astronomy (Radio-Gamma), Graviton astronomy (GWs)Pneumatic Astronomy:Acoustic waves, magnetohydrodynamic wavesParticle Astronomy:Cosmic-rays, neutrinos, meteoritesDirect Techniques:Space probes, manned exploration
Radio Astronomy -5214RAAS6YEM radiation is emitted by accelerated chargedparticles.Thermal emission depends only on thetemperature of the emitting object.Non-thermal emission does not depend ontemperature.Photon frequency proportional to energy.λ =cν=EhcEmission Mechanisms
Radio Astronomy -5214RAAS6YBrightness temperature: the equivalenttemperature of a blackbody emitting the sameintensity.Brightness Temperature
Radio Astronomy -5214RAAS6YContinuum vs. Line EmissionContinuum: wide-range of particle energies.Line: discrete energies due to transitions in atomsor molecules.
EM SpectrumOnly optical/IR and radio pass through the atmosphere.Radio window: 1cm - 30m / 10MHz - 30 GHz (or more)Radio Astronomy -5214RAAS6Y
Radio Astronomy -5214RAAS6YThermal EmissionProcesses
Radio Astronomy -5214RAAS6YThermal EmissionAny object with a temperature above 0K emitsthermal radiation.Temperature is related to particle motion.
Radio Astronomy -5214RAAS6YBlackbodyTemperatureA true blackbodyonly absorbsradiation and reflects none.Blackbody reaches a thermalequilibrium and re-radiates in thecharacteristic “blackbodyspectrum”.
Radio Astronomy -5214RAAS6YBlackbody RadiationIntensity and spectrumdependsonly on temperature.Blackbody spectrum describedby Planck’s law.Even relatively cool objects (e.g.the Earth) peak well above theradio band (in infrared).
Radio Astronomy -5214RAAS6YStefan-Boltzman Law: total emitted energyincreases rapidly with temperature.Wien’s Law: peak frequency depends ontemperature.Blackbody Radiation
Radio Astronomy -5214RAAS6YBlackbody Radiation1 GHzJust see the “tail” in radio.
Radio Astronomy -5214RAAS6YCMB: The Perfect BlackbodyThe CMBR has a thermal blackbody spectrum at atemperature of 2.72548±0.00057 KSpectrum peaks around 200GHz
Radio Astronomy -5214RAAS6YThermal Emission from DustBlackbody radioemitters are extremelycool, < 10K.

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synchrotron emission

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