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c Swinburne University of Technology, 2010 43 OF 1 AGE P : A. Koekemoer, R. Schilizzi, G. Bicknell and R. Ekers Credit (ATCA)/ATNF HET608-M03A02: Mechanisms ofand Non-Thermal Radio Emission: Emission Thermal Mechanisms of RadioRadio Emission Emission: Thermal and Non-Thermal c Swinburne University of Technology, 2010 43 OF 2 AGE P blackbody ). thermal processes, which depend on the of the emitter (eg. non-thermal processes, which do not depend on the temperature of the emitter (e.g. and the importance of plotting spectra; 1. ); and 2. HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal – – processes that result in continuum emission, including: flux • • We will also learn about polarisation and Faraday rotation. In the last Activity, we lookedinvestigate: at processes that resulted in radio line emission. In this Activity, we will Summary c Swinburne University of Technology, 2010 43 OF 3 AGE P ), hydrogen . , such as the movement of (i.e. the 21 cm line of atomic . spin-flip transition . between energy levels and the HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal radio wavelengths and changes in the rotational states of Let’s now time to turn our attention to continuum emission at radio These primarily involved changes in the quantum state of an In the last Activity we lookedat at a number of processes that resulted in the creation of emission lines Classifying emission types c Swinburne University of Technology, 2010 43 OF 4 AGE P . looks like: electromagnetic spectrum visible wavelengths . at each amount of Is there more blue light? Or more green light? Or more red light? HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal relative Unfortunately, we cannot provide aconcept of “true-colour” colour image when we of move a out radio ofEven if the continuum, we visible as could part make there of an the the is analogous radio no continuum image, such it is limited in that it cannot easily show As a quick reminder, this is what continuum emission at The visible continuum c Swinburne University of Technology, 2010 43 OF 5 AGE P detected are coloured purple. I NGC253 is shown on the right in false colour. Regions with : Koribalski, Whiteoak “& Houghton (1995) spiral galaxy Credit ) we can assign a different colour depending on the intensity of the signal. are coloured red, while regions with very little H I HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal emission line lots of H A 21 cm line image of the One solution is tocolour use to a each “false-colour” image. wavelength. For Or a for continuum an image, image we at can one assign particular a wavelength different (e.g. a specific radio Paint by numbers c Swinburne University of Technology, 2010 43 OF 6 AGE P or wavelength versus and the red cooler ones. stars . HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission flux Flux (Vertical) vsrepresents wavelength hotter (horizontal). The blue line But before we go toolearn much a further, little we more need about flux. to An alternative methodof is to plot a spectrum The exact shapethe of peak wavelength the withslope the spectrum of most (such the flux spectrum), orgreat as can the deal provide of usmechanism. with information a about the emission Plotting a spectrum c Swinburne University of Technology, 2010 43 OF 7 AGE P ) that has a , L, radio telescope luminosity ). How does that affect the energy that is ) and a finite frequency range over which 2 is the (total) Hz second per (in units of star We observe the source usingfinite a collecting detector (in (e.g. unitsit a of can m detect actually detected from the source? Jansky and there is finite HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal bandwidth for detection The dish has finite collecting area Consider the simple casedirections. The where total we energy emitted have by a the source which is emitting energy in all measured in Watts [W]. Flux and the c Swinburne University of Technology, 2010 43 OF 8 AGE P (using our hypothetical multi-wavelength 2 2 L πR 4 = F , passing through this sphere is: F , radiant flux will intercept a fraction of the total luminosity depending on how far the telescope is , the flux and luminosity L is defined as the total radiation energy crossing a unit area per unit time within a particular HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal telescope In this specialtelescope case, with an the infinite units observing bandwidth!) ofThis flux is an would example of be the W/m inverse square law. The Radiant from the source. Flux narrow frequency range.space We in can series of think concentric of spheres. theIgnoring energy the from frequency the dependenceluminosity for source the as moment, spreading at out a through sphere of radius R from the source with c Swinburne University of Technology, 2010 43 OF 9 AGE P HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal The flux detected by acollecting telescope area is of much a less telescope, thansurface such the area as total of the flux the dish imaginary of of sphere the surrounding a source. the radio This source! telescope, is is because very the much smaller than the Flux at the telescope c Swinburne University of Technology, 2010 43 OF 10 AGE P ). Jy 1 − Hz 2 − W m 26 − 1 Jy = 10 over which the measurement is made. Hertz to get the answer in Jansky (often abbreviated as 26 it is common to see fluxes quoted in Jansky, where: . is the number of is per square metre. 1 2 is the total amount of watts which is the number of Joules per second collected. − − HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal W m Hz radio astronomy • • • So to work out thesquare flux metres of and a divide source byradio we source the would in bandwidth measure the (in the sky! Hz). power in ThisSo watts, would we divide multiply be by by a the 10 tiny number number of for every known Now this seemsastronomy like a tiny amountHere: of energy, but as we’ll see later it is appropriate for radio In The Jansky c Swinburne University of Technology, 2010 43 OF 11 AGE P HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal We can also plot aflux spectrum over a at different radio frequency wavelengths, or as wavelength all range. we need to do is measure the amount of Now that we have defined thetime flux we from will a plot source, the let’s spectrum: look at our visible light continuum again. This Plotting a spectrum c Swinburne University of Technology, 2010 43 OF 12 AGE P : galaxy . 1 absorption lines to learn more about absorption lines HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal Click here In this spectrum the emission lines are1 very prominent whereas the absorption lines are more subtle. It has both emission and This is the of a typical spiral Emission lines c Swinburne University of Technology, 2010 43 OF 13 AGE P , continuum emission results from the accelerations ofcharged . . electromagnetic radiation HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal non-thermal radiation thermal radiation • • particles. Butquantised while energies, continuum line emission emission resultsquantised from and results processes so where the from the photons atomic emitted energy may exchange processes haveThere is a are not that continuous two energy main only distribution. types of have continuum emission: very specific Like all Continuum emission c Swinburne University of Technology, 2010 43 OF 14 AGE P of the emitter. and non-thermal synchrotron temperature blackbody radiation involves other processes. In this Activity we are going to look at two . means that the radiation is dependent solely on the 1 HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal Other types of radio thermal radiation include free-free radiation, and other types of non-thermal radio radiation includeradiation maser in a emission. later Activity.mechanisms (Note that that We do there will not are many emitBremsstrahlung, learn of radio both types photons, of more of which such thermal about emit as and X-ray thermal photons.) these non-thermal Compton radiation scattering types and of non-thermal continuum radiation At short radio wavelengths, thermaldominate emission at sources long dominate radio the wavelengths. sky, while non-thermalAs process we shall see, theeasy shape to of determine the the emission spectrum mechanism of1 of thermal a and source. non-thermal radiation differs, making it Thermal Thermal radiation Non-thermal radiation specific examples of continuum radiation: thermal c Swinburne University of Technology, 2010 43 OF 15 AGE P all in solids . blackbody have some internal motion: the . 2 thermal radiation C ◦ HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal = 0 K = -273 All objects with above absolute zero Thermal radiation and blackbodies are vibrating and thehotter molecules the in body, the gases faster are the vibrations zooming or aroundSince the accelerating and more charged collisions bumping particles occur. into (like thebodies each electrons above other. absolute in zero atoms) The emit emit electromagnetic radiation, Thermal radiation just means that asimplest body’s emission (and spectrum most is ideal) determined case by of its2 thermal temperature. emission The is that of a c Swinburne University of Technology, 2010 43 OF 16 AGE P wavelengths; all . In turn, such an object will a smooth spectrum of radiation which peaks at HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal the higher the temperature ofshorter the the blackbody, the wavelength atenergy is which emitted. the maximum they emit some energy at the higher the temperature ofmore the energy blackbody, it the emits; and • • • An object which isradiation capable which of happens absorbingperfect to all blackbody of fall the emit on it isa frequency called (or wavelength) a object’s dependent temperature. only on the Blackbodies also have the following characteristics: Perfect blackbodies c Swinburne University of Technology, 2010 43 OF 17 AGE P is inversely proportional to the max λ . 6 10 × T 898 is in . 2 T = max λ As the source temperature increases the peakstars wavelength are decreases. blue and This cool is stars why are hot red. of the . T is in nanometers and the temperature λ HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal where There are two important laws that describeWien’s blackbody radiation. law states that the peak of the radiation spectrum Wien’s law temperature c Swinburne University of Technology, 2010 43 OF 18 ) AGE P P 4 emits per second (i.e. its luminosity or power, AσT A = P , which is the Stefan-. 4 − K 2 − m by: 1 − T J s 8 − 10 × Flux vs wavelength for a selection ofcurve blackbodies. is As reduced the dramatically. temperature decreases the area under the = 5.67 σ HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal Here The Stefan-Boltzmann law relates the energyenergy output output from is a blackbody just toradiates. the its temperature, area where under the the flux-wavelength curve.The total The amount of hotter energy a a body blackbody of is, area the more it The Stefan-Boltzmann law is related to temperature c Swinburne University of Technology, 2010 43 OF 19 AGE P (and planets emits thermal radiation that peaks in the region. Universe HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal In the radio rangetemperatures less we than 10 will K. The only entire be able to detect thermal radiation from very cool sources, with wavelength, with a temperature of only 2.7 K. We can use both the Stefan-Boltzmann lawsource and Wien’s once law its to luminosity determine the is temperature known. of aWien’s thermal law shows us that hotpeople!) stars emit emit thermal thermal radiation radiation in in the the visible band, while cool c Swinburne University of Technology, 2010 43 OF 20 AGE P non-thermal emission depends on the temperature of the emitting source, thermal emission HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal While Non-thermal radiation depends on other things, suchfield as strength. the relative proportions of excitedExamples states of of non-thermal atoms and radiationscattering. magnetic include Both synchrotron synchrotron and radiation, maser emissionthem maser are both important emission, in in some and radio detail. astronomy Compton emission so In to we this the will Activity look next we at Module. will learn about synchrotron radiation, and leave maser c Swinburne University of Technology, 2010 43 OF 21 AGE P passes synchrotron radiation. HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal through a magnetic field, the particleThis spirals along change the in field their lines in directionthey helical of will paths. therefore motion emit means radiation. that they are accelerating, and If a charged particle (usually electrons) travelling near the This type of radiation is called Since there will be manyradiation emitted electrons covers with a a wide range frequencyemission. of range and energies so that synchrotron encounter radiation the is magnetic seen as field, continuum the Synchrotron radiation c Swinburne University of Technology, 2010 43 OF 22 AGE P Typical thermal spectrum. Typical synchrotron spectrum. Compare this to a typical thermal spectrum: HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal Note that we have made a change from wavelength to frequency. Note that synchrotron radiationphotons. has When plotted relatively on a few log-log high scale energy it’s continuum photons spectrum looks compared like to this: low energy Synchrotron vs thermal c Swinburne University of Technology, 2010 43 OF 23 AGE P at that instant. of the speed spectrum has a frequency that is determined by the HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal As the electrons spiral aroundEach the emitted magnetic field, they emits radiation over a range of . Synchrotron emission The spectrum of synchrotronelectrons. emission results from summing the emission spectra of individual c Swinburne University of Technology, 2010 43 OF 24 AGE P . α as a function of frequency F ν 10 . log α ∼ F 10 log follow this link the ‘spectral index’. α . 1 we say it has a steep spectral index. is a constant, we call 0 we say it has a negative spectral index. α 0 we say the source has a flat spectral index. telescopes HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal ∼ ) by the formula: α < α < < α ν Since synchrotron radiation is strongest atradio low frequencies (long wavelength) it can be detected with If If If We say that the flux has a ‘power law dependence’ on frequency given by where For a reminder of the logarithmic scale, ( If over some finite frequency range we can describe the amount of flux Spectral Index c Swinburne University of Technology, 2010 43 OF 25 AGE P HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal It is emitted over a wide range ofIt energies, producing is a highly wide polarised energy (whichpolarisation spectrum. providing we’ll information discuss about in the a magnetic fields moment), of with the the source. degree and orientation of the It is extremely intense andbe highly coming collimated from radiation, a which thin means cone. that the radiation seems to • • • Some properties of synchrotron radiation include: Synchrotron properties c Swinburne University of Technology, 2010 43 OF 26 . AGE P ) with black holes remnants Crab , and near and the quiet galaxies Nebula (which are remnants of massive stars), around emanating from active pulsars jets and 3C273, the galaxy M31, and the quasar supernovae HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal planets with strong magnetic fields, in the Here we compare thesynchrotron emission spectrum from of the twonebula and thermal Cassiopeia sources A. The (Orion shape of the two types of emission clearly differ. Synchrotron radiation can befields. observed anywhere there areThis fast-moving occurs, for electrons example, and in magnetic c Swinburne University of Technology, 2010 43 OF 27 AGE P is a self-propagating disturbance of electric and magnetic electromagnetic wave HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal Recall that an Polarisation fields at right angles to each other, with both fields perpendicular to the direction of propagation. c Swinburne University of Technology, 2010 43 OF 28 AGE P , which means that the wave vibrates perpendicular to the direction of transverse wave HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal or in any intermediate orientation. side to side in the horizontal plane, If a beam of lightthe is electric field), travelling it out can of vibrate the either screen up towards and you down in (consider the for vertical now plane, just Light is a Properties of light propagation. c Swinburne University of Technology, 2010 43 OF 29 AGE P . linearly Unpolarised light plane polarised vibrating of electromagnetic radiation. If the amplitude polarisation defines the electric field . Light comprises of oscillating electric and magnetic fields , and can range from partially to totally polarised. HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal in all directions perpendicular to theunpolarised direction of propagation. Such light is called Normally, a ray of light contains a mix of waves of the same The orientation of the vibrations have greaterpolarised amplitude in a preferredIf direction, the electric then field always the vibrates in light just a is single said plane, the to wave is be said to be c Swinburne University of Technology, 2010 43 OF 30 AGE P . . elliptically polarised circularly polarised , and the radiation is HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal ellipse If the amplitude of vibration remains theis same, called the electric field will trace out a circle and the radiation On the other hand, if the amplitude changes during propagation, the electric field traces out an The plane of vibration can also rotate as it propagates. c Swinburne University of Technology, 2010 43 OF 31 AGE P – depends on what is happening to the . elliptical polarised . unpolarised The atoms willrandomly, generally so be the direction orientated offields will the also electric be random and thebe light will If the atoms are aligned, their electricwill fields also be aligned and thesuch a radiation source from will be HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal The type of polarisation – whetheratoms linear, (or circular electrons) or in the source. A light beam isatoms usually in made the source. up of the superposition of many light waves emanating from many Why is light polarised? c Swinburne University of Technology, 2010 43 OF 32 AGE P planetary HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal Circular polarisation is an indicationthe light of is the emitted presence (making of electronsand spiral visible strong along emission magnetic on from fields helical pulsars, paths). supernova in remnants Examples the and are region magnetic found where stars. in the radio . Lightpolarised. scattered off interstellar grains that are aligned can also become partially For example, light scattered in a gas becomes partially polarised, which occurs in Polarising light c Swinburne University of Technology, 2010 43 OF 33 AGE P . Faraday rotation HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal The plane of polarisationthrough of a magnetised linearly plasma. polarised This effect radiation is is called rotated when the radiation propagates Faraday rotation (The reason that the magnetic fieldof rotates the the electromagnetic polarisation wave is which dueof to requires this a some Unit!) complex high-level propagation mathematics process that is beyond the scope c Swinburne University of Technology, 2010 43 OF 34 AGE P of the plasma; density travelled through the medium. The magnetic field must be aligned with the direction distance HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal the mean electron the mean magnetic field strength; the square of the wavelength of the radiation;the and of wave propagation. • • • • The amount of Faraday rotation depends on four things: The magnetic field must be aligned with the direction of wave propagation. c Swinburne University of Technology, 2010 43 OF 35 AGE P in the galaxy. dust . And... neutral hydrogen HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal A component of synchrotron radiationmagnetic at field. long wavelengths (low frequency) dueA to blackbody the spectrum galaxy’s at short wavelengths (high frequency)21 due cm to line emission from Possibly some absorption lines as well. • • • • On the next page, you can see the composite spectrum. The combined emission from a source,result detected in over a a range composite of of wavelengths (or all frequencies), the might For processes example, we a have typical looked normal at. galaxy has a spectrum that contains: Putting it all together c Swinburne University of Technology, 2010 43 OF 36 AGE P HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal If we only observe theradio source portion at of visible the wavelengths,source. spectrum we provides would us only get with a many part clues of about the the picture. nature and The properties of the Composite spectrum for a typicalradio to normal visible: galaxy observed over a wide range of frequencies, from Composite radio spectrum c Swinburne University of Technology, 2010 43 OF 37 AGE P : A. Koekemoer, R. Schilizzi, G. Bicknell and R. Ekers (ATCA)/ATNF Credit Optical/Radio composite image of the powerful PKS 2356-61 HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal The different emission mechanisms can dominate different regions of your source. c Swinburne University of Technology, 2010 43 OF 38 AGE P HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal So now that we know theobserve various it? mechanisms that produce continuum radiation, how doUnlike we line actually emission, we dodoing not have continuum a observations. specific frequency Ideally,we’d to to like tune gain to our observe the radio over optimal receiver as amountcorrelators to wide and a when of receivers, range we information we of are from are frequencies usually as our possible. limited source, to BecauseSo, for of some example, the set you design frequency might of range. be radio interestedcm in and the total 3.5 flux cm, from which alllike radiation the you in “7 then your mm call source continuum” between the and 2.5 “3cm the “6 continuum cm flux continuum”. density”. Thus you’ll often see terms Observing continuum radiation c Swinburne University of Technology, 2010 43 OF 39 AGE P HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal To get a full spectrumacross of a your range sourcetelescopes, across of or the (iii) radio entire at telescopes bands.This radio with range dual must would orIn be multi-frequency reality require observing this you done is capacity. to very either rare. observe (i) at different times, (ii) at different The full spectrum? c Swinburne University of Technology, 2010 43 OF 40 AGE P . Milky Way HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal Interstellar atoms and moleculesdiscovered emit was line the emission 21 cm athave line radio been of wavelengths. discovered. atomic We hydrogen, The have andWay learned first thanks since a to radio then lot these line many radio about interstellar lines. the molecules structure and composition of the Milky • Now that we’ve learnedinvestigated some of a the mechanisms little of radio aboutin emission, the what the types radio of range? radio sources actually end emit radiation ofThere are the several sources electromagnetic of spectrum radio emission and in our have galaxy, the Radio sources c Swinburne University of Technology, 2010 43 OF 41 AGE P , are also strong radio emitters. They too have supernova remnant neutron stars : Chandra Cassiopeia A Credit Radio image of , which are the remains of exploded stars, are sources of strong radio , which are rapidly rotating HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal very strong magnetic fields and their radio emission is also probably synchrotron. Pulsars Supernova remnants emission. They have verydue strong to magnetic synchrotron radiation. fields and so their radio emission is thought to be • • More sources c Swinburne University of Technology, 2010 43 OF 42 AGE P , which is the left over thermal radiation CMB ! or 4881 in Giant elliptical NGC Coma spiral galaxies radio sources), which are extremely distant strong radio sources. stellar . , which are usually giant elliptical and irregular galaxies. They emit more than a cosmic microwave background (or quasi- Big Bang HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal There is also a smallknown amount as of background the radio noisefrom coming the from every direction in the sky, Quasars Radio galaxies million times the radio energy of • • • We’ll learn more about these and other radio sources in the next Module. Extragalactic sources of radio emission include: Still more sources c Swinburne University of Technology, 2010 43 OF 43 AGE P sources of radio emission. galactic HET608-M03A02: Mechanisms ofand Non-Thermal Radio Radio Emission: Emission Thermal In this Activity, we have discussedwithin flux a - particular the narrow total frequency range radiation - energyWe and crossing plotting also a specta unit discussed (flux area against continuum per wavelength). particles. emission unit time processes In which continuum result emissioncontinuous energy from the distribution. the energy We acceleration is lookednon-thermal not of at emission quantised charged thermal such and as emission synchrotron from so radiation. blackbody the radiationWe photons as also emitted well discussed as have polarisation, a Faraday in rotation, which which occurs the when direction theit plane of propagates of through vibration polarisation a of of magnetised linearly plasma. radiation polarised are lightIn restricted, is the rotated next and as Module we will learn about Summary