Natural Sources of Radio RET 2013 MIT HAYSTACK OBSERVATORY

Transcription

1 Natural Sources of Radio RET 2013 MIT HAYSTACK OBSERVATORY

2 Learning Objectives NGSS Performance Expectations Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction. Force = ma acceleration of a charge is the primary mechanism for EM radiation emission We will investigate the nature of those forces leading to emision.

3 Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter..

4 Lecture Outline Types of emission Thermal emission Background, Blackbody radiation Emission spectra analysis Non-thermal emission

5 Lecture 1: Overview Thermal Emission Radiative Transfer process overview Foundation of Thermal Emission Kinetic molecular theory Types of thermal emission Blackbody Emission Free-Free emission Spectral Line emission Atomic Molecular

6 Radia&ve Source Processes Blackbody emission Free- free radia&on Spectral line emission Radia&ve Transfer processes Observed light Cyclotron and Synchrotron radia&on

7 Any process that will accelerate a charged par&cles will produce EM radia&on This could be a free electron traveling through the vaccum of space and being affected by a magne&c field and thus accelerated It could be a bound electron or proton and the mo&on associated with thermal energy is causing quick accelera&ons associated with that mo&on. The Kine&c molecular theory states all maker is made of &ny par&cles in constant mo&on o o The constant mo&on will generate EM radia&on We call this type of emission, thermal emission

8 The type of radia.on tells us something about the source Thermal emission Blackbody radia&on Spectral line emission Free- free radia&on Non- Thermal Cyclotron emission synchrotron emission MASERs

9 All macroscopic (everyday) objects emit EM radia&on at all &mes!! (if T > 0 K) explaina&on: The Kine&c Molecular Theory,KMT» all maker is made up of &ny par&cles (atoms, molecules, sub- atomic par&cles) in constant mo&on.

10 Temperature is a direct measure of average kine&c energy of all microscopic par&cles. Velocity vector Atom or molecule Distribu&on of the # of par&cles at each level of kine&c energy # of molecules T Average Kine&c Energy

11 Wein's Law» Wavelength of peak emmission λ 1/Temperature Wavelength of peak emission is inversely propor&onal to the Temperature. Higher Temp == lower λ (blue) Lower Temp == Higher λ (red)

12 » Recall that the EM spectrum ranges from frequencies of 1 cycle per second (1 Hz) to

13 » Stephan's Law The power output from the surface of a blackbody radiator is propor&al to the Temperature to the 4 th power P σ T 4

14 » The KMT represents par&cles as moving at a distribu&on of Kine&c energy» Accelera&ng charges create EM waves, The different accelera&ons produce different frequencies» A blackbody spectrum represents the distribu&on of EM radia&on and changes with temperature» Link to Starter Ac&vity: Imagine each student traveling randomly and they were carrying a flashlight that changed color depending on their speed. An observer from distance would see a combina&on of all the different colors represented by the different speeds. If put through a simple spectrometer or prism it would produce a spectrum. That s the blackbody spectrum.

15 Lecture 2: Spectral line analysis Wave nature of light Particle nature of light Spectroscopy for absorption and emission processes

16 Spectral Line emission (spectroscopy) Radiation can be examined with a simple spectrometer

17 Interaction principle The way that atoms and molecules absorb and emit radiation can tell us something about their nature or identity. Demo: Hydrogen emission Continuous spectrum Absorption spectrum Emission spectrum

18 Electron moved from ground state to elevated state. Absorption UV Photon Electron falls down to ground state again A photon is emiked equal in energy to the difference between ground state and excited state. Emission

19 Each transi&on from higher to lower state emits a photon of a certain energy and therefore wavelength

20 » The emission spectra of an element provides a fingerprint that allows scien&sts to deduce its presence from the observa&on of the specta Analogy: Bar code» Detec&ng composi&on The composi&on of an object is determined by matching its spectral lines with laboratory spectra of known atoms and molecules

21 » Link to Unit Starter: What if every element and molecule has a specific set of seats available on the bleachers: + You would only see a specific # of emission lines as electrons move up and down into them? That s exactly how atoms and molecules work. They have a fingerprint that is their absorp&on/emision spectrum that is unique to that element if you look for the transi&ons that should set it apart from all the others. The cataloguing of these transi&on loca&ons and energies in the lab has helped scien&sts find many atomic and molecular species in the night sky remotely.

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23 - Both the proton and the electron are going to have an individual spin The spin of both can therefore be in the same direc&on (aligned) or in opposite direc&ons (an&- aligned) Because of quantum mechanics, it turns out when the spins are aligned, the hydrogen is higher in energy hkp://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/hydrogenlineparallel.svg/500px- HydrogenLineParallel.svg.png

24 - Even though the Aligned version is higher in energy, its electron s&ll exists in the S orbital Instead, the aligned version compared to the an&- aligned version has hyperfine structure

25 - It is possible for hydrogen to jump from its higher energy aligned state to the lower energy an&- aligned state Very unlikely to happen: o probability of s 1 o &me it takes for a single isolated H atom to undergo this transi&on is ~ 10,000,000 yrs When it does happen, it releases a specific wavelength of light... o Care to guess what that wavelength is?

26 - The energy gap between the hyperfine structures directly corresponds to the 21- cm wavelength ( MHz) This wavelength was predicted by Jan Oort and Hendrick C. van de Hulst in 1944 Discovered by Edward Mills Purcell and Harold Irving Ewen in 1951 hkp://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/green_banks_- _Ewen- Purcell_Horn_Antenna.jpg/321px- Green_Banks_- _Ewen- Purcell_Horn_Antenna.jpg

27 - So what's the point? What can be done with this informa&on? First use of this was in 1952 where the first maps of neutral hydrogen in our galaxy were made These maps using the doppler shiq of the 1420 MHz spectral line revealed the spiral structure of our galaxy hkp://upload.wikimedia.org/wikipedia/commons/thumb/4/43/eso- VLT- Laser- phot- 33a- 07.jpg/320px- ESO- VLT- Laser- phot- 33a- 07.jpg hkp://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/ngc_6384_hst.jpg/320px- NGC_6384_HST.jpg

28 » So we have seen that if maker is moving in any way, charged par&cles are being accelerated» If charges are being accelerated EMR photons are being produced» The power and spectral distribu&on of those photons depends on The Temperature of the material.» Therefore: We can detect the temperature of materials in space by analyzing the light coming to us on earth.!

29 Lecture 3 NON-THERMAL EMISSION AND OTHER WEIRDNESS

30 This energy distribution can be modelled very accurately. Everything resembling this shape is called THERMAL radiation. Remember that the temperature of an object can be inferred from the peak wavelength of the blackbody spectrum. λ~1/t

31 Comparison of Thermal vs. Non-Thermal radiation Non-thermal Thermal Think of intensity as the number of photons In thermal radiation, most photons are at the peak frequency, thus you can relate that to the Temperature (average kinetic energy) In non-thermal you can t do that Non-thermal Thermal Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy 101 Spring Minnesota State University, 6 Jan

32 Direct observations leading to new insights Particle physics studies the properties of the fundamental particles of matter. Uses very high energy Alows us to discover how particles behave at these high energies. Non-thermal emission processes were discovered in this way.

33 From these types the synchrotron radiation seemed to fit the models for non-thermal sources The non-thermal emission properties were used to model the spectra of quasars and other radio sources. The spectra of these could be explained with the models

34 Synchrotron Radiation First discovered in a Bell Laboratory particle accelerator called a synchrotron (1947) The power law distribution was very different from the Maxwellian-Planck distribution in that it increased with higher frequency High energy sources could then be detected by this unusual spectral feature especially at x-ray and gamma-ray bands.

35 Examples of Astrophysical Synchrotron Radiation The bluish region in the center of the crab nebula is caused by synchrotron radiation The bluish jet from M87 is emerging from the AGN core

36 Case Study: Blazars (yes, that is an actual group of objects in astronomy) In 1963 Maarten Schmidt discovered quasars using radio wave measurements Quasars Quasi-star radio sources Quasars are: Very distant (100s of billion LY) Very bright (about the same amount of light as our entire galaxy) Highly Variable (changing in periods of days to years) This was a discovery that confirmed the big bang cosmological model over the static universe model.

37 Blazars cont. Blazars are radio quiet but have red shifts similar to quasars and are therefor very distant. Blazars are originally named BL Lac objects from observations of the star BL lacertae Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy 101 Spring Minnesota State University, 6 Jan

38 Other Sources of Non-Thermal (Synchrotron) Radiation: MASERS Microwave Amplification by Stimulated Emission of Radiation Emissions from a particular transition are used as a pump for sustained emission from other molecules Added together the radiation becomes amplified Fish, Vincent L., and Loránt O. Sjouwerman. "GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL MASERS IN ONSALA 1." The Astrophysical Journal (2010): Web.

39 MASERs cont. Requirements for interstellar MASERs Low density Less than 10 4 cm -3 This is very difficult to achieve in the Lab but is very high density for interstellar media But high gain Lots of particles in the path along the line of site Therefore, we need large regions in space to form masers cm 3

40 Summary of Non-Thermal Sources Non-Thermal sources have a different energy distribution function. Basically everything that doesn t look like this is non-thermal Synchrotron radiation observed in particle accelerators explains the spectra of distant quasars Observations of non-thermal radiation has lead to important discoveries of Active Galactic Nuclei (AGN)

41 1. "Astronomy: A Beginner's Guide to the Universe" 7th ed. Chaisson, E.; McMillan, S. Pearson Educa&on inc p hkp://physics.nist.gov/cgi- bin/cuu/value?me search_for=electron+mass 3. Outer Space is not Empty: A Teaching Unit in Astrochemistry. RET 2004 Haystack Observatory MIT. Wesley Johnson and Roy Riegel.

42 4. Course: ASTR 122: Birth, Life and Death of Stars hkp://jersey.uoregon.edu/~imamura/122/astro.122.html 5. hkp:// 6. hkp://galileo.phys.virginia.edu/classes/241l/emwaves/emwaves.htm 7. hkp:// 8. hkp://scienceworld.wolfram.com/physics/brightnesstemperature.html 9. Eskridge, Paul. "Ac&ve Galac&c Nuclei." Notes for Week 14 Astronomy 101 Spring Minnesota State University, 6 Jan Web. 24 July <hkp://frigg.physastro.mnsu.edu/~eskridge/astr101/ week14.html>. 10. Fish, Vincent L., and Loránt O. Sjouwerman. "GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL MASERS IN ONSALA 1." The Astrophysical Journal (2010): Web.