1. Swinburne Online Education Introduction to Particle Physics and High Energy Astrophysics © Swinburne University of Technology Module 8: High Energy Astrophysics Activity 1: High Energy Processes high energy photon e-e-

2. Summary In this Activity we will learn about some of the high energy processes that both produce high energy emissions and also high energy interactions that can occur between photons and elementary particles that are important in high energy astrophysics. Specifically, we will learn about: blackbody radiation; bremsstralung radiation; Compton and inverse Compton scattering; pair production; and synchrotron radiation.

3. Introduction So far in this Unit we have learned a great deal about particle physics and the detection of elementary particles. Let’s now apply our new found particle physics knowledge to study high energy processes in astrophysics. In this Activity we’ll look at some of the processes that produce high energy particles - both by thermal and non- thermal processes. We’ll also look at some of the ways that high energy photons interact with matter. Then in the next few Modules we’ll look at high energy processes in the Sun and other stars including white dwarfs and pulsars, and then move on to X-ray and gamma ray astronomy.

4. High Energy Radiation Electromagnetic radiation is produced whenever a charged particle is accelerated. The greater the acceleration, the higher the energy of the emitted photon. There are two types of electromagnetic radiation: thermal radiation - which depends on the temperature of the emitting source - and non-thermal - which does not depend on the source temperature. The shape of the spectrum (which is radiation flux plotted again frequency) allows us to determine the source’s emission mechanism.

5. The simplest case of thermal emission is that of a blackbody - which absorbs all radiation that falls on it and in turn emits a smooth spectrum of radiation. The wavelength peak of the spectrum depends only of the temperature of the source. Blackbody radiation obeys two important laws: Wien’s Law: which relates the peak wavelength to the source temperature, and basically the hotter the source the shorter the peak wavelength (the higher the energy). Stefan-Boltzmann’s Law: which relates the total amount of energy emitted per second to the source temperature, and again, the hotter the source, the more energy produced. Flux Wavelength blackbody spectrum

6. So what sort of temperatures and energies are involved in high energy thermal radiation? First let’s review a few equations related to electromagnetic radiation. To calculate the thermal source temperature we use Wien’s law: 2.898 x10 6 nm max T = The photon energy and momentum are related to frequency via: E = h and Momentum = h / c where h is Planck’s constant Recall that wavelength  and frequency are related via: c = where c is the speed of light Click hereClick here to be reminded about the physical constants and units used. This means that when a photon loses energy and momentum, its frequency decreases.

7. Gamma radiation is generally defined by photons with  1.24 MeV and T >10 8 K. Such high energy photons are created in nuclear reactions and other very high energy processes. X-rays are those photons in within the wavelength range 0.001 nm < < 10 nm, with 124 eV < E < 1.24 MeV and 10 6 K < T < 10 8 K. These high energy photons are created, for example, in supernovae remnants and the solar corona, as well as in the hot gas between galaxy clusters. Armed with these equations, we can now see what thermal temperatures and energies are involved in high energy radiation:

8. Types of Thermal Radiation You will already be familiar with atomic excitation and de-excitation as a means of producing photons. Thermal atomic excitation is generally via collisions, which excite atoms, and as they de-excite, they emit photons. The hotter the medium, the higher the kinetic energy of the impacts, and the higher the resulting photon energy. photon

9. H+H+ e-e- If a negative electron approaches a positive ion, they will be attracted to each other and the strong electric force will alter the trajectory of the electron (i.e. accelerating it), which leads to electromagnetic radiation being emitted: This type of emission form is called free-free emission, or thermal bremsstrahlung - which is German for “braking radiation”. As well as atomic excitation, another thermal process is bremsstrahlung radiation, which occurs when free electrons interact with ions in, for example, the hot atmospheres of stars.

10. The frequency range of the radiation depends on how much the electron’s trajectory is bent by the interaction with the positive ion. This depends on several things, including the relative velocities of the two bodies, which in turn depends on the temperature of the gas, which is why free-free emission is a thermal process. An example of high energy thermal bremsstrahlung is the X-ray emission from giant elliptical galaxies and hot intercluster gas. X-ray image of hot intercluster gas in Hydra A

11. As an introduction to particle scattering, let’s begin with Thomson scattering, which is actually low energy scattering between a photon and an electron. Thomson Scattering The interaction is elastic, which means that the photon and electron just both bounce off each other, changing their direction, but there is no exchange of energy.  photon Photon (no change in wavelength) e-e- electron

12. As an introduction to particle scattering, let’s begin with Thomson scattering, which is actually low energy scattering between a photon and an electron. Thomson Scattering The interaction is elastic, which means that the photon and electron just both bounce off each other, changing their direction, but there is no exchange of energy. photon  Photon (no change in wavelength) e-e- electron

13. If the energy of the scattering photon is increased, then there will be an exchange of energy, and hence and frequency change after the interaction. Increasing the Energy But as long as the energy of the incident photon is lower than the rest mass energy of the stationary electron, that is: then the interaction can always be described as Thomson scattering. h << m e c 2 (in the reference frame of centre of momentum) Ok - let’s now look at what happens if h > m e c 2.

14.  In high energy astrophysics there are many inelastic photon interactions, whereby the photon energy changes after the scattering. Compton Scattering In Compton scattering, a photon of high energy collides with a stationary electron and transfers part of its energy and momentum to the electron, decreasing its frequency in the process. high energy photon lower energy photon e-e- electron at rest higher energy electron

15. This process was discovered first in 1923 by Arthur Compton, who realised that the wavelength of X-ray radiation increased after the scattering with stationary electrons. The change in wavelength of the photon after Compton scattering is given by: where  is the scattering angle of the photon.  (1 – cos  ) h mec2mec2 In the optical band, this effect is pretty negligible, but in the X-ray the wavelength shift can be as high as 10 . The increase in wavelength during Compton scattering results in a corresponding decrease in the the energy of the photon. The energy is transferred to the electron in the form of kinetic energy or motion.

16. In inverse Compton scattering, a high energy electron transfers both energy and momentum to a lower energy scattering photon. Inverse Compton Scattering The Compton effect and the inverse Compton effect are exactly the same process, only the order of energy and momentum transfer is reversed. photon gains energy lower energy photon electron loses some energy high energy electron In astrophysics, inverse Compton scattering is actually more important that Compton scattering.

17. In astrophysical situation, the inverse Compton effect occurs when a low energy photon, such as in the cosmic microwave background, bounces off a relativistic electron. PKS 2356-61 Such relativistic electrons are produced in supernovae and active galactic nuclei. Crab Nebula

18. The Inverse Compton Spectrum If we plot the spectrum of an inverse Compton source, we can easily see that it is quite different to a thermal spectrum. At low frequencies, the scattered radiation increases proportionally with frequency, while at high frequencies, it drops down below a maximum frequency. / 0 arbitrary unit log 10 I( ) arbitrary unit max / 0

19. E max = (h ) max  4  2 h 0 Relativistic effects become important in inverse Compton scattering since it involves high energy (and therefore fast moving) electrons. The maximum energy gained by photons via inverse Compton scattering is equal to its initial energy multiplied by the square of twice the Lorentz factor (where the Lorentz factor squared is given by  2 =1 / [1-(v/c) 2 ] and v is the electron velocity): In general, the frequency of the scattered photon is approximately given by   2 0. In many astronomical sources there are electrons with   100 –1000, and therefore inverse Compton scattering is the main radiation process, scattering low energy photons up to very high energies.

20. As we have seen in the Activity Lend Me a Lepton, every particle has an antiparticle, and if they encounter each other they will annihilate and give off gamma rays. Pair Production For this to occur, the photon energy (h ) must be at least the rest mass (mc 2 ) of the electron and positron, given by h > 2 m e c 2 = 2 x 0.51 MeV = 1.02 MeV. The reverse of this process can also occur: pair production is the formation of an electron and position from a high energy photon (usually in the vicinity of an atomic nucleus). e-e- e+e+ 

21. For X-ray and gamma ray energies above 1.02 MeV, pair production is one of the most important types of interaction with matter, and one of the principle ways that gamma rays are absorbed in matter. If the photon energy is greater than 2 m e c 2, the excess energy goes into the kinetic energy of the particle pair. Electron-positron pairs in bubble chamber tracks. In the presence of a magnetic field, the electron-positron pair will follow curved arcs away from each other. This is in fact how pair production was first discovered in 1933.

22. Synchrotron Emission photons fast-moving electron helical path B When electrons encounter a magnetic field, they spiral along the field lines in a helical path. This means that their direction is constantly changing, and hence they are accelerating and therefore emit radiation. This radiation is called synchrotron radiation.

23. The total power, P, emitted by a particle moving inside a magnetic field is proportional to the magnetic energy density U B (U B  B 2 ): P  U B, which means that the stronger the magnetic field, the stronger the emitted radiation. This relation can be reversed and used to determine the magnetic field strength producing the observed synchrotron radiation. Synchrotron radiation is emitted over a wide range of energies, producing a continuum spectrum. The region of maximum emission depends on the energy of the electron and the strength of the magnetic field.

24. Because the electrons are travelling at relativistic speeds, they don’t escape in all directions. In fact, the faster the electrons are travelling, the more collimated the radiation beam. Source 1  Beam of radiation v Electron velocity As v  c,  increases, so 1/  decreases and the beam becomes more collimated. Synchrotron radiation is highly collimated in the direction of the velocity of the charged particles due to relativistic effects. Collimated Radiation

25. We will return to this idea in the Module of Gamma Ray Bursts. Beaming can result in anomalously high radiation. The strong gamma ray sources that we see, for example, could either be extremely strong sources of radiation that emit in all directions equally; or it could simply be that the charged particle that produce the radiation are moving towards us and thus the radiation is beamed in our direction, making it seem a lot stronger than it really is.

26. Since there will be many electrons with a range of energies that encounters the magnetic field, the radiation emitted covers a wide frequency range and so synchrotron radiation is seen as continuum emission. … which is very different from thermal emission that exhibits a typical blackbody spectrum... log 10 F log 10 The resulting spectrum of synchrotron radiation looks like this: F = flux density = frequency Synchrotron Spectrum

27. log 10 F log 10 Since synchrotron radiation is strongest at low frequencies, it can be detected with radio telescopes This straight line behaviour can be represented by the formula where  is a constant. We say that the flux has a ‘power law dependence’ on frequency: F ~ - . log 10 F ~ -  log 10

28. Above the critical frequency, c, the spectrum drops exponentially. The shape of the overall spectrum actually comes from the sum of each electron’s contribution. Individual electrons spiraling around the magnetic field lines emit a spectrum that peaks at one particular frequency, c : log 10 / c log 10 F c Log 10 / c Sum of individual contributions Summing the individual contributions of many electrons gives the resulting synchrotron spectrum:

29. Note that at low frequencies, the flux of the synchrotron emission does not increase without any limit – the spiralling electrons begin to re-absorb the photons with low energies. Synchrotron Self Absorption This corresponds to a turn-over in the spectrum, known as synchrotron self absorption. log 10 F Log 10 / c Turn-over

30. Synchrotron radiation can be observed anywhere there are fast-moving electrons and magnetic fields. This occurs, for example, in supernova remnants and pulsars, around planets with strong magnetic fields, in the jets emanating from active galaxies, and near black holes. Vela Supernova Remnant M82: outflow of ionised gas streaming out of the galaxy

31. Summary In this Activity, we have had a look a both thermal and non- thermal processes that produce high energy photons, as well how these photons interact with matter. We learnt about free-free emission, Compton and inverse Compton scattering, as well as pair production and synchrotron radiation. Now that we understand some of these high energy processes, we can look at some specific situations of high energy astrophysics and explore them in more detail. In the next Activity, we will investigate high energy processes in the Sun.

32. Image Credits VLT image of the Crab nebula http://antwrp.gsfc.nasa.gov/apod/image/9911/crab_vlt_big.jpg http://antwrp.gsfc.nasa.gov/apod/image/9911/crab_vlt_big.jpg Chandra observations of a galactic cluster Hydra A - NASA/CXC/SAO http://chandra.harvard.edu/photo/0087/index.html http://chandra.harvard.edu/photo/0087/index.html The Milky Way http://antwrp.gsfc.nasa.gov/apod/image/0006/southerncross_gb.gif http://antwrp.gsfc.nasa.gov/apod/image/0006/southerncross_gb.gif Electron-positron pairs in bubble chamber tracks. http://teachers.web.cern.ch/teachers/archiv/hst2000/teaching/resource/bubble/gtj/gtj.htm Radio-Optical Image of radio galaxy PKS 2356-61, ATNF http://wwwatnf.atnf.csiro.au/research/images/pks2356.html http://wwwatnf.atnf.csiro.au/research/images/pks2356.html Vela Supernova Remnant http://antwrp.gsfc.nasa.gov/apod/image/vela_roe.gif M82 http://crux.astr.ua.edu/gifimages/m82r.html http://crux.astr.ua.edu/gifimages/m82r.html

33. End of Activity Press the ESC (Escape) key to return to the home page for this Module.

35. Physical Constants and Units The energy of elementary particles is quoted in electronvolts (eV) where 1 eV = 1.602  10 -19 Joule = 1.602  10 -12 erg In high energy astrophysics, we deal with high energy particles, and the energies may give given in terms of: 1 kiloelectron-volt = 1 KeV = 10 3 eV 1 megaelectron-volt = 1 MeV = 10 6 eV 1 gigaelectron-volt = 1 GeV = 10 9 eV 1 tetraelectron-volt = 1 TeV = 10 12 eV Speed of light = c = 2.9979  10 8 m/s Planck’s constant: h = 6.6261  10 -34 J s

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