M.Phys., M.Math.Phys., M.Sc. MTP Radiative Processes in Astrophysics and High-Energy Astrophysics

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1 M.Phys., M.Math.Phys., M.Sc. MTP Radiative Processes in Astrophysics and High-Energy Astrophysics Professor Garret Cotter Office 756 in the DWB & Exeter College

2 Welcome to M.Phys. & M.Math.Phys. Astrophysics There are some recent changes to the course in response to feedback: We have re-organised the logical structure, the course now starts with radiative processes and then moves on to more phenomenological topics. The size of the lecturing team has been reduced. I will cover the whole of Michaelmas term, giving the lectures on the first two topics, Radiative Processes & High Energy. No doubt there will be some teething troubles, please get in touch with any problems and (constructive!) criticism. I use a combination of slides and blackboard. Wherever possible derivations will be done on the board to allow everyone time to follow them. I am always happy to answer questions - always feel free to ask at the end of the lecture, or to if you want to arrange a time have a longer discussion.

3 Lectures this term are always at noon in the Dennis Sciama Lecture Theatre Radiative Processes in Astrophysics Week 1 Mon Wed Fri Week 2 Mon Wed Fri Week 3 Mon Wed Fri Week 4 Mon Wed Fri High-Energy Astrophysics Week 4 Mon Wed Fri Week 5 Mon Wed Fri Week 6 Mon Wed Fri Week 7 Mon Wed Fri Week 8 Mon Wed Fri

4 Synopsis: Radiative Processes The EM spectrum in astrophysics; temperature and radiation brightness. Spectroscopy, forbidden and allowed transitions, cosmic abundances Two-level atom, A, B and C coefficients and their useful regimes, thermal populations, IR fine structure, critical density, mass estimates Recombination andionization, Stromgren sphere, ionization balance, effective temperature estimates. Three-level atom: electron temperature and density. Absorption lines, equivalent width, curve of growth, column densities. The interstellar medium. Atomic and ionic absorption lines, abundance of gas, molecules and dust. Hyperfine transitions: 21cm line of HI Interstellar extinction, dust, equilibrium and stochastic processes The sun. Ionization and sources of opacity, radiative transfer, the Gray atmosphere, limb darkening, absorption line formation.

5 Synopsis: High-Energy Astrophysics Supernova blast waves; shocks. Acceleration of particles to ultra-relativistic energies. Synchrotron emission; total power; spectrum; self-absorption; spectral ageing Accretion; properties of accretion discs Eddington luminosity; evidence for black holes. Relativistic jets; models of jet production; relativistic projection effects; Doppler boosting. Cosmic evolution of AGN; high-energy background radiation and cosmic accretion history of black holes. Bremsstrahlung; inverse-compton scattering; clusters of galaxies; Sunyaev-Zel dovich effect. Cosmic rays and very high energy gamma rays; Cherenkov telescopes.

6 Books Radiative Processes Radiative Processes in Astrophysics by Rybicki, George B., Lightman, Alan P., graduate level text, comprehensive. Physics and Chemistry of the Interstellar Medium by Sun Kwok. Physics of the Interstellar and Intergalactic Medium by Bruce Draine. High-Energy Astrophysics High Energy Astrophysics by Malcolm Longair. Very comprehensive, advanced undergrad/graduate level. The Physics of Extragalactic Radio Sources, David de Young Accretion Power in Astrophysics, Frank, King & Raine

7 Philosophy Many of the physical processes we will study in this course have exact mathematical descriptions which are either very lengthy, or can only be solved by numerical methods. My aim is to focus on showing you as many interesting advanced physics topics in the time available, and we will often make approximations justifiable approximations to get to a shorter mathematical description of the situation. You will learn a lot about how physics is actually done as a part of this process! But I will provide you with links to the relevant fully detailed research papers where possible.

8 Observing the whole EM spectrum (and beyond) Modern astronomical telescopes and detectors span the observational range from tens of metres in the radio to hundreds of TeV for the highest energy gamma rays. The classification of different astronomical wavebands is generally driven by the technology used in the detectors. Radio (from 10 MHz to 100 GHz) very highest spatial resolution because coherent detection of the EM field allows interferometry. Millimetre, sub-millimetre and far-infrared ( 0.3 mm to 10 µm). Bolometers onboard satellites and high-altitude terrestrial sites.

9 Infrared (10 µm to 1 µm) and optical (1 µm to 0.3 µm). Almost all of traditional astronomy. Most stars put out most of their energy in this range. Unsurprisingly the human eye is adapted to use these wavelengths! Ultraviolet (0.3 µm to 3 nm). Satellite-borne instruments are needed because the atmosphere is opaque now; but we can still use essentially ordinary telescopes. X-rays (3 nm to m; 0.4 kev to 100 kev). Satelliteand rocket-borne instruments are needed. Special grating-incidence mirrors are used to focus X-rays.

10 Gamma-rays ( 100 kev up to hundreds of GeV ). Again telescopes are satellite-borne. Use similar detectors to particle physics experiments. Very high-energy photons and particles entering the Earth s atmosphere produce Cherenkov radiation. This is detected by very large light bucket telescopes which don t need finely-figured mirrors. Beyond the EM spectrum: gravitational waves, neutrinos

11 W.M. Keck observatory, Mauna Kea, Hawai i.

12 Keck telescope 10-m segmented mirror.

13 European Southern Observatory, Cerro La Silla, Atacama Desert, Chile August 2011

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16 Average optical/uv spectra of quasars Note non-blackbody spectrum with prominent emission lines.

17 Very Large Array in New Mexico. Radio interferometer, 73 MHz 43 GHz. 27 antennae, baselines up to 36 km.

18 The sky as it would appear if we could see at 1.4 GHz. Almost all the sources lie far beyond the Milky Way. (Credit: NRAO / AUI / NSF)

19 Radio spectra of some sources from the 3C survey Frequency / MHz

20 ROSAT (Röntgensatellit) X-ray observatory

21 ROSAT all-sky survey. Note the fairly uniform background which is caused by unresolved extragalactic point sources

22 X-ray zoom-in from the high-resolution Chandra satellite. We can now see the individual X-ray sources.

23 X-ray spectrum of intracluster plasma in the Coma cluster of galaxies Again note non-blackbody and emission lines.

24 Fermi Gamma-Ray Space Telescope, launched June

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26 CTA - proposed array telescope for observing high-energy Cherenkov showers.

27 Primary!-ray ~ 10 km Particle Shower» Air-shower... ~ 100 m Cherenkov technique (figure from Prof J. Hinton, CTA collaboration).

28 Active galaxies: broad-band spectra

29 Observed spectrum of high-energy particles hitting the atmosphere.

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31 Gravitational waves

32 Surface brightness/specific intensity Traditionally in astronomy the observed flux of stars is given in magnitudes: m star = fluxvega 2.5 log 10 flux star mes fainter than Vega through a so, a star 100 times fainter than Vega through a V-band filter has V = 5. However this does not express the flux as a physical quantity. One of the most commonly-used physical units is the Jansky (Jy, named after the discoverer of radio waves from the Milky Way): 1 Jy = W Hz 1 m 2 -ban i.e. the units are power collected per square metre of telescope collecting area per unit bandwidth of the receiving device. As a yardstick, Vega has a flux density of about 3600 Jy at 500 nm. (For fun: The faintest stars the human eye can see at a dark site are about magnitude 6. How much power does the eye receive from them? How many photons does it take to trigger a light- sensitive cell in the eye? Take the integration time to be 20 ms and the starlight to be spread over 10 cells in the eye.)

33 In virtually all of optical astronomy stars are effectively point sources (their angular size is much smaller than the 1 arcsec limit imposed by atmospheric turbulence). However when we look at the diffuse interstellar medium, or at extragalactic sources, often we deal with objects whose angular extent can be resolved, and this leads us to a perhaps more fundamental quantity, the surface brightness or specific intensity. This is the flux density per solid angle, and has SI units of Power developed in the detector, per unit bandwidth of the detector, W Hz -1 m -2 Sr -1 per square metre of telescope collecting area, per unit solid angle of sky from which the telescope is collecting energy. This is also sometimes called simply brightness and the formal SI term is spectral radiance. Notation is conventionally Iν. You will often also see it in units such as Jansky per square arcsec, or Jansky per square arcminute.

34 Hand-held infrared thermometer measures temperature via σt 4. Thermocouples were the traditional devices used for this purpose, but they are unsuitable for continuous measurement because they rapidly dissolve.

35 Receiver bandwidth Delta nu Beam solid angle Omega Antenna area A Power received by system

36 Brightness and temperature Thought experiment: take two cavities of different size and shape, but both filled with black-body radiation at the same temperature T. Suppose there is an aper- ture between the cavities and that in this aperture there is a filter which transmits only through some narrow range of frequencies. T T We know from the Zeroth Law that there is no heat transfer between box 1 and box 2 so Iν,1 = Iν,2 i.e. for black-body radiation Iν can only be a function of T and ν. So brightness and temperature are intimately linked. The brightness of a black- body as a function of frequency is given by the Planck function. Filter

37 frequency is given by the Planck func B = 2h 3 1 h exp 1 c 2 k B T This you will be familiar with the B ν blackbody, as the Iν we just defined. here is the same thing, for a It gives the flux per unit frequency through an element of area from an element of solid angle. So if we know Iν for an astronomical object from observation, we can calculate the temperature of a blackbody that would have the same brightness at that frequency: the brightness temperature. Jargon alert: Pedantically, one should use Bν when referring to the brightness of a black-body and Iν when referring to the brightness of an object with an arbitrary spectrum. But don t be surprised if you sometimes see them interchanged. That s astronomers for you. I learned very early the difference between knowing the name of something and knowing something. - Feynman

38 Extremes of brightness Most stars radiate approximately as black-bodies, ranging from 3000 K for M- type red dwarfs to K for O-type supergiants. We will spend a lot of time in the next few weeks discussing the wealth of physics can can be derived from the emission and absorptions features where stellar spectra deviate form the black-body. But when we go to both long (radio) and short (X-ray) wavelengths, we find objects which have brightnesses far in excess of those achieved by blackbody processes in stars. Broadly speaking, when we move on to high-energy astrophysics in the second half of term, we will encompasses physical process and objects where the energies and temperatures involved are far in excess of those observed in normal stellar systems (often implying that the energy source is not nuclear fusion). And we often find cases where the temperatures and densities of the material involved lend themselves to spectral energy distributions that are greatly different from a black-body.

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