High-Energy Astrophysics Lecture 1: introduction and overview; synchrotron radiation. Timetable. Reading. Overview. What is high-energy astrophysics?

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1 High-Energy Astrophysics Lecture 1: introduction and overview; synchrotron radiation Robert Laing Lectures: Week 1: M 10, T 9 Timetable Week 2: M 10, T 9, W 10 Week 3: M 10, T 9, W 10 Week 4: M 10, T 9, W 10 Classes Week 4 Week 6 Reading Essential High Energy Astrophysics, M. Longair, CUP; Volume 2 (Stars, the Galaxy and the interstellar medium). Good on physical processes; observational material now dated. Recommended High Energy Astrophysics, Longair, Volume 1. Useful background. Active Galactic Nuclei, J. Krolik, Princeton UP. Up-todate, advanced, especially good on theory. CGS. Active Galactic Nuclei, I. Robson, Wiley-Praxis. Out-ofprint, but should be in some college libraries. Accretion Power in Astrophysics, Frank, King & Raine, CUP. Advanced, very good on accretion in stars. Overview What is High-Energy Astrophysics? Further reading Radiation processes Other physical processes Observational techniques What objects do we study? What is high-energy astrophysics? Some useful numbers High-energy particles or plasma Very hot (e.g K plasma in clusters of galaxies) Non-thermal (e.g.. energy spectrum is a power law, not a Maxwell-Boltmann distribution) High-energy (and frequency) radiation (X, gamma rays) Enormous energy release 1 parsec (pc) 3 x m 1 solar mass 2 x kg 1 electron volt (ev) = 1.6 x J 1 arcsec = 1/3600 degree Powers of 10: kilo k 10 3 mega M 10 6 giga G 10 9 tera T peta P exa E

2 Radiation processes Continuum radiation Synchrotron - ultra-relativistic electrons spiralling in a magnetic field, Inverse Compton - scattering of photons by high-energy electrons Bremsstrahlung (free-free) - acceleration of electrons in electrostatic fields of ions and nuclei. Line radiation Photoioniation Recombination Physical processes Fermi acceleration at strong shocks - a mechanism which can produce a power-law energy spectrum to very high energies. Accretion - the infall of material onto a compact object, releasing large amounts of gravitational potential energy. Strong gravitational fields - around collapsed objects such as neutron stars and black holes. Special relativistic phenomena - in astrophysical jets with flow speeds close to c. Astronomical objects (1) Astronomical objects (2) Explosive events Dead stars Supernova - collapse of a star at the end of its life, leaving a remnant (white dwarf, neutron star or black hole) with the explosive release of gravitational energy. Gamma-ray bursts - enormously energetic, brief pulses of high-energy radiation. Active galaxies Galaxies whose nuclei contain supermassive black holes. Accretion is the fundamental power source. Relativistic jets are formed close to the nucleus and propagate to vast distances Pulsars - isolated, rotating neutron stars, which produce regular, pulsed radio emission. Accreting binary stars - In which matter from a normal star accretes onto a compact object (white dwarf, neutron star or black hole). Supernova remnants - debris of a supernova explosion. Hot plasma K plasma Found in massive galaxies and clusters of galaxies Observational techniques (1) Observational techniques (2) All accessible wavelengths of the electromagnetic spectrum provide useful information. Radio (10 MH - 30 GH or 30m - 1cm) Highest spatial resolution. Primarily interferometric (resolution λ/d can be 0.1 micro-arcsec for VLBI). Synchrotron radiation. Millimetre (1cm - 0.1mm) and far-infra-red (0.1 mm - 10 µm) emission from cold dust. Satellites and bolometer arrays on ground-based telescopes; interferometry. Near-infrared (10-1 µm) and optical (1-0.3 µm) Stellar emission, gas at 10 4 K. Hubble Space Telescope and large-aperture ground-based telescopes. Ultra-violet ( µm) Nuclear continuum from active galaxies. X-ray (3 x x m, kev ) Hot plasma in galaxy clusters, radiation from accretion disks and inverse Compton scattering from jets. Gamma rays (3 x m, 400 kev - 10 TeV ). Inverse Compton scattering and pion decay in jets and supernova remnants. Neutrinos Gravitational waves 2

3 Radio Observations Cangaroo II VLA: aperture synthesis array. 27 x 25m antennas 74 MH - 43 GH Resolution 0.25 arcsec at 8.4 GH What is an AGN? 1. A galaxy nucleus containing an accreting black hole. 2. A galaxy with some of the following phenomena associated with its nucleus: (a) Very small angular sie/high surface brightness. (b) Galactic or higher luminosity. (c) Broad-band continuum. (d) Strong emission lines. (e) Variability. (f) High linear polariation. (g) Bright and/or extended radio emission. How do we find AGN? Optical colour Optical emission lines Infra-red emission X-rays γ-rays Radio emission... All of these techniques have their own selection effects, almost always redshift-dependent. Classification of AGN Accretion in Active Galaxies Three key parameters: 1. Luminosity from the accretion process (-> thermal continuum, photoioniation, emission lines from IR to X- ray). 2. Luminosity of the jet (non-thermal emission, particularly radio but also at high energies). 3. Orientation (obscuration and beaming). The relative amount of energy radiated by jet and accretion-related processes varies by orders of magnitude (radio-loud and radio-quiet). 3

4 Seyfert Galaxy Quasars Circinus spiral (Seyfert 2) Bright stellar nucleus Strong emission lines Usually (but not always) spiral hosts. X-ray emission Bright, quasi-stellar nucleus; strong emission lines; usually with broad permitted lines. Line spectra Broad-line region Ioniation cone Obscuration and unified models 1 2 4

5 The Eddington limit Eddington limit Central source radiates, therefore exerting an outward force on the accreting gas. Assuming Thomson opacity only, this sets a maximum luminosity L Edd for the central source, above which radiation overpowers gravity: L Edd = 4πGMµ e /σ T = 1.51 x (M/M sun ) W Eddington accretion rate Given an efficiency η, the accretion rate for Eddington luminosity is L edd / c 2 η = 3M 8 (η/0.1) -1 M sun / year Implied black hole mass for Eddington luminosity: Accretion: key points Fundamental power source from gravitational potential energy. Infalling material has angular momentum, so forms a disk. Continuum radiation from the accretion disk is thermal, (range of T) and peaks in the ultra-violet. Broad-line region (1 pc) Obscuring torus absorbs nuclear radiation and re-emits in far-infrared, Narrow-line region (- 20 kpc) AGN: W => M sun Jets in Active Galaxies Weak radio galaxy Tail Jets Tail 3C 31 (VLA 1.4GH; 5.5 arcsec FWHM) Powerful radio galaxy A powerful radio-loud quasar 5

6 VLBI observations of M 87 VLA Collimation scale <100 R S VLBA Optical X-ray Radio The jet in 3C273, probably a mixture of synchrotron and inverse Compton emission. End-on jets: BL Lac objects and OVV quasars Blaars Jet points almost directly at us: Doppler beaming gives very bright, highly variable, broadband, polaried continuum. BL Lac objects have emission lines which are very weak compared with the continuum (often intrinsically weak). Optically violently variable (OVV) quasars have typical quasar emission-line spectra. Jets: key points Variability Collimated on very small scales (<50 R S ) Very low-density plasma Ultrarelativistic (γ > 10 5 ) electrons and perhaps positrons emitting synchrotron radiation Mildly relativistic (Γ 10) flow on small scales Can propagate to large distances from the galaxy (up to 2 Mpc) Disk Disk Jet Disk 6

7 Black holes in active galaxies Water masers in NGC 4258 If an object becomes sufficiently compact and massive, light cannot escape from it. This is a black hole. The Schwarschild radius R S = 2GM/c 2 Water masers in NGC 4258: 3.6 x 10 7 solar masses within 0.1 pc (VLBA). Resolved gas kinematics e.g. M87: 2.4 x 10 9 solar masses within 18 pc (HST). Individual stellar velocities Milky Way (3 x 10 6 solar masses within 0.01 pc) Gas kinematics in M87 Image of predicted Fe line emission from around a black hole Stellar explosions A recent supernova A After Before Remnant (X-rays) 7

8 Gamma-ray bursts Gamma-ray burst location and physics Brief pulses of gamma rays Energy spectrum GRB s occur in distant galaxies A possible formation mechanism - hypernovae Dead stars and their remnants A shell supernova remnant - Cas A A pulsar-driven supernova remnant - the Crab Nebula Crab Nebula - around the pulsar Radio Near infra-red Optical Optical (Hubble) X-ray (Chandra) 8

9 Accretion onto compact stars Jets from stellarmass black holes in accreting binary systems - micro-quasars X-ray emission from hot gas Cosmic-ray energy spectrum X-ray Optical Continuum radiation processes Synchrotron radiation Extended radio emission from active and normal galaxies (including our own), supernova remnants. Optical and X-ray emission from jets and pulsardriven supernova remnants. What is the emission mechanism? Broad-band (smooth spectrum, no lines) Roughly power-law spectrum S(ν) ν -α High linear polariation (up to 70%) Synchrotron radiation generated by high-energy (relativistic) electrons spiralling in a weak magnetic field. 9

10 Radiation from an accelerated charge - 1 Heuristic derivation (due to J.J. Thomson; see Longair vol 1, p. 62) Charge q stationary at origin O of inertial frame; then small acceleration v in time t. Think about field lines attached to charge. Inside a sphere of radius ct, field lines are radial and centred on new position of charge. Outside it, they are radial and centred on O. Hence the field lines must kink in a shell of thickness c t => circumferential field component. Geometry: E θ /E r = v t sinθ/c t (θ w.r.t. acceleration) E θ = qa sinθ/4πε 0 c 2 r (Coulomb; a = acceleration) Radiation from an accelerated charge - 2 Dipole moment => pulse of EM radiation is produced. Energy flow/area/time given by Poynting flux E x H = E 2 /(µ 0 ε 0 ) 1/2 Energy loss rate per unit solid angle: -(de/dt) dω = (q 2 a 2 sin 2 θ/16π 2 ε 0 c 3 ) dω Integrate over solid angle: -(de/dt) = q 2 a 2 /6πε 0 c 3 This is Larmor s formula Rigorous treatment (Longair, vol 1, p. 66) gives the same result Radiation from an accelerated charge - 3 Polar diagram is a dipole Radiation is polaried with E along projected acceleration vector 10

Active Galactic Nuclei

Active Galactic Nuclei Optical spectra, distance, line width Varieties of AGN and unified scheme Variability and lifetime Black hole mass and growth Geometry: disk, BLR, NLR Reverberation mapping Jets