Download High-Energy Astrophysics Lecture 1: introduction and overview

High-Energy Astrophysics
Lecture 1: introduction and overview;
synchrotron radiation
Timetable
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Lectures:
Week 1: M 10, T 9
Robert Laing
Week 2: M 10, T 9, W 10
Week 3: M 10, T 9, W 10
Week 4: M 10, T 9, W 10
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Classes
Week 4
Week 6
Reading
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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.
Overview
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What is High-Energy Astrophysics?
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Further reading
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Radiation processes
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Other physical processes
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Observational techniques
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What objects do we study?
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.
What is high-energy astrophysics?
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Some useful numbers
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1 parsec (pc) ≈ 3 x 1016 m
Very hot (e.g. 108 K plasma in clusters of galaxies)
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1 solar mass ≈ 2 x 1030 kg
Non-thermal (e.g.. energy spectrum is a power law, not a
Maxwell-Boltzmann distribution)
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1 electron volt (eV) = 1.6 x 10-19 J
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1 arcsec = 1/3600 degree
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Powers of 10:
High-energy particles or plasma
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High-energy (and frequency) radiation (X, gamma rays)
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Enormous energy release
kilo
k 103
mega M 106
giga
G 109
tera
T 1012
peta
P 1015
exa
E 1018
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Radiation processes
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Physical processes
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Fermi acceleration at strong shocks - a mechanism which
can produce a power-law energy spectrum to very high
energies.
Inverse Compton - scattering of photons by high-energy
electrons
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Accretion - the infall of material onto a compact object,
releasing large amounts of gravitational potential energy.
Bremsstrahlung (free-free) - acceleration of electrons in
electrostatic fields of ions and nuclei.
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Strong gravitational fields - around collapsed objects such
as neutron stars and black holes.
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Special relativistic phenomena - in astrophysical jets with
flow speeds close to c.
Continuum radiation
Synchrotron - ultra-relativistic electrons spiralling in a
magnetic field,
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Line radiation
Photoionization
Recombination
Astronomical objects (1)
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Explosive events
Astronomical objects (2)
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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.
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).
Gamma-ray bursts - enormously energetic, brief pulses of
high-energy radiation.
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Hot plasma
Accretion is the fundamental power source.
106 - 108 K plasma
Relativistic jets are formed close to the nucleus and
propagate to vast distances
Found in massive galaxies and clusters of galaxies
Observational techniques (1)
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Supernova remnants - debris of a supernova explosion.
Active galaxies
Galaxies whose nuclei contain supermassive black holes.
Dead stars
All accessible wavelengths of the electromagnetic
spectrum provide useful information.
Radio (10 MHz - 30 GHz or 30m - 1cm) Highest
spatial resolution. Primarily interferometric
(resolution λ/D can be 0.1 micro-arcsec for VLBI).
Synchrotron radiation.
Observational techniques (2)
Ultra-violet (0.3 - 0.003 µm) Nuclear continuum from
active galaxies.
X-ray (3 x 10-9 - 3 x 10-12 m, 0.4 - 400 keV ) Hot
plasma in galaxy clusters, radiation from accretion
disks and inverse Compton scattering from jets.
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.
Gamma rays (3 x 10-12 - 10-19 m, 400 keV - 10 TeV ).
Inverse Compton scattering and pion decay in jets
and supernova remnants.
Near-infrared (10 - 1 µm) and optical (1 - 0.3 µm)
Stellar emission, gas at 104 K. Hubble Space
Telescope and large-aperture ground-based
telescopes.
Neutrinos
Gravitational waves
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Radio Observations
Cangaroo II
VLA: aperture synthesis
array.
27 x 25m antennas
74 MHz - 43 GHz
Resolution 0.25 arcsec
at 8.4 GHz
What is an AGN?
How do we find AGN?
1. A galaxy nucleus containing an accreting black hole.
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Optical colour
2. A galaxy with some of the following phenomena
associated with its nucleus:
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Optical emission lines
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Infra-red emission
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X-rays
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γ-rays
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Radio emission
(e) Variability.
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...........
(f) High linear polarization.
All of these techniques have their own selection
effects, almost always redshift-dependent.
(a) Very small angular size/high surface brightness.
(b) Galactic or higher luminosity.
(c) Broad-band continuum.
(d) Strong emission lines.
(g) Bright and/or extended radio emission.
Classification of AGN
Accretion in Active Galaxies
Three key parameters:
1. Luminosity from the accretion process (-> thermal
continuum, photoionization, emission lines from IR to Xray).
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).
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Seyfert Galaxy
Quasars
Bright stellar nucleus
Strong emission lines
Usually (but not always)
spiral hosts.
X-ray emission
Circinus spiral (Seyfert 2)
Bright, quasi-stellar
nucleus; strong
emission lines; usually
with broad permitted
lines.
Line spectra
Broad-line region
Ionization cone
Obscuration and unified models
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2
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The Eddington limit
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Eddington limit Central source radiates, therefore
exerting an outward force on the accreting gas.
Assuming Thomson opacity only, this sets a maximum
luminosity LEdd for the central source, above which
radiation overpowers gravity:
Accretion: key points
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Fundamental power source from gravitational potential
energy.
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Infalling material has angular momentum, so forms a
disk.
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Continuum radiation from the accretion disk is thermal,
(range of T) and peaks in the ultra-violet.
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Broad-line region (1 pc)
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Obscuring torus absorbs nuclear radiation and re-emits
in far-infrared,
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Narrow-line region (- 20 kpc)
LEdd = 4πGMµe/σT = 1.51 x 1031 (M/Msun) W
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Eddington accretion rate Given an efficiency η, the
accretion rate for Eddington luminosity is
Ledd / c2 η = 3M8(η/0.1)-1 Msun / year
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Implied black hole mass for Eddington luminosity:
AGN:
1036
-
1040
W =>
105
-
108
Msun
Jets in Active Galaxies
Weak radio galaxy
Tail
Jets
Tail
3C 31 (VLA 1.4GHz; 5.5 arcsec FWHM)
Powerful radio galaxy
A powerful radio-loud quasar
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VLBI observations of M 87
VLA
Collimation scale
<100 RS
Optical
VLBA
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
Blazars
Jets: key points
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Collimated on very small scales (<50 RS)
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Very low-density plasma
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Ultrarelativistic (γ > 105) electrons and perhaps
positrons emitting synchrotron radiation
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Jet points almost directly at us: Doppler beaming gives
very bright, highly variable, broadband, polarized
continuum.
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BL Lac objects have emission lines which are very
weak compared with the continuum (often intrinsically
weak).
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Optically violently variable (OVV) quasars have
typical quasar emission-line spectra.
Variability
Disk
Disk
Mildly relativistic (Γ ∼ 10) flow on small scales
Can propagate to large distances from the galaxy (up to
2 Mpc)
Jet
Disk
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Black holes in active galaxies
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If an object becomes sufficiently compact and massive,
light cannot escape from it. This is a black hole.
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The Schwarzschild radius RS = 2GM/c2
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Water masers in NGC 4258: 3.6 x 107 solar masses
within 0.1 pc (VLBA).
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Resolved gas kinematics e.g. M87: 2.4 x 10 9 solar
masses within 18 pc (HST).
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Individual stellar velocities Milky Way (3 x 10 6 solar
masses within 0.01 pc)
Water masers in NGC 4258
Gas kinematics in M87
Image of predicted Fe line emission from
around a black hole
Stellar explosions
A recent supernova - 1987A
After
Before
Remnant (X-rays)
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Gamma-ray bursts
Brief pulses of gamma rays
Gamma-ray burst location and physics
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)
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Accretion onto compact stars
Jets from stellarmass black holes
in accreting binary
systems micro-quasars
X-ray emission from hot gas
X-ray
Cosmic-ray energy spectrum
Optical
Continuum radiation processes
Synchrotron radiation
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Extended radio emission from active and normal
galaxies (including our own), supernova remnants.
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Optical and X-ray emission from jets and pulsardriven supernova remnants.
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What is the emission mechanism?
Broad-band (smooth spectrum, no lines)
Roughly power-law spectrum S(ν) ∝ ν-α
High linear polarization (up to 70%)
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Synchrotron radiation generated by high-energy
(relativistic) electrons spiralling in a weak magnetic
field.
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Radiation from an accelerated charge - 1
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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/Er = ∆v t sinθ/c ∆t (θ w.r.t. acceleration)
Radiation from an accelerated charge - 2
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Dipole moment => pulse of EM radiation is
produced. Energy flow/area/time given by Poynting
flux |E x H | = E2/(µ0ε0)1/2
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Energy loss rate per unit solid angle:
-(dE/dt) dΩ = (q2a2sin2θ/16π2ε0c3) dΩ
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Integrate over solid angle:
-(dE/dt) = q2a2/6πε0c3
This is Larmor’s formula
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Rigorous treatment (Longair, vol 1, p. 66) gives
the same result
E θ = qa sinθ/4πε0c2r (Coulomb; a = acceleration)
Radiation from an accelerated charge - 3
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Polar diagram is a dipole
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Radiation is polarized with E along projected
acceleration vector
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