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Flashcards in Radiative Processes Deck (40):

Cosmic rays

High energy particles (protons neuctrons nuclei up to Z > 60) that pervade universe and carry relativistic energies. Observatories consist of water tanks with photomultiplier tubes in that record correlated bursts of life as EAS occurs (extensive air showers)
Considered separate from highest energy gamma rays


Gamma ray bursts

Bursts of gamma rays, uniformly distributed in sky
Longer duration bursts are >2s and related to collapse of massive stars that have evolved rapidly and expelled a lot of their outer atmosphere.
Shorter class are much rarer, emit much more energetic gamma rays and have much less luninous after glows, blieved to be from neutron star-neutron star mergers
GRBs also appear more at redshifts 1.5-3 where star formation is most intense.
Dominated by synchotron and compton radiation process in initial stages.



Massive galaxies that exhibit strong, non-thermal emission from a small region at their centre. Often highly variable, polarised, bright over most of the EM spectrum. Require presence of supermassive (>10^6 M_sun) black hole


Observational properties of cosmic rays

Isotropic on sky at E < 10^19.5 eV
Power law energy distribution
Distribution of nuclei consistent with solar abundances with excpetion of He, N, Ar, Ne which are deficient.


Greisen-Zatsepin-Kuzmin (GZK) limit

CMB provides cross-section for interactions with UHECR to create resoncance that decays to pi pion and proton or neutron. In local universe, expected path length is 500-100 Mpc.


First order fermi acceleration

Exact method of CR acceleration not fully understood but most probably mechanism is FOFA. Works by bounding particles across strong, relativistic shock. Most likely site of acceleration of CRs is strong shock formed in early stages of supernova explosion. Outside of galaxy most likely sources of acceleration are in AGN, radio galaxies and clusters of galaxies.


Supernova Remnants

Two classes of supernovae, Type I and Type II - type I don't exhibit H emission lines, type II do but there are sub-classes: Ia, Ib and Ic and then type II


Type Ia

Believed to occur in binary system when mass is transferred onto C-O white dwarf until it reaches Chandrasekhar limit and begins uncontrolled fusion of C and O. Predictable peak brightness.


Type Ib and Ic SN

Believed to occur when massive star reaches end of its nuclear fuel supply and implodes.


Type II SN

Believed to be related to death of massive star but unlike Ib or Ic, explode before ejecting outer atmosphere, so retain H in ejecta. In initial rapidly expanding shock CR acceleration is believed to be occuring.


Stages of type II SN

Piston phase
Sedov-Taylor Phase
Snowplough Phase
Subsonic Phase


AGN types

Seyfert Type 1 - broad permitted lines (mainly H balmer), narrow forbidden lines and strong coninuum that increases to shorter wavelengths.
Seyfert type 2 - narrow permitted lines, narrow forbidden lines but weak continuum emission

There are also subclasses, and Seyferts are less than 1% of galaxies.


AGN unification

Want to classify the objects by matching them to common geometry - Black hole with large, thick torus and accretion disk around it that is view at different angles.


Black hole masses - estimating

Stellar motion
Stellar Dynamics
Reverberation Mapping
Line widths


Magorrian relation

Mass of BH scales with mass of host galaxy


Cosmological context of AGN

Oerall level of AGN activity traced to Universe being 1GYr old and steep decline since - peak in activity coincides with peak in star formation activity in galaxies and in GRB events.


Superluminal motion

Some objects appear to move faster than c but this is because of the angle it travels at.
Sizes also appear smaller


What sets upper limit to AGN luminosity?

Eddington limit - limit to what can power SMBH is amount of material that can reach it without being driven away by radiation pressure.


Insterstellar medium

Stuff between stars - consists of gas (ionised and not), dust, radiation and magnetic fields and cosmic rays.


Flux & EM energy through dA & Energy density

F = L/4 pi R^2

dE = F dA dt

u_v = (4 pi / c ) J_v


Larmor eq

Holds for non-relativistic velocities
P = (q^2/6 pi epsilon 0 c^3) . |r^2|


Compton Processes

Relativistic interction of a photon and charged particle lies at hart of complexity of high energy astrophysics. Use four-momentum


Compton balance

Balance of energy gain/loss for photons in relatvistic plasma.
At extreme case where photons are more energetic than electrons, electrons always gain energy. For lower energy photons in more energyetic plasma all interactions need to be considered from rest frame.


Synchotron emission

Helical motion of e or ion in B field leads to accelertion of charge and therefore EM radiation - has radius and therefore characteristic feq. or gyrofrequency
v_g = 1/t = qB/2 pi m
This cyclotron emision is polarised but strongly dependent on viewing angle.


Synchotron self-compton

Once synchotron photons are emitted, they are free immediately to interact with relativistic electrons through inverse compton scattering to higher energies. Believed to power most of the emission from relativistically beamed AGN just as BLLacs and blazars


Synchotron self-absorption

In most compact AGN, temps exceed 10^12 K . Lowest freq. radio spectrum can be severly truncated as emission falls within Rayleigh-Jeans part of BB emission curve for the source. Synchotron self-absorption is truncation.



"Braking radiation" - movement of relativistic electrons past positively charged ions results in brief but significant acceleration of electron and hence radiation of a photon.
The total X-ray luminosity of a system scales as L ∝ T^0.5 ρ^2


Gas in cores of clusters of galaxies

Density squared nature of Bremsstrahlung means as density increases emissivity increases. Increased X-ray emission can radiate all thermal energy of gas in core on timescale much shorter than age of system. Radiative cooling of gas lowers temp but as it is in hydrostatic eq. density must incrase to compensate - slow infall of gas. Cores of many clusters show these cooling flows.


What process dominates at highest energy emission?

Inverse compton processes


Evidence for relativistic motion of AGN cores

Superluminal motion of jets
Broad emission lines
Time variability shorter than light crossing time
Very bright emission temperatures
Relatvisitically broadened X-ray lines


Sungaev-Zel'dovic effect

CMB passing through clusters of galaxies for example is up scattered to higher energies by energetic electrons. inverse compton scattering. Higher electron density and T increase this


Polarisation alpha

Alpha = 0 unpolarised
Alpha = pi/2 maximum polarisation

Star observed through dust will be not polarised when rays come directly to viewer, maximum polarisation when it changes to right angle


Compton scattering v inverse

Covers case where photons change energy after interaction with particle. If photon loses energy:Compton scattering, gains energy: inverse CS


Example of mass estimate of BH for passive and active and +vs/-vs for each

Passive: stellar motion, very accurate but very time consuming and only for one object
Active: reverberation mapping, accurate but time consuming and requires modelling


Where and how are CRs accelerated in our galaxy

Cosmic rays are accelerated in the mildly relativistic shocks generated in supernovae
in their initial expansion phase. The process is first-order Fermi acceleration.


What is source of radio emission in normal and active galaxies?

In normal, supernovae and shocks
In active, accretion onto SMBH


Which process dominates the highest energy

Inverse compton processes


Radio sources Faranoff-Riley

FRI - lobes fade gradually with distance from nucleus, frequently have 2 jets nad one of low radio power
FRII - most luminous, brigtest emission at ends of lobes. Often only have one visibly jet, high power. Dominate mor distant samples.


2 main sources of emission in relativistic jet

Synchotron emission - radio/IR/optical - depends on on no. electrons
Inverse compton scattering - X-ray/gamma ray - depends on no. of electrons and seed photons


Difficulty in locating GRBs

Transient events
Need rapid targetting
Most successful method to measure position
Observe with ground based telescopes when opticall bright
Rapid fading
Many telescopes tend to react in few hours to alert
Communication of information critical