Radiative Processes Flashcards
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.
AGN
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.
Bremsstrahlung
“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
emission?
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
