Radiative Processes

Question 1

Cosmic rays

Answer 1

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

Question 2

Gamma ray bursts

Answer 2

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.

Question 3

AGN

Answer 3

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

Question 4

Observational properties of cosmic rays

Answer 4

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.

Question 5

Greisen-Zatsepin-Kuzmin (GZK) limit

Answer 5

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.

Question 6

First order fermi acceleration

Answer 6

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.

Question 7

Supernova Remnants

Answer 7

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

Question 8

Type Ia

Answer 8

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.

Question 9

Type Ib and Ic SN

Answer 9

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

Question 10

Type II SN

Answer 10

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.

Question 11

Stages of type II SN

Answer 11

Piston phase
Sedov-Taylor Phase
Snowplough Phase
Subsonic Phase

Question 12

AGN types

Answer 12

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.

Question 13

AGN unification

Answer 13

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.

Question 14

Black hole masses - estimating

Answer 14

Stellar motion
Stellar Dynamics
Reverberation Mapping
Line widths

Question 15

Magorrian relation

Answer 15

Mass of BH scales with mass of host galaxy

Question 16

Cosmological context of AGN

Answer 16

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.

Question 17

Superluminal motion

Answer 17

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

Question 18

What sets upper limit to AGN luminosity?

Answer 18

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

Question 19

Insterstellar medium

Answer 19

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

Question 20

Flux & EM energy through dA & Energy density

Answer 20

F = L/4 pi R^2

dE = F dA dt

u_v = (4 pi / c ) J_v

Question 21

Larmor eq

Answer 21

Holds for non-relativistic velocities

P = (q^2/6 pi epsilon 0 c^3) . |r^2|

Question 22

Compton Processes

Answer 22

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

Question 23

Compton balance

Answer 23

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.

Question 24

Synchotron emission

Answer 24

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.

Question 25

Synchotron self-compton

Answer 25

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

Question 26

Synchotron self-absorption

Answer 26

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.

Question 27

Bremsstrahlung

Answer 27

"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

Question 28

Gas in cores of clusters of galaxies

Answer 28

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.

Question 29

What process dominates at highest energy emission?

Answer 29

Inverse compton processes

Question 30

Evidence for relativistic motion of AGN cores

Answer 30

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

Question 31

Sungaev-Zel'dovic effect

Answer 31

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

Question 32

Polarisation alpha

Answer 32

``` 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

Question 33

Compton scattering v inverse

Answer 33

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

Question 34

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

Answer 34

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

Question 35

Where and how are CRs accelerated in our galaxy

Answer 35

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

Question 36

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

Answer 36

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

Question 37

Which process dominates the highest energy | emission?

Answer 37

Inverse compton processes

Question 38

Radio sources Faranoff-Riley

Answer 38

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.

Question 39

2 main sources of emission in relativistic jet

Answer 39

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

Question 40

Difficulty in locating GRBs

Answer 40

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