Chapter 6

Question 1

What is Doppler Boosting?

Answer 1

If a material is moving at relativistic speeds, radiation from a source within the jet will be beamed in the forward direction of the jet with an observed increase in intensity

Question 2

With which images are knotty jets seen the clearest?

Answer 2

Under radio and infrared images

Radio monitoring of the “knots” in the jets show they are moving

Question 3

What is magnetic field in a galactic plane parallel and perpendicular to?

Answer 3

Parallel to jet close to nucleus

Perpendicular to jet further away

Question 4

What does a boost in intensity of radio emission depend on?

Answer 4

Spectral index

Question 5

Why do some components of a source appear to separate at velocity?

Answer 5

The particle beam is aimed almost directly at the Earth

Question 6

How can large fluxes of relativistic plasma
and magnetic fields survive so far from the
galactic nucleus?

Answer 6

The radio spectrum is much flatter in the “head” of a lobe than in the “tail” suggesting the head contains particles energised more recently
The plasma is being replenished

Question 7

What is the nucleus a source of?

Answer 7

Double beam

Question 8

What are radio galaxies?

Answer 8

Giant elliptical galaxies with strong radio emission

Question 9

Where does the emission from a radio galaxy come from and stretch to?

Answer 9

From nucleus and often from pair of symmetric lobes that extend for several kpc on both sides of nucleus

Question 10

What features do quasars have?

Answer 10

Host galaxy barely detectable

Nearly featureless spectrum from radio to hard X-ray

Variable emission across whole spectrum

Question 11

What are the two types of quasars?

Answer 11

Radio-loud quasars (strong radio emission)

Radio-quiet quasars (weak radio emission)

Question 12

What do Active Galaxies have?

Answer 12

Nuclei exhibiting quasar-like phenomena: point like sources of non-thermal radiation varying in intensity on timescales from years to days

Question 13

What does the correlation between the mass of a black hole and the optical luminosity of the bulge of its host galaxy?

Answer 13

It suggests a link between galaxy formation and black hole formation

Question 14

What is the sphere of influence?

Answer 14

The region where gravitational potential of the black hole is larger than kinetic energy of surrounding stars

(but angular momentum conservation means BH growth takes long time and gas falls in instead)

Question 15

How can a black hole’s mass be determined?

Answer 15

From motion of surrounding matter (the dynamics of stars governed by gravity or of gas (influenced by pressure too)

Question 16

What is relaxation time?

Answer 16

Time required for gravitational interactions to alter orbits

Question 17

What relaxation time do most galaxies have?

Answer 17

Greater than the age of the universe (this time must elapse before a group of stars collapses into the BH

Question 18

What causes in falling gas that powers AGNs?

Answer 18

Galaxy mergers

Question 19

What does more accretion lead to?

Answer 19

A higher accretion luminosity which in turn leads to a larger radiation pressure acting on the in falling material

Question 20

What happens when a luminosity exceeds a certain value (Eddington Luminosity) ?

Answer 20

The outward radiation force becomes larger than the force of gravity and the accretion is halted
Eddington luminosity is the upper limit beyond which radiation pressure would blow away infalling matter

Question 21

What is the rate of increase of BH mass limited by>

Answer 21

The radiation pressure

Question 22

What is Big Blue Bump?

Answer 22

Thermal emission from the disc peaks in the U.V.

Question 23

What has to happen fro matter to be accreted onto the BH?

Answer 23

Angular momentum has to be transported out through the disc

Question 24

What is the big blue bump evidence for?

Answer 24

For the existence of a hot disc, bright in the U.V., superimposed on a power law
spectrum from the central object.

Question 25

What happens in the halo?

Answer 25

UV photons inverse compton scatter to x-rays in halo

X-ray hitting accretion disk can be scattered back but also absorbed

Question 26

What causes line broadening to be asymmetric?

Answer 26

Gravitational redshift: emission lines from the inner edge of the disc near the last stable orbit around the BH appear at
lower energies than those from the outer edge of the disc

Question 27

What does the iron line profile allow?

Answer 27

To measure the BH spin

Question 28

What are blazars?

Answer 28

AGN characterised by high optical polarisation (spectral lines are swamped by synchrotron radiation)

Question 29

What do blazars exhibit?

Answer 29

They are radio loud
Exhibit superluminal motion
They vary in brightness at all frequencies and are labelled as “optically violent variables”

Question 30

What can boost chances of seeing gamma rays?

Answer 30

Relativistic effects in a jet can boost our chances of seeing gamma rays as radiation from a region (blob) traveling towards us at v = b cos(q )c appears exceptionally bright

Question 31

What is the effect of doppler boosting?

Answer 31

Emitting region almost keeps up with the photons it emits. The
observed luminosity is greater than the intrinsic luminosity

Question 32

What is used to power the jet?

Answer 32

The accretion energy from a black hole

Question 33

What does the optical spectrum of quasars show?

Answer 33

Emission lines of Bulmer sequence of hydrogen where electrons are recombining with proton and emitting a photon at every point they drop down an energy level