Introduction to Extra-galactic Radio Sources & Apparent Superluminal motion

Anupreeta MoreMy sincere thanks to Dr. Saikia (NCRA, Pune)

Contents

● Features of an Extra-galactic radio source● Fanaroff-Riley Classification● Apparent Superluminal motion & its explanation● Relativistic Beaming● Summary

Features of an Extragalactic Radio source

A) Core ~ mas B) Jets ~ pc-kpc

C) Hotspots ~ kpc D) Lobes – (lobe to lobe) ~ 100 kpc

A

C

B

D

Fanaroff-Riley ClassificationR = dist. between brightest regions

total extent of the sourceL(178 MHz) ~ 2x1025 W/Hz/rad2

Class FRI Class FRII

● jet dominated● turbulent, subsonic jets● weaker total radio power● associated with large cD

galaxies located in rich clusters

● hotspot & lobe dominated

● collimated, supersonic jets

● stronger total radio power

● associated with more isolated large galaxies

FRI FRII 3C272.1 3C47

3C465 3C83.1B 3C296 1.4 GHz 1.38 GHz 1.5GHz

Images of FRI sources

Images of FRII sources

VLBI maps of 3C273A second look

C

Observer

c t

v t v t cos

c t-v t cos

Explanation of apparent superluminal motion

After time t,

distance covered along the line of sight: v t cos ө transverse distance covered : v t sin ө delayed time as seen by the observer : t (1- cos ө )

Hence for the observer,the apparent transverse velocity is

vapp = v t sin ө / t (1- cos ө )

app = sin ө / (1- cos ө )

A) For a fixed value of ,

at = cos i.e. ~1/

app(max) =

Lorentz factor

> 0.707 app > 1

i.e apparent superluminal motion

B) For a fixed value of app ,

at cot-1app

min app / (1 + app2)1/2

minapp

2

As increases , increases as --> 1

max = 2 cot-1app

Relativistic Beaming

For an object moving relativistically at a small angle to the line of sight to the observer, we find the flux to be enhanced which is called Relativistic Beaming

For a spherically symmetric source with a power law spectrum, F() I() & F()

the observed flux is boosted by

Fobs() = D Frest()

where D = 1 / (1 – cos )

The ratio of observed flux of a relativistically

moving blob approaching at an angle to the

one receding ( is given by,

Fapp = (1 + cos )

Frec (1 – cos )

Fapp

Frec

Observer

Summary

1. FRI & FRII sources may be intrinsically different or have different host galaxy environments

2. Orientation effects and Relativistic Beaming - explain SL motion & one-sided jets respectively - help in building unified models