AGN jets

1
AGN jets

• G. Henri
• Laboratoire d Astrophysique de Grenoble, France

2
Outline

• Phenomenology
• AGN zoology
• Radio jets
• Superluminal motion, Relativistic beaming
• Unification
• Other wavelengthes
• Emission processes
• Hadronic vs leptonic models
• Synchrotron emission
• Inverse Compton emission
• g-ray absorption
• Physical parameters
• Jet generation
• Blandford-Znajek process (rotating BH)
• Blandford-Payne (accretion disk)
• 2-flow models
• Particle acceleration

3
Astrophysical Jets an ubiquitous phenomenon

• Jets appear almost universally in association
  with accretion

Active Galactic Nuclei
Young Stellar Objects
X-ray binaries
Probably also GRBs (see Waxman lectures)
4
Active Galactic Nuclei

• AGNs 10 galaxies
• Strong activity from point-like source at the
  center of the galaxy
• Luminous UV continuum
• Optical emission lines
• Intense X-ray emission
• In 10 of AGNs intense Radio emission
  (radio-louds), Strongly variable, polarized -gt
  Non thermal process
• Radio louds often seen in gamma-rays -gt TeV
• Usually in giant elliptical galaxies (but less
  true _at_ high z)
• 90 classified as  radio quiets 

5
Central Engine

• Current paradigm AGNs are powered by Super
  Massive Black Holes (SMBH) accreting surrounding
  interstellar matter .
•  Fiducial  numbers
• Mass M 107 - 10 10 Msol, take M8 M/108 Msol
• Radius (Schwarzschild ) Rg 2GM/c2 3 1013 M8 cm
• (2 u.a., or 2 .10-4 pc)
• Light crossing time tg Rg/c 1000 M8 s (20 min)

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Luminosity

• Eddington luminosity
• Balance gravitational attraction on protons
• With radiation pressure on electrons
• gives maximal Eddington luminosity
  erg.s-1
• Equiv. Black Body temperature Teq 5.105 M8-1/4
  K (not much larger than a O star)
• Equipart. Magn Field Beq 4.104 M8-1/2 G (
  solar spot)

7
What do we see exactly?

• Jets are observed with various techniques/waveleng
  htes
• Historically radio observations
• Single dish, low angular resolution, no imaging
  relativistic jets inferred from rapid variability
  (Rees, 1966)
• Direct imaging by Interferometry (VLA, VLBI)
• Optical-IR observations HST, Spitzer..
• X-ray imaging CHANDRA
• g-ray indirect evidence (CGRO, Atmospheric
  Cerenkov Telescopes)

8
Quasars
Large radio surveys -gt discovery in the 50s of
strong, stellar-like (point) radio-sources (3rd
Cambridge Catalogue 3Cxxx.) QUAsi StellAr
Radiosource QUASAR Unknown emission lines until
M. Schmidt realized they were highly redshifted,
indicating cosmological distances (and enormous
luminosities 1046 erg/s)
QUASAR are now thought to be most luminous cores
of active galaxies. Optical surveys yielded lot
of radio-quiets counterparts QSO
9
Radio spectra

• Most often, radio spectral distribution close to
  a power law
• Fn dF/dn F0 n-a
• a spectral index
• usually between 0,5 and 1 (steep spectra)
• But can be also lt 0,5 (flat spectra)

10
Radio imaging

• Most of jets observations come from radio
  interferometry

Angular resolution dq l/D
l from m downto mm D extend up to a few 103 km
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Radio images
VLA angular resolution 1 arcsec ( optical
telescopes) Correspond to linear resolution d
kpc (normal galaxy 30 kpc) Reveal huge
collimated structures from radiogalaxies,
extending up to Mpc
2 different morphologies (Fanaroff-Riley)
lobes
jet
Hot spot
FR I
FR II
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FRI vs FR II
FR I
Less powerful (Lrad lt 1042 erg.s-1) Core-dominated
, gradually fading Weakly collimated Spectral
index a grows with distance
(core acceleration)
FR II
lobes
More powerful (Lrad gt 1042 erg.s-1) Edge-dominated
, hot spots Strongly collimated, weakly
dissipative Spectral index a diminishes with
distance
jet
Hot spot
(acceleration in terminal shocks)
13
VLBI observations

• VLBI, VLBA baseline 103 km, angular
  resolution lt 1 mas ( optical interferometry)
• Correspond to linear resolution d lt pc
• Possible to see direct motion in timescales
  years

For most quasars apparent superluminal
velocities V 5 to 10 c
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Superluminal motions

• No magics, predicted by M. Rees in 1966.
• Due to the variation of the light travel time for
  a moving blob

q
bmax Gb
q
15
Doppler amplification
Relativistic motion amplifies also frequencies
and intensities
Doppler factor
Frequency
Time scales
Specific intensity
For power law spectra
thus
-gt jets seen at small angles will be much
brighter and variable
16
Beaming cone
and
Use
to show
Hence
for
G10
d, bapp
-gt jets seen at small angles will be much
brighter and variable
q (deg)
Explain also VLBI jets one-sidedness
usually gt 106 !
F(jet)/F(counterjet) G 62a
17
Lorentz factors distribution
In principle, detection of relative velocity AND
brightness contrast (if measurable), can
determine b and q, and if jets are symmetric.
More practically, bapp sets a .lower limit to Gb
. Statistical studies necessary
Vermeulen Cohen 1995 compatible with G10
Depends weakly on cosmological parameters
Some rare objects with G gt 25 (max 40 ?)
q
18
Evidence for different ejection velocities
Two-sided jet _at_ kpc scale
One sided jet _at_ pc scale superluminal motion,
vapp 6 c
e.g. 1928134 (Hummel et al. 1992)
Either q is varying by 50 Or Gb is varying
from 7 to 1.08.
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Unification models
Beamed quasars should be a small fraction of a
larger set of unbeamed objects radio-galaxies
Statistical studies suggest that weak FR I
galaxies are the  parent population  of weak
quasars named  BL Lacs  (relatively) low
radio power weak or absent accretion disk
thermal UV continuum and fluorescent
lines Population numbers and relative
brightness suggest
G 3 to 5 (Padovani, Urry)
Bright quasars could be associated with FR II
galaxies.
20
Jets at other wavelengths
Optical jets 4 well known historical cases
M87 (close radio galaxy in Virgo cluster)
3C273 (closest quasar)
Almost 40 known with HST (rapidly growing , see
http//home.fnal.gov/jester/optjets/)
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Superluminal motion in optical jets
Only observed in closest object M87 (Biretta et
al. 1999)
Vapp 6 c
22
X-ray jets
Imaging capabilities of CHANDRA has allowed to
discover a large number of X-ray jets (gt 80)
http//hea-www.harvard.edu/XJET/
3C273 jet
Complex structures, interpretation still
discussed.
Radio (Merlin)
Optical (HST)
X-ray (Chandra)
23
g-ray sources
CGRO has discovered more than 80 AGNs with EGRET
gt 100 MeV All associated with radio sources
No direct imaging but g-ray transparency argument
indicates a strong Doppler boosting
24
Extreme TeV sources
Atmospheric Cerenkov Detectors can detect VHE
photons in the 100 GeV - 10 TeV range
PKS 2155 -304
HESS telescopes (Namibia)
Several AGNs detected at TeV energies, all are BL
lacs or FRI radio-galexies
Jets are visible throughout the entire
electromagnetic spectrum !
25
Jets in microquasars
X-ray binaries binary stars associating a
galactic black hole/Neutron star with a normal
star,
Low Mass X-ray binary accreting through the Roche
lobe High Mass XRB accretion through strong
wind of a high mass star
26
Jets in microquasars
GRS 1915105

• Relativistic ejections during flares
  superluminal velocities
• (Mirabel Rodriguez 94)

• Compact jets observed during low-hard  plateau 
  states
• Brightness contrast implies b 0.1 to 0.5
  (Dhawan 00)

Again, hint for different velocities
27
Observed spectra
Most radio-loud objects show double-peaked broad
band spectra, usually strongly variable
 Red   Objects
 Blue   Objects
First Bump radio-optical Second Bump X-ray
g-ray
First Bump radio-X-ray Second Bump g-ray ( ?
TeV )
28
Radiative processes in jets

• Jet emission can not be thermal
• Thought to be due to non-thermal processes
  originating from ultrarelativistic particles (see
  F. Halzen lectures)
• NB particles must have a much larger random
  Lorentz factor gr than the bulk Lorentz factor G
  (for TeV blazars, gr 106 wheras G a few 10).
• Lot of questions concerning
• The nature of particles (relativistic hadrons,
  electrons, pairs)
• The acceleration mechanism
• The dominant radiative processes

29
General pictures

• 2 broad classes of models
• Hadronic models assume a population of highly
  relativistic protons (linked with the problem of
  UHECR) with g 106 to 108
• Produce pions by various mechanisms (pp, pg)
• p0 ? gg (high energy component)
• p,- ? µ,- ? e,- producing synchrotron
  radiation
• Leptonic models implying the formation of a
  single population of ultrarelativistic leptons
• low energy component synchrotron radiation
• High energy component Inverse Compton process
• Conceptually simpler, explains correlated
  variabilities often (but not always !) observed
  currently preferred.

30
Particle acceleration
Most often assumed to be due to internal shocks
in a relativistic jet.
Acceleration through 1st order Fermi acceleration
(see Kirks lectures) Produces naturally n(g)
N0 g-p p 2 to 3
Credit R. Kollgaard
31
Synchrotron emission
Low energy component thought to be Incoherent
Synchrotron Radiation from ultra relativistic
leptons. Basics of synchrotron radiation (see
also F. Halzen lectures) Monoenergetic particle
with energy
Helical motion in a B field with a pitch angle
With frequency
Acceleration produces électromagnetic emission
cyclotron (non rel.) or synchrotron (rel.)
radiation
32
Basics of synchrotron radiation
Typical synchrotron frequency
Power emitted by a single particle
Averaging on sina (using ltsin2agt 2/3)
(NB although often used, may be not true with B
monodirectional assumes tangled B field)
Synchrotron loss time
33
Emission by a particle distribution
Monoenergetic (delta function)
Quasi-maxwellian distribution
Power law distribution
gives
with
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Maximal synchrotron frequency
Maximum synchrotron frequency
gmax usually determined by acceleration
mechanism and fastest loss time But there is an
absolute maximum value where
Rearranging a little bit gives (exercise)
Where a1/137 is the fine structure constant.
Gives hnmax 60 MeV for e-/e and 120 GeV for
protons (NB can be multiplied by d if bulk
relativistic motion) Compatible with hardest
synchrotron spectra (lt MeV )
35
Effect of synchrotron reabsorption
Synchrotron reabsorption occur when radiation
temperature exceeds particle temperature
(even with non thermal distribution)
Reabsorption limits Tn at most equal to Tp
Tp
Tn
Log(n)
nabs
36
Reabsorbed spectra
Power law Absorbed flux a n 5/2
Monoenergetic or pile-up Absorbed flux a n 2
37
Stratified jet
Flat spectrum sources addition of different
regions with different absorption frequencies
At a given frequency, one observes a compact,
self-absorbed photosphere, with optically thin
components
38
The high energy component
High energy component probably Inverse Compton
process elastic e-photon scattering
e(gmc2) hn (esmc2) ? e(gmc2) hn (emc2)
In the e- initial rest frame, appears like a
normal Compton process. ? Use a double Lorentz
transform (direct and backwards) to describe the
process in the observer frame.
39
The high energy component
High energy component probably Inverse Compton
(IC) process Elastic e-photon scattering
e(gmc2) hn (esmc2) ? e(gmc2) hn (emc2)
In the e- initial rest frame, appears like a
normal Compton process photon looses
energy. Use a double Lorentz transform to
describe the process in the observer frame
photon can gain energy !
40
Theory of IC process (basics..)
Transformed to e- rest frame (Âe), , the photon
energy becomes ges mc2
2 regimes Thomson (classical) regime ges ltlt 1
(i.e. photon energy lt 511 keV ) photon
scattered elastically in (Âe) total cross
section sT. In (Â), photon final energy is
g2es mc2 (small loss for e-)
Klein Nishina (quantum) regime ges gtgt 1 (i.e.
photon energy gt 511 keV ) photon scattering
strongly inelastic in (Âe) total cross section
sT ln(ges)/ges ltlt sT In (Â), photon final
energy is g mc2 (catastrophic loss for e-)
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Main results on IC process
In the Thomson regime
Typical frequency
Power emitted by a single particle in an
isotropic field
Inverse Compton loss time
(much like synchrotron radiation can be viewed
as Inverse Compton scattering on virtual
longitudinal photons of static magnetic field !)
42
Source of soft photons
Relativistic electrons will scatter ANY soft
photon in the Thomson regime (KN regime much
weaker Consider only photons with energy lt
mc2/g )

• Photons from the accretion disk external IC
  process
• Photon from synchrotron radiation Synchrotron
  Self Compton
• (SSC ) process
• Photons from Cosmological Microwave Background
  (2.7 K)

43
Source of soft photons
Sources of photons contribute proportionally to
the energy density Wpheff (limited to Thomson
Regime es lt 1/g )

• CMB negligible up to kpc scale ( 107 rg)
• Photons from accretion disk can be dominant for
  bright quasars, close to the black hole ( lt 100 -
  1000 rg)
• SSC should be the dominant process for
  intermediate range, or for low luminosity BL
  Lacs.

44
Limits on brightness temperature
Synchrotron loss
So
Inverse Compton Loss
But successive Inverse Compton generations will
produce higher order Compton losses amplified by
the same factor (Wph/WB)
Catastrophic losses if Wph/WB gt1 , ie. LIC gt Lsyn
Limits the brightness temperature Tn lt 10 12K for
a static source
Observed brightness temperature Tn 10 15 K -gt
Doppler boosting
45
IC cooling time
Recall

• with

For a typical emission radius R
Gives
With the compactness
For near Eddington luminosity
so
Needs very fast acceleration !!!!
46
g-ray opacity
Further constraint for g-ray emission must be
transparent with respect to pair production
process.
gg -gt ee-
Photons with energy e mec2 are mainly absorbed by
photons with energy e-1 mec2 (threshold for pair
creation in the barycentric frame) N.B.
Convenient rule high energy photons with energy
E are absorbed by low energy  soft  photons
with l(mm) E (TeV).
Assume a spatial photon density per unit volume
and reduced energy n(e), and a spherical source
with a radius R.
then
47
g-ray opacity
Take MeV dominated objects (red quasars)
For MeV photons e 1
absorbed by themselves
(g-ray compactness)
R can be estimated for a static source by the
variability time scale tvar R/c
48
Unveiling the g-ray jet
3C279 LMeV 1048 erg.s-1 Tvar 2 days
tgg 102 !!
Must give up the hypothesis of a static
source And/or spherical geometry.
Justifies again a jet origin.
49
A (over?)simple SSC model
G
Assume a spherical, homogeneous region filled
with magnetic field and relativistic particles
with a characteristic (random) Lorentz factor g0,
with a constant bulk Lorentz factor G
q
B
R
5 free parameters R, B, n, g0 , d
4 spectral observables 2 frequencies and 2
fluxes
One free parameter
50
How to estimate physical parameters
Assume for simplicity a pure Thomson regime
Peak Synchrotron frequency
Yields g0 and Bd
Peak Inverse Compton frequency
Synchrotron luminosity IC luminosity
Yields Nd2
Yields Rd
Degeneracy can be lifted by assuming the value of
d, or by using another independant constraint.
51
Further constraints
RdCst
R/dc tvar
logd
tgglt1
dmin
logR
52
A  Lorentz factor  crisis?
For TeV blazars (Mrk 501, PKS 2155), the above
method yields very high Lorentz factors (30 to 50)

• Much larger than observed superluminal velocities
  (lt 10)
• Much larger than those extracted from unification
  models (statistics AND brightness contrast -gt 3
  to 5 )
• Very difficult to understand the detection of
  unbeamed radiogalaxies (M87)
• Difficult to produce theoretically !

One-zone models oversimplified?