Blazar Variability

1
Blazar Variability the Radio Galaxy/Cosmology
Interface

• Paul J. Wiita
• Georgia State University, Atlanta, USA
• Winter School on Black Hole Astrophysics
• APCTP, Pohang, January 17-20, 2006

2
OUTLINE

• Blazar Basics
• Accretion Disks in AGN Recent Evidence for
  their Presence Basic Timescales A Few
  Important Instabilities Spiral Shocks
• Aspects of Jet Produced Variations Coherent
  Emission Slow Knot Speeds vs.
  Ultrarelativistic Jets
• Radio Galaxies Trigger Extensive Star
  Formation Spread Magnetic Fields and
  Metals into IGM

3
Blazar Characteristics

• Rapid variability at all wavelengths
• Radio-loud AGN
• BL Lacs show extremely weak emission lines
• Optical polarization ? synchrotron domination
• Double humped SEDs RBL vs XBL?
• Core dominated quasars (optically violently
  variable and high polarization quasars) clubbed
  w/ BL Lacs to form the blazar class
• Population statistics indicate that BL Lacs are
  FR I RGs viewed close to jet direction (Padovani
  Urry 1992)

4
Long-term Blazar Lightcurve(Optical monitoring
at Colgate U.- Balonek)
5
Long-term Radio MonitoringAller Aller, U
Michigan
6
Microvariability Intraday Variability
too!Romero, Cellone Combi Quirrenbach et al
(2000)
7
Blazar SED 3C 279 (Moderski et al. 2003)
Left hump peak in mm or FIR, from
synchrotron Right hump peak in gamma-rays, from
Inverse Compton off seed photons From disk, from
jet itself or from broad line clouds
8
Orientation Based Unification Picture
9
Evidence for Accretion Disks in Blazars
Big blue bump in AO 0235164 (Raiteri et al.
astro-ph/0503312)
10
More New Evidence for Accretion Disks
Optically thick hidden Balmer edge now claimed
to be seen in several quasars.

• Ton 202 polarized flux with face-on Kerr disk
  model fitted to it (Kishimoto et al. 2004)

11
Why Quasi-Keplerian and Disk-like?Quasi-black
body fits to disk spectraBroad K? lines for
NLS1sVariable Double peaked lines here H?
lines Strateva et al, AJ (2003)Jets
probably require disks as launching pads
12
Accretion Flow Geometries

• Quasi-accepted picture L/LE determines disk
  thickness and extent toward BH very high
  L/LE ? geometrically optically thick
  intermediate L/LE ? cold optically thick,
  geometrically thin low L/LE ? optically thin
  hot flow interior to some transition
  radius.

13
Key Timescales for Accretion

• With R r/3RS, a quasi-Keplerian flow, h the
  thickness and ? the viscosity parameter, the
  fastest expected direct variations are on
  dynamical times of hours for SMBHs (e.g. Czerny
  2004).

M8MBH/108M?
Radial sound transmission time
Thermal and viscous timescales
For thin disks, h?0.1r
14
How fast can the cold disk be removed?

• Transition radius changes, either by evaporation
  or substantial outflow
• Either way, disk T must go up to about virial T
  and enough energy to do this must be stored
• For an ?-disk, tevaptvisc , but more generally,

For AGN gt 103yr, so if disk appears to disappear
quickly, probably from suppression of energy
dissipation (I.e., MRI instability turned off,
perhaps by some ordered B field.)
15
Longest Timescale?

• Governed by rate at which outer disk is fed
• Probably the rate at which gas is injected into
  the core of a galaxy (bars within bars to drive
  inward?)
• Dominated by galactic mergers (probably major)
  and timescales gt 107 years can exceed 108
  yr Does harassment (mere passage) work?
• Does the AGN self-regulate, with its energy
  injection halting the inflow of gas? (Hopkins et
  al. 2005a,b,c)
• Most likely depends on whether quasi-isotropic
  winds star-burst supernovae OR narrow
  jets carry off most kinetic energy from AGN.

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DISK INSTABILITIES

• ? many of them. How many are important,
  especially for blazars?
• Radiation pressure instability
• Magneto-rotational instabilities
• Flares from Coronae
• Internal oscillatory modes (diskoseismology)
• Avalanches or Self-Organized Criticality
• Spiral shocks induced by companions or
  interlopers
• Key point even if blazar emission dominated by
  jets, disk instabilities may feed into jets

17
Radiation Pressure Instability

• Long known that ?-disks are unstable if radiation
  pressure dominated (Shakura Sunyaev 1976)
• AGN models should be Prad dominated over a wide
  range of accretion rates and radii
• Computed variations are on tvisc(100RS)
  (Janiuk et al. 2000 Teresi et al 2004)
• May have been seen in the microquasar GRS
  1915105 (over 100s of sec).
• Scaled to AGN masses significant outbursts, but
  over years to decades all the way from X-rays
  through IR.

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SPH simulation of Shakura-Sunyaev
instability (Teresi, Molteni Toscano, MNRAS
2004)
19
MRI Induced Variations

• Magneto-Rotational Instabilities (e.g. Balbus
  Hawley ApJ, 1991) are commonly agreed to be
  present
• Probably produce effective disk ? 0.01-0.10

Total (solid), magnetic stress (dashed) and fluid
(dotted) viscosities at a disk center (Armitage
1998, ApJL) ? Also produce changes in dissipation
and accretion rate ? Some disk clumping, but not
destruction (profile changes?)
20
Turbulence in a Magnetized Disk
Distant views of inner disk _at_ inclinations of 55
and 80o

• Integrated flux for inclinations of (top to
  bottom) 1, 20, 40, 80O for a hot simulation
  using Zeus and pseudo-Newtonian potential
• (Armitage Reynolds, MNRAS 2003)
• Significant fluctuations develop on a few
  rotational timescales (hours for 108M?).

21
Spiral Shocks in Disks

• Perturbation by smaller BH can drive spiral
  shocks
• Significant flux variations ensue on orbital
  timescales of the perturber (Chakrabarti Wiita,
  ApJ, 1993)

Perturbers w/ 0.1 and 0.001 MBH
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Spiral Shocks and Line Variations

• This type of shock provides the best fits to
  changes in double hump line profiles seen in
  about 10 of AGN (Chakrabarti Wiita 1994)

Model vs. data for 3C 390.3 H? broad lines in
1976 1980. Expected variations.
23
Flares and Coronae

• Plenty of debate over the relative contribution
  of disk coronal flares to X-ray (predominantly)
  and other band (secondarily) emission and
  variability.
• Clearly an important piece of the Seyfert
  variability but probably usually a small piece of
  blazar emission.
• Total energy releasable from low density coronal
  flares is probably too small unless avalanche
  or self-organized criticality process is
  triggered, perhaps propagating inward within a
  disk (Mineshige et al. 1994 Yang et al. 2000)
  easily produces correct PSD.
• But flares can provide low level X-ray variations
  visible when other activity is minimal maybe
  produce a bit of optical variability too.

24
Jet Variations in Blazars

• This is the dominant idea, but it still is not
  well modeled. SOMETHING changes outflow
  rate, velocity, B-field structure. Waves can
  steepen into shocks.
• Relativistic shocks propagating down jets can
  explain much of the gross radio through optical
  variations via boosted synchrotron emission.
  Accretion disk fluctuations could drive them.
• Turbulence, instabilities, magnetic
  inhomogeneities can probably explain the bulk of
  rapid variations.
• Inverse Compton models SSC, External
  Compton, Mirror Model , Decelerating Jets, can
  explain particular high energy variations wrt low
  energy ones, though no model seems able to cover
  all observations (multiple IC photon sources?)

25
Shock-in-jet model new components (Aller, Aller
Hughes 1991)
26
Turbulence in a Jet ? Rapid Variations(Marscher
Travis 1996)
27
Synchrotron vs. Coherent Emission

• Do any compact radio sources show intrinsic
  TBgt1011K? (More realistic self-absorbed source
  equipartition inverse Compton catastrophe limit
  3x1010 Singal Gopal Krishna 1985 Readhead
  1994)
• IDV at cm ? big Lorentz factor is necessary (if
  intrinsic) as simple measurements often give
  TB1021K
• To avoid it, a size ? larger is allowed if
  plasma approaches us with ?gtgt 1. So solid angle
  up ?2.
• TB intrinsic boosted by ? wrt source frame so
  total help of ?3 available BUT still need ?103
  for enough help
• Such huge ?s prevent too many X-rays, but at the
  cost of low synchrotron radiative efficiencies
  and thus demand very high jet energy fluxes
  (Begelman et al. 1994)

28
But what really produces radio IDV?

• It seems most IDV is due to refractive
  interstellar scintillation (e.g.,
  Kedziora-Chudczer et al. 2001)
• Then TB,intrinsic1013K, so ??30 solves this
  problem
• However, space VLBI couldnt resolve many of
  these sources, so TB could be much higher
  (Kovalev)
• A recent claim that the blazar J18193845 shows
  diffractive scintillation ? ? ? 10?as and
  TB,intrinsicgt(gt)1014K (Macquart de Bruyn 2005)
• If true, it demands ?gt103 if incoherent
  synchrotron emission is the radiation mechanism,
  and the energy problem returns

29
Coherent Radiation Could Solve Problems

• If strong Langmuir turbulence develops in AGN
  jets then coherent mechanisms can produce needed
  huge TB without requiring extreme Lorentz factors
  (e.g., Baker et al. 1988, Krishan Wiita 1990,
  Benford 1992).
• One possibility a pump field can be scattered
  off a collective mode of a relativistic electron
  beam Stimulated Raman Scattering for a density
  n, area A, electron Lorentz factor ? and bunching
  fraction ?

For n109cm-3, ?103, A1032 cm2, ?0.5 Lo 1046
erg/s BUT problems with absorption of masers
hard to solve
30
What Type of Coherent Radiation?

• Above models implicitly assumed ?plasmagt
  ?cyclotron but some only required mild
  population inversions.
• Begelman, Ergun and Rees (2005) have argued that
  the opposite, ?c?? ?p is more likely
  in blazar jets.
• Employ small-scale magnetic mirrors, arising from
  hydromagnetic instabilities, shocks or
  turbulence any could provide good conditions for
  numerous transient cyclotron masers to form
• Current into mirror inhibits motion of es along
  flux tube. Maintaining current demands
  parallel E field and accelerates es
  Accelerated es along converging flux tubes ?
  population inversion needed for cyclotron maser
  
  Maser pumped by turning kinetic and
  magnetic energy into j?E work
• Synchrotron absorption is serious but high TB
  maser photons can escape from a boundary layer
  giving TB,obs 2x1015K (?/10)4 R

31
Magnetic Mirror Cyclotron Maser
Current carrying magnetic mirror on
quasi-force-free flux rope. Parallel E field
maintains electron flow through mirror. Parallel
potential magnetic mirror turns initial
electron distribution into a horseshoe shaped one
(shell in 3-D) Conditions mirror ratio
R5, Current Jzm30mA/m2 (Jz06mA/m2) Epar500
keV, consistent w/ Te100 keV n100
cm-3 (Begelman et al. 2005)
32
Modest Superluminal VLBI Speeds

• Only semi-direct probe of extragalactic jet
  speed VLBI knot apparent motions gt 30
  subluminal for TeV blazars (Piner Edwards 2004
  Giroletti et al. 2004)
• ? low ?2-4 contradict usual blazar estimates
  IDV

1ES 1959650 _at_ 15 GHz 3 epochs Natural (top)
vs. Uniform (bottom) weighting (Piner Edwards
2004) vapp-0.1 /- 0.8 c
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TeV Blazars want High Doppler factors

• To avoid excessive photon-photon losses variable
  TeV emission demands ultrarelativistic jets
  (Krawczynski et al. 2002) with 15lt ? lt 100
• Taking into account IR background absorption
  strongly implies 45 lt ? needed in unreddened
  emission (e.g. Kazanas Wagner)
• Evidence for TB,intrinsic gt 1013K in IDV sources
  would also imply ? gt 30
• While rare (Lister), some vapp gt 25c components
  are seen (Piner et al.) in EGRET blazars.
• Substantial apparent opening angles are seen for
  some transversely resolved knots.
• GRB models usually want ? gt 100

34
How to Reconcile Fast Variations with Slow Knots?

• Spine-sheath type systems fast core gives
  variations via IC and slower outer layer seen in
  radio (Sol et al. 1989 Laing et al. 1999
  Ghisellini et al. 2004)
• Rapidly decelerating jets between sub-pc (?-ray)
  and pc (VLBI knot) scales (Georganopoulos
  Kazanas 2003)
• Viewing angles to within 1o could work in an
  individual case but ? too many slow knots.
• Differential Doppler boosting across jet of
  finite opening angle can make the weighted
  probable vapp surprisingly small (Gopal-Krishna,
  Dhurde Wiita 2004)
• Motions can reflect pattern, not physical, speeds

35
Conical Jets w/ High Lorentz Factors

• Weighted ?app vs ? for ? 100, 50, 10 and
  opening angle 0,1,5 and 10 degrees, with blob
  ?3 boosting
• Probability of large ?app can be quite low
  for high ? if opening angle is a few degrees

36
High Gammas Yet Low Betas

• ?app vs ? for jet and prob of ?app gt ? for
  opening angles 0, 1, 5, 10 degrees and ? 50,
  10 (continuum ?2 boosting)
• Despite high ? in an effective spine population
  statistics are OK
• Predict transversely resolved jets show different
  ?app

37
Finding Jet Parameters

• Determining bulk Lorentz factors, ?, and
  misalignment angles, ?, are difficult for all
  jets
• Often just set ? 1/ ?, the most probable value
• Flux variability and brightness temperature give
  estimates

?S change in flux over time ?obs Tmax
3x1010K ?app from VLBI knot speed ? is
spectral index
38
Conical Jets Also Imply

• Inferred Lorentz factors can be well below the
  actual ones
• Inferred viewing angles can be substantially
  underestimated, implying deprojected lengths are
  overestimated
• Inferred opening angles of lt 2o can also be
  underestimated
• IC boosting of AD UV photons by ?10 jets would
  yield more soft x-rays than seen (Sikora bump)
  but if ?gt50 then this gives hard x-ray fluxes
  consistent with observations
• So ultrarelativistic jets with ?gt30 may well be
  common

39
Inferred Lorentz Factors
?inf vs. ? for ?100, 50 and 10 for ?5o P(?)
and lt ?infgt
40
Inferred Projection Angles

• Inferred angles can be well below the actual
  viewing angle if the velocity is high and the
  opening angle even a few degrees
• This means that de-projected jet lengths are
  overestimated

41
Radio Lobes in the Quasar Era

• The dramatic rise in both star formation rate and
  quasar densities back to z gt 1 motivates
  investigation of a possible causal connection.
• Radio lobes affected a large fraction of the
  cosmic web in which galaxies were forming at 1.5
  lt z lt 3
• Most powerful radio galaxies (RGs) are only
  detectable for a short fraction of their total
  lifetimes, so the volumes filled by old,
  invisible, lobes are extremely large.
• The co-moving density of detected RGs was roughly
  1000 times higher at 2 lt z lt 3 than at z 0.
• These RG lobes need only fill much of the
  "relevant universe", the denser portion of the
  filamentary structures containing material that
  is forming galaxies, not the entire universe
  much easier for these rare AGN!

42
RGs Suffer Restricted Visibility

• All recent models of RG evolution (Kaiser et al.
  1997 Blundell et al. 1999 -- BRW Manolakou and
  Kirk 2002 Barai Wiita 2006) agree that radio
  flux declines with increasing source size because
  of adiabatic losses, and with redshift because of
  inverse Compton losses off the CMB.
• Jets of power Q0, through a declining power-law
  density, n(r) has total linear size D with a0
  the core radius (10 kpc), n0 the central density
  (0.01 cm-3), and ? 1.5.
• Many properties of low frequency radio surveys
  (3C, 6C, 7C) can be fit if typical RG lifetimes
  are long (up to 500 Myr) and if the jet power
  distribution goes as Q0-2.6 (BRW).
• For RGs at z gt 2, most observable lifetimes (?)
  are only a few Myr, even if the jet lifetimes (T)
  are 100s of Myr

43
P-D Tracks for Different Models (Barai Wiita
2005)
44
Radio Luminosity Functions

• Powerful (FR II) RGs were nearly 1000 times more
  common between redshifts of 2 and 3 (Willott et
  al. 2001).
• RLF is flat for about a decade in radio power
  P151 gt 1025.5 W Hz1 sr1 , where FR II sources
  are most numerous.
• With the correction factor (T/? 50) we find at
  z 2.5 the proper density of of powerful RGs
  living for T is
• ? 4 x 105(1z)3 T5 Mpc3 (? log P151)1
  with T5 T/(5 x 108 yr).
• Integrate over the peak of the RLF and take into
  account generations of RGs over the 2 Gyr length
  of the quasar era.
• We find (Gopal-Krishna Wiita 2001) the total
  proper density of intrinsically powerful RGs is
  about ? 8 x 10 3 Mpc-3

45
Radio Luminosity FunctionWillott et al. 2001 FR
II vary much more than FR I
46
Models Data Agree Adequately (BW for MK)
47
The Relevant Universe
The web of baryons traced by the WHIM at z0 in a
100 Mpc3 box (Cen 2003) RGs nearly all form in
these filaments and so most of the radio lobes
will be confined to them
48
Radio Lobes Penetrate the Relevant Universe

• During the quasar era, only a small fraction of
  the baryons had yet settled into the
  proto-galactic cosmic web roughly 10 of the
  mass and 3 of the volume (Cen Ostriker 1999).
• Thus RG lobes have a big impact if they pervade
  only this filamentary "relevant universe", with
  volume fraction ? 0.03.
• Assuming BRW parameters and integrating over
  beam power and z, we find the fraction of the
  relevant universe filled during the quasar era by
  radio lobes
• ? 2.1? T518/7 ?1 (5/RT)2, is gt 0.1
• if T gt 250 Myr and RT (RG length to
  width) 5.

49
Overpressured Lobes Can Trigger Extensive Star
Formation

• RG lobes remain significantly supersonic out to D
  gt 1 Mpc.
• Their bow shocks will compress cooler clouds
  within the IGM (e.g., Rees 1989 GKW01),
  triggering extensive star formation.
• Much of the "alignment effect" (McCarthy et al.
  1987) is thus explained.
• Recent numerical work that includes cooling
  (Mellema et al. 2002 Fragile et al. 2003, 2004)
  confirms that RG shocked cloud fragments become
  dense enough to yield massive star clusters (Choi
  et al. 2006).
• Hence, RGs may accelerate the formation of new
  galaxies and in some cases produce them where
  they wouldnt have formed in the standard
  picture.

50
Jet/Cloud Interaction Simulation
When cooling is included powerful shocks leave
behind dense clumps that can yield major star
clusters (Mellema et al.)
51
Relativistic Jet/Cloud 3-DSimulation Density,
Pressure, Lorentz factor(Choi, Wiita Ryu
2006)
52
Magnetization of the ICM/IGM

• We showed (GKW01) that during the quasar era the
  RGs could inject average magnetic fields of 108
  G into the IGM. Such field strengths within the
  filaments are supported by observations (Ryu et
  al. 1998 Kronberg et al. 2001).
• Very different arguments based on total accretion
  energy extracted via BHs and on the assumptions
  of isotropized magnetized bubbles also lead to
  similar conclusions that significant B fields
  from AGN can fill much of the IGM (Kronberg et
  al. 2001 Furlanetto Loeb 2001) and can have
  major impact on star formation (Rawlings Jarvis
  2004 Silk 2005).

53
Metalization of the IGM

• Substantial metal abundances have been found in
  Lyman-break galaxies at z gt 3 and in damped Ly-?
  clouds.
• Gopal-Krishna Wiita (2003) have shown that the
  giant RGs can sweep up significant quantities of
  metals from host galaxies.
• These can seed the young galaxies, often
  triggered by the lobes, with metals.
• Subsequent generations of radio activity could
  further disperse metals produced in early
  generations of stars in those newly formed
  galaxies.

54
CONCLUSIONS

• Accretion disks are present and they must
  contribute something to optical, UV, and X-ray
  variability in all AGN.
• Jet emission may include or be dominated by
  coherent processes.
• We can reconcile slow TeV blazar VLBI motions
  with high Lorentz factors.
• Radio galaxies can fill much of the universe in
  the quasar era they can trigger substantial star
  formation (even new galaxies) spread both
  metals magnetic fields into the IGM