Xray and gammaray observations of active galaxies as probes of their structure

1
X-ray and gamma-ray observations of active
galaxies as probes of their structure
Greg Madejski Stanford Linear Accelerator Center
and Kavli Institute for Particle Astrophysics
and Cosmology

• Outline
• Two classes of active galaxies - (1) Isotropic
  emission dominant
• (2) Relativistic jet emission dominant
• Isotropic emitter (black hole and the accretion
  disk) as the source of the jet
• Emission processes and content the relativistic
  jet
• Key questions about the nature and the origin of
  the jet
• Future observational prospects in the high energy
  regime towards
• the answers GLAST and the future X-ray missions
  AstroE2, NuSTAR

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Compton Gamma-ray Observatory
Featured instruments sensitive from 40 keV
(OSSE) up to nearly 100 GeV (EGRET)
3
Global observational differences between

• Radio-quiet and jet-dominated
  active galaxies active galaxies (a. k. a.
  blazars)
• no strong
• MeV GeV emission MeV GeV emission
• Strong signatures of Only weak
  signatures
• circumnuclear matter
• (symmetric emission lines)
• No strong compact radio Radio, optical,
  X-ray cores and jets
• structure
• Both classes are rapidly variable, requiring
  simultaneous observations
• spectra and variability patterns can reveal
  structure and physical processes responsible for
  emission
• X-rays and g-rays vary most rapidly presumably
  originate the closest to the central engine (?)
• Study of the isotropic emission as well as the
  jet should reveal the details of formation,
  acceleration, and collimation of the relativistic
  jets

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Radio, optical and X-ray images of the jet in M 87

• Jets are common in AGN and radiate in radio,
  optical and X-ray wavelengths
• Blazars are the objects where jet is pointing
  close to the line of sight
• In many (but not all) blazars, the jet emission
  dominates the observed spectrum

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Unified picture of active galaxies

• Presumably all AGN have the same basic
  ingredients a black hole accreting via
  disk-like structure
• In blazars the jet is most likely
  relativistically boosted and thus so bright that
  its emission masks the isotropically emitting
  central engine
• But the nature of the isotropically emitting AGN
  should hold the clue to the nature of the
  conversion of the gravitational energy to light
• Again, X-ray and g-ray emission varies most
  rapidly potentially best probe of the
  close-in region

Diagram from Padovani and Urry
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Radio galaxy M87 (Virgo-A) studied with the HST
Weighing the central black hole
Seyfert galaxy NGC 4258 studied using H2O
megamaser data

• Black holes are a common ingredient of galaxies
• When fed by galaxian matter, they shine or
  produce jets or both
• The BH mass is very important to know L the
  accretion rate in Eddington units

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High-energy spectra of isotropically-emitting
AGN Example is an X-ray bright Seyfert 1
galaxy IC 4329a

• Asca, XTE, OSSE data
• for IC 4329a
• (from Done, GM, Zycki 2003)
• Average OSSE / Ginga spectrum of 20 AGN looks
  essentially the same

• General description of the broad-band intrinsic
  X-ray spectrum of a non-jet (isotropically-emit
  ting) AGN is a power law, photon index 2,
  with exponential-like cutoff at 200 keV

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Effects of the orientation to the line of sight
in AGN
The spectrum of the object depends on the
orientation with respect to the line of sight
soft X-rays are (photo-electrically) absorbed by
the surrounding material
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X-ray Background Spectrum (from G. Hasinger)
from Gilli 2003
E5
keV still lots of work...
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Heavily obscured AGN hiding in the dust
Important ingredient of the Cosmic X-ray
Background?

• The origin of the diffuse Cosmic X-ray Background
  is one of the key questions
• of high energy astrophysics research
• Most likely it is due to a superposition of
  individual AGN, at a range of Lx, z
• Spectrum of the CXB is hard, cannot be due to
  unobscured AGN (Seyfert 1s)
• - but it can be due AGN with a broad range of
  absorption in addition to a range of Lx, z

RXTE PCA HEXTE

• Absorbed (Seyfert 2) active galaxy NGC 4945
• - Anonymous in radio, optical, but the X-ray
  spectrum taken by us (Done, GM, Smith) reveals
  one of the brightest 100 keV AGN in the sky
• - Sources similar to NGC 4945 at a range of Lx, z
  and absorption can make up the CXB
• - BUT for this, one needs most of the AGN to be
  heavily absorbed Whats the absorption geometry?
  

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Astrophysical jets and blazars what are
blazars?

• Radio-loud quasars, with compact, flat-spectrum
  radio cores which also reveal some structure
• The structures often show superluminal expansion
• Radio, IR and optical emission is polarized
• Blazars are commonly observed as MeV GeV
• g-ray emitters ( 60 detected by EGRET)
• In a few objects, emission extends to the TeV
  range
• Rapidly variable in all bands including g-rays
• Variability of g-rays implies compact source
  size, where the opacity of GeV g-rays against
  keV X-rays to e/e- pair production would be
  large - opaque to their own emission!
• Entire electromagnetic emission most likely
  arises
• in a relativisitc jet with Lorentz factor Gj
  10, pointing close to our line of sight

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Example of radio map of a blazar 3C66B
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EGRET All Sky Map (100 MeV)
3C279
Cygnus Region
Vela
Geminga
Crab
PKS 0528134
LMC
Cosmic Ray Interactions With ISM
PKS 0208-512
PSR B1706-44
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Broad-band spectrum of the archetypal GeV blazar
3C279

• Data from Wehrle et al. 1998

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Example broad-band spectrum of the TeV-emitting
blazar Mkn 421

• Data from Macomb et al. 1995

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Blazars are variable in all observable bands

• Example X-ray and GeV g-ray light curves from
  the 1996 campaign to observe 3C279

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The blazar sequence

• Work by G. Fossati, G. Ghisellini,
• L. Maraschi, others (1998 and on)
• Multi-frequency data on blazars
• reveals a progression
• As the radio luminosity increases
• Location of the first and second peaks
• moves to lower frequencies
• Ratio of the luminosities between
• the high and low frequency
• components increases
• Strength of emission lines increases

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Radiative processes in blazars

• What do we infer? We have some ideas about the
  radiative processes
• Polarization and the non-thermal spectral shape
  of the low energy component are best explained
  via the synchrotron process
• The high-energy component is most likely due to
  the inverse Compton process by the same
  relativistic particles that produce the
  synchrotron emission
• Relative intensity of the synchrotron vs. Compton
  processes depends on the relative energy density
  of the magnetic field vs. the ambient soft
  photon field
• The source of the seed photons for the
  up-scattering process is diverse - it depends on
  the environment of the jet
• BUT WE STILL DON'T KNOW HOW THE JETS ARE
  LAUNCHED, ACCELERATED AND COLLIMATED

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From Sikora, Begelman, and Rees 1994

• Source of the seed photons for inverse Compton
  scattering can depend on the environment
• It can be the synchrotron photons internal to the
  jet (the synchrotron self-Compton model
• - This is probably applicable to BL Lac objects
  such as Mkn 421
• Alternatively, the photons can be external to the
  jet (External Radiation Compton model)
• - This is probably applicable to blazars hosted
  in quasars such as 3C279

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Example of an object where ERC may dominate
3C279(data from Wehrle et al. 1998)
SSC or ERC?

• Example of an object where SSC may dominate Mkn
  421
• (data from Macomb et al. 1995)

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Moderski, Sikora, GM 2003 Blazejowski et al.
2004

• For the External Radiation Compton models, the
  ultraviolet flux from Broad Emission Line
  regions is not the only game in town
• Infrared radiation specifically, AGN light
  reprocessed by dust - might also be important,
  especially in the MeV-peaked blazars (Collmar et
  al.)
• Sensitive hard X-ray through soft g-ray
  observations will be crucial to resolve this,
  since IR should be Compton-upscattered to
  energies less than GeV

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Modelling of radiative processes in blazars

• In the context of the synchrotron models, emitted
  photon frequency is
• ns 1.3 x 106 B x gel2 Hz
• where B is the magnetic field in Gauss
• and gel is the electron Lorentz factor
• The best models have B 1 Gauss, and gel for
  electrons radiating at the peak of the
  synchrotron spectral component of 103 106,
  depending on the particular source
• Degeneracy between B and gel is broken by
  spectral variability spectral curvature, at
  least for HBLs (Perlman et al. 2005)
• The high energy (Compton) component is produced
  by the same electrons as the synchrotron peak
  and ncompton nseed x gel2 Hz
• Still, the jet Lorentz factor Gj is 10, while
  Lorentz factors of radiating electrons are gel
  103 106
• Thus, one of the central questions in blazar
  research is
• HOW ARE THE RADIATING PARTICLES ACCELERATED?

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Interpretation of the observational data for
blazars

• PARTICLE ACCELERATION
• The most popular models invoke the Fermi
  acceleration process in shocks forming via
  collision of inhomogeneities or distinct plasma
  clouds in the jet (internal shock model, also
  invoked for GRBs)
• This can work reasonably well the
  acceleration time scale tacc to get electron up
  to a Lorentz factor gel can be as short as 10-6
  gel B-1 seconds, while the cooling time (due to
  synchrotron losses) is 5 x 108 gel-1
  B-2 seconds, perhaps up to 10 times faster for
  Compton cooling, so accelerating electrons to gel
  up to 106 via this process is viable (but by no
  means unique!)
• INTERNAL SHOCK SCENARIO MODEL
• This model assumes that the central source
  produces multiple clouds of plasma and ejects
  them with various relativistic speeds those
  clouds collide with each other, and the collision
  results in shock formation which leads to
  particle acceleration
• A simple "toy model" that reproduces
  observations well assumes two clouds of equal
  masses, with Lorentz factors G1 and G2 with G1 G2 (G1 and G2 1)
• From G2 and G1 one can infer the efficiency
  (fraction of kinetic power available for particle
  acceleration)
• Recent simulations reproducing well the X-ray
  light curves of Mkn 421 (and applicable to other
  objects) (Tanihata et al. 2003) imply that the
  dispersion of G cannot be too large
• However, the small dispersion of G implies a
  low efficiency (problem - as huge kinetic luminosities of
  particles are required
• MY OWN PREJUDICE IS THAT THE JETS ARE LAUNCHED
  AS MHD OUTFLOWS, AND ARE INITIALLY DOMINATED BY
  POYNTING FLUX
• WE NEED TO UNDERSTAND DISSIPATION/PARTICLE
  ACCELERATION AS WELL AS THE DISK JET
  CONNECTION

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Broad line region providing the ambient UV
Accretion disk and black hole
Time -

• Diagram for the internal shock scenario
  colliding shells model G2 G1, shell 2
  collides with shell 1

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Interpretation of the observational data for
blazars

• PARTICLE ACCELERATION
• The most popular models invoke the Fermi
  acceleration process in shocks forming via
  collision of inhomogeneities or distinct plasma
  clouds in the jet (internal shock model, also
  invoked for GRBs)
• This can work reasonably well the
  acceleration time scale tacc to get electron up
  to a Lorentz factor gel can be as short as 10-6
  gel B-1 seconds, while the cooling time (due to
  synchrotron losses) is 5 x 108 gel-1
  B-2 seconds, perhaps up to 10 times faster for
  Compton cooling, so accelerating electrons to gel
  up to 106 via this process is viable (but by no
  means unique!)
• INTERNAL SHOCK SCENARIO MODEL
• This model assumes that the central source
  produces multiple clouds of plasma and ejects
  them with various relativistic speeds
• The clouds collide with each other, and the
  collision results in shock formation which leads
  to particle acceleration
• A simple "toy model" that reproduces
  observations well assumes two clouds of equal
  masses, with Lorentz factors G1 and G2 with G1 G2 (G1 and G2 1)
• From G2 and G1 one can infer the efficiency
  (fraction of kinetic power available for particle
  acceleration)
• Recent simulations reproducing well the X-ray
  light curves of Mkn 421 (and applicable to other
  objects) (Tanihata et al. 2003) imply that the
  dispersion of G cannot be too large
• However, the small dispersion of G implies a
  low efficiency (problem - as huge kinetic luminosities of
  particles are required
• MY OWN PREJUDICE IS THAT THE JETS ARE LAUNCHED
  AS MHD OUTFLOWS, AND ARE INITIALLY DOMINATED BY
  POYNTING FLUX
• WE NEED TO UNDERSTAND DISSIPATION/PARTICLE
  ACCELERATION AS WELL AS THE DISK JET
  CONNECTION

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Content of the jet

• Are blazar jets dominated by kinetic energy of
  particles from the start, or are they initially
  dominated by magnetic field (Poynting flux)?
  (Blandford, Vlahakis, Wiita, Meier, Hardee, )
• There is a critical test of this hypothesis, at
  least for quasar-type (EGRET) blazars
• If the kinetic energy is carried by particles,
  the radiation environment of the AGN should be
  bulk-Compton-upscattered to X-ray energies by the
  bulk motion of the jet
• If Gjet 10, the 10 eV, the H Lya photons
  should appear
• bulk-upscattered to 102 x 10 eV 1 keV
• X-ray flare should precede the g-ray flare
  (precursor)
• X-ray monitoring concurrent with GLAST
  observations is crucial to settle this
• A lack of X-ray precursors would imply that the
  jet is particle-poor and may be dominated by
  Poynting flux

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Future of g-ray observations GLAST
Features of the MeV/GeV g-ray sky Diffuse
extra-galactic background (flux 1.5x10-5
cm-2s-1sr-1) Galactic diffuse and galactic
sources (pulsars etc.) High latitude
(extragalactic) point sources blazars and new
sources? - typical flux from EGRET sources 10-7 -
10-6 cm-2s-1
EGRET all-sky survey (galactic coordinates) E100
MeV
Need an instrument with a good sensitivity
and a wide field of view GLAST currently
under construction - will be launched in 2007
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GLAST LAT instrument overview
Si Tracker pitch 228 µm 8.8 105 channels 12
layers 3 X0 4 layers 18 X0 2 layers
Grid ( Thermal Radiators)
3000 kg, 650 W (allocation) 1.8 m ? 1.8 m ? 1.0
m 20 MeV 300 GeV
CsI Calorimeter Hodoscopic array 8.4 X0 8
12 bars 2.0 2.7 33.6 cm
LAT managed at SLAC
Flight Hardware Spares 16 Tracker Flight
Modules 2 spares 16 Calorimeter Modules 2
spares 1 Flight Anticoincidence Detector Data
Acquisition Electronics Flight Software

• cosmic-ray rejection
• shower leakage
• correction

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Schematic principle of operation of the GLAST
Large Area Telescope

• g-rays interact with the hi-z material in the
  foils, pair-produce, and are tracked with silicon
  strip detectors
• The instrument looks simultaneously into 2
  steradians of the sky
• Energy range is 30 MeV 300 GeV, with the
  peak effective area of 12,000 cm2
• This allows an overlap with TeV observatories

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GLAST LAT Science Performance Requirements Summary
31
Sensitivity of GLAST LAT
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GLAST LAT has much higher sensitivity to weak
sources, with much better angular resolution
GLAST
EGRET
33
GLAST LATs ability to measure the flux and
spectrum of 3C279 for a flare similar to that
seen in 1996 (from Seth Digel)
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The future is (almost) hereNext high energy
astrophysics satellite Astro-E2 will be
launched in June/July 2005 Astro-E2 will
have multiple instruments X-ray calorimeter
(0.3 10 keV) will feature the best energy
resolution yet at the Fe K line region, also good
resolution for extended sources (gratings cant
do those!) - but the cryogen will last only 3
years Four CCD cameras (0.3 10 keV, lots of
effective area) to monitor X-ray sources when the
cryogen expires Hard X-ray detector, sensitive
up to 700 keV
NEAR FUTURE Astro-E2
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Principal Investigator is Fiona Harrison
(Caltech) the NuSTAR team includes Bill Craig,
GM, Roger Blandford at SLAC/KIPAC Steve
Thorsett, Stan Woosley at UCSC Columbia, Danish
Space Res.Inst., JPL, LLNL,
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NuSTAR was recently selected for extended study,
with the goal for launch in 2009 (Fiona
Harrison/Caltech, PI)

• Its the first focusing
• mission above 10 keV
• (up to 80 keV)
• brings unparalleled
• sensitivity,
• angular resolution, and
• spectral resolution
• to the hard x-ray band
• and opens an entirely new region of the
  electromagnetic spectrum for sensitive study it
  will bring to hard X-ray astrophysics what
  Einstein brought to soft X-ray astronomy

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Hardware details of NuSTAR
NuSTAR is based on existing hardware developed
in the 9 year HEFT program
Based on the Spectrum Astro SA200-S bus, the
NuSTAR spacecraft has extensive heritage. NuSTAR
will be launched into an equatorial orbit from
Kwajalein.
Orbit 525 km 0 inclination
The three NuSTAR telescopes have direct heritage
to the completed HEFT flight optics.
The 10m NuSTAR mast is a direct adaptation of the
60m mast successfully flown on SRTM.
NuSTAR det-ector modules are the HEFT flight
units.
Launch vehicle Pegasus XL
Launch date 2009
Mission lifetime 3 years
Coverage Full sky
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NuSTAR science point sources

• One of the main goals of NuSTAR is to conduct a
  census of hard X-ray sources over a limited part
  of the sky
• What are the hard X-ray properties of AGN?
• How do they contribute to the peak of the CXB?

Spectrum of NGC 4945, a heavily obscured active
galaxy
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Spectrum of the blazar PKS 1127-145

• Best probe of the content of the jet will be the
  hard X-ray / soft gamma-ray observations,
  simultaneous with GLAST
• Bulk of the radiating particles is actually at
  low energies, inferred only from hard X-ray
  observations

40
Extended sources with Astro-E2 Hard X-ray
Detector, and with NuSTAR

• Besides compact sources such as AGN and binaries,
  diffuse sources are also great targets
• In supernova remnants, hard X-rays might point to
  the origin of cosmic rays
• Examples Cas-A, Kepler on the right
• Hard X-ray emission from clusters is also
  expected via energetic electrons (inferred from
  radio data) by Compton-scattering the CMB (see
  Abell 2029 on the right)

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Thank you!