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
Compton Gamma-ray Observatory
Featured instruments sensitive from ~ 40 keV (OSSE)
up to nearly 100 GeV (EGRET)
Global observational differences between:
Radio-quiet
active galaxies
and
jet-dominated active galaxies
(a. k. a. blazars)
no
MeV – GeV emission
strong
MeV – GeV emission
Strong signatures of
circumnuclear matter
(symmetric emission lines)
Only weak signatures…
No strong compact radio
structure
Radio, optical, X-ray cores and jets
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
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
Unified picture of active galaxies
•
Presumably all AGN have the
same basic ingredients: a
black hole accreting via disklike 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
Weighing the central black hole
Radio galaxy M87 (Virgo-A) studied
with the HST
•
•
•
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
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-emitting) AGN is a power law,
photon index ~ 2, with exponential-like cutoff at ~ 200 keV
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
X-ray Background Spectrum (from G. Hasinger)
Revnivtsev et al., 2003 RXTE
XMM LH resolved
Worsley et al. 2004
from Gilli 2003
=> E<2 keV XRB resolved (Chandra, XMM); at E>5 keV still lots of work...
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… What’s the absorption geometry?
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
Example of radio map of a blazar 3C66B
EGRET All Sky Map (>100 MeV)
3C279
Cygnus
Region
Vela
Geminga
Crab
Cosmic Ray
Interactions
With ISM
LMC
PSR B1706-44
PKS 0208-512
PKS
0528+134
Broad-band spectrum of the archetypal GeV blazar 3C279
Data from Wehrle et al. 1998
Example: broad-band spectrum of the TeV-emitting
blazar Mkn 421
Data from Macomb et al. 1995
Blazars are variable in all observable bands
Example: X-ray and GeV g-ray light curves from the 1996 campaign to
observe 3C279
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
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
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 selfCompton” 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
SSC or ERC?
Example of an object
where SSC may
dominate: Mkn 421
Example of an object
where ERC may
dominate: 3C279
(data from Macomb et al. 1995)
(data from Wehrle et al. 1998)
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
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?
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 – (< 0.1%) so there might be a 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 THE DISK – JET CONNECTION
AS WELL
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
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 – (< 0.1%) so there might be a 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
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-Comptonupscattered 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
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.)
EGRET all-sky survey (galactic coordinates) E>100 MeV
* High latitude (extragalactic)
point sources – blazars and new
sources? - typical flux from
EGRET sources 10-7 - 10-6 cm-2s-1
* Need an instrument with a good sensitivity and
a wide field of view
* GLAST – currently under construction - will be
launched in 2007
GLAST LAT instrument overview
g
Si Tracker
ACD
pitch = 228 µm
8.8 105 channels
12 layers × 3% X0
+ 4 layers × 18% X0
+ 2 layers
Segmented
scintillator tiles
0.9997 efficiency
 minimize self-veto
Grid (& Thermal
Radiators)
CsI Calorimeter
e+
Hodoscopic array
8.4 X0 8 × 12 bars
2.0 × 2.7 × 33.6 cm
 cosmic-ray rejection
 shower leakage
correction
e–
3000 kg, 650 W (allocation)
1.8 m  1.8 m  1.0 m
20 MeV – >300 GeV
LAT managed at
SLAC
Flight Hardware & Spares
Data
acquisition
16 Tracker Flight Modules + 2 spares
16 Calorimeter Modules + 2 spares
1 Flight Anticoincidence Detector
Data Acquisition Electronics + Flight Software
Schematic principle of operation of the GLAST
Large Area Telescope
* g-rays
interact with
the hi-z material in
the foils, pairproduce, 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
GLAST LAT Science Performance Requirements Summary
Parameter
Requirement
Peak Effective Area (in range 1-10 GeV)
>8000 cm2
Energy Resolution 100 MeV on-axis
<10%
Energy Resolution 10 GeV on-axis
<10%
Energy Resolution 10-300 GeV on-axis
<20%
Energy Resolution 10-300 GeV off-axis (>60º)
<6%
PSF 68% 100 MeV on-axis
<3.5°
PSF 68% 10 GeV on-axis
<0.15°
PSF 95/68 ratio
<3
PSF 55º/normal ratio
<1.7
Field of View
>2sr
Background rejection (E>100 MeV)
<10% diffuse
Point Source Sensitivity(>100MeV)
<6x10-9 cm-2s-1
Source Location Determination
<0.5 arcmin
Sensitivity of GLAST LAT
GLAST LAT has much higher sensitivity to weak
sources, with much better angular resolution
GLAST
EGRET
GLAST LAT’s ability to measure the flux and spectrum of
3C279 for a flare similar to that seen in 1996
(from Seth Digel)
NEAR FUTURE: Astro-E2
* The future is (almost) here:
Next 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 can’t
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
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,
NuSTAR was recently selected for extended study, with the
goal for launch in 2009 (Fiona Harrison/Caltech, PI)
It’s 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
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.
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 detector
modules are
the HEFT
flight units.
Orbit
525 km 0° inclination
Launch vehicle
Pegasus XL
Launch date
2009
Mission lifetime
3 years
Coverage
Full sky
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
Spectrum of NGC 4945, a heavily obscured active galaxy CXB?
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
Extended sources with Astro-E2 Hard Xray Detector, and with NuSTAR
•
Besides compact
sources such as
AGN and binaries,
diffuse sources are
also great targets:
•
In supernova
remnants, hard Xrays 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 Comptonscattering the CMB
(see Abell 2029 on
the right)
Thank you!