Fermi AGN:
Open Questionsand Looking Forward
Lukasz Stawarz
KIPAC/SLAC
Stanford University
Special thanks to R.D. Blandford, as well as to my Fermi colleagues
C.C. Cheung, J. Kataoka, G. Madejski, and D. Paneque, for many discussions.

Active Galactic Nuclei (AGN)•

AGN activity is powered by the enhanced accretion unto supermassive black
holes (SMBHs) residing in galaxy centers. As such, AGN enable us to study
the extreme physics of SMBHs, their accretion disks, and their surrounding
media.

•

AGN activity is triggered by galaxy mergers/interactions. As such, AGN activity
is directly linked to the structure formation in the Universe. But AGN are not
only passive witnesses/by-products of galaxy formation! Outflows, jets, andhigh-energy radiation produced in AGN may substantially influence thesurrounding (galactic and intergalactic) medium, modifying therefore thestructure formation itself via some complex feedback process. Studying how
AGN evolve with redshift is therefore important for understanding
cosmological evolution of galaxies in general.

•

AGN are established sources of broad-band electromagnetic emission, and thehigh-energy γ-ray photons in particular. Maximum energies of ultrarelativistic
particles produced thereby exceed by orders of magnitude maximum energies
accessible in our accelerators. As such, AGN enable us to study fundamental
properties of subatomic particles, cosmic-ray acceleration, and the physics
of ionized collisionless magnetized plasma, which is not accessible in ourlaboratories, but which constitutes a significant part of the baryonic
Universe.

I. AGN Zoo
AGN come in many many flavors… They differ in the properties of their large-scale
environments, in the properties of their host galaxies, in the accretion rates andaccretion fuels, in the structure and state of the circumnuclear matter, andfinally in the properties of their outflows:
•

•
•

•

•

Quasi-Stellar Objects (quasars or QSOs; ~ 10-7 Mpc-3)
Radio-quiet quasars (RQQs)
Radio-loud quasars (RLQs)
Flat Spectrum vs Steep Spectrum Radio Quasars (FSRQs vs SSRQs)
BL Lacertae Objects (BL Lacs; ~ 10-7 Mpc-3)
Radio Galaxies (RGs; ~ 10-6 Mpc-3)
Broad Line vs Narrow Line Radio Galaxies (BLRGs vs NLRGs)
Fanaroff Riley class I vs class II (FR Is vs FR IIs) - but not only!
WATs, NATs, XRGs, DDRGs, HYMORS, GPS/CSOs, CSS/MSOs…
Seyfert Galaxies (Sys; ~ 10-4 Mpc-3)
Type 1 Seyferts - Type 2 Seyferts (Sy 1s - Sy 2s)
Narrow-Line Seyferts (NLSys)
Low-Luminosity AGN (LLAGN; > 10-3 Mpc-3)
Low-Ionization Nuclear Emission-Line Region Galaxies (LINERs)“Regular” Spiral Galaxies…

AGN Unification
GENERAL OPEN QUESTIONS:
What controls the observed diversity of AGN?
Is our current understanding of the AGN unification sufficiently good?
Why only some AGN are radio loud? What controls jet production efficiency in different
types of AGN?
Urry & Padovani
Unification Scheme(s)•anisotropic obscuration of a nuclear emission(Sy 1s  Sy 2s)•
relativistic beaming of a jet emission(FSRQs/BL Lacs  SSRQs  BLRGs  NLRGs/FR Is)•
accretion rate
(QSOs/FR IIs  BL Lac/FR Is)•
black hole mass
(Sys  NL Sys)•
age of a radio structure(CSOs  MSOs  RGs)•
Spin of SMBHs(RL  RQ)

OPEN QUESTIONS FOR FERMI (I):
What are the γ-ray properties of different types of AGN?
Are radio quiet AGN γ-ray emitters at some level?
Is the γ-ray emission of RL AGN shaped by the jet properties (on small and large scales)
and/or by the properties of the accreting matter?
(Fermi AGN: FSRQs, BL Lacs, FR Is, NLSys)

AGN Phenomenon
1)
2)
3)

All galaxies host SMBHs in their centers (106-1010M)All SMBHs accrete at some level, and all show some AGN-like activity (1036-1048 erg/s)
Radio quiet AGN are not radio silent!
Mrk 766

Mrk 279

Seyferts, LINERs & Spirals: nuclear and extended radio emission
due to the jet or the starburst actvity? [Ho et al.]1045+352

Broad Absorption Line Quasars, believed to beradio quiet as a class, do produce relativistic jets
[Siemiginowska et al.]

PG 0157+001

Radio Quiet Quasars: nuclear radio emission due to thejet activity, accretion disk/disk coronae, or
uncollimated slow disk outflows?

E1821+643

Radio Quiet Quasars may be sometimes associatedwith relatively low-power FR I jets [Blundell et al.]

Broad-Band AGN Spectra
4)

All AGN are established sources of radio-to-X-ray emission (a mixture of different
thermal and non-thermal components). However, the energy range >100 keV is hardly
explored in this context…

?
Koratkar & Blaes

Template γ-ray spectra
for different types of AGN
constructed with Fermi/LAT?
GeV

Need for a careful investigation/identification of low-flux Fermi/LAT sources,
stacking analysis for different classes of AGN, etc.

γ-ray Emission of RQ AGN?
One can indeed expect some γ-ray emission from non-jetted AGN due to theefficient particle acceleration taking place in the turbulent and magnetizedaccretion disks/disk coronae, as possibly observed in some Galactic sources.
Zdziarski et al.

Cygnus X-1
detected atradio, 1-10 MeV
and possibly also
0.1-1 TeV
photon energies

Galactic Center
Sgr A* detected atradio, near infrared,
X-ray, and TeV
photon energies

Liu et al.

Already Detected?

Di Cocco et al.

Fermi/LAT has detected Narrow Line
Seyfert galaxy PMN J0948+0022.
Previously, NL Sys have been considered asradio quiet in general. The particular source
PMN J0948+0022 is radio loud, being
characterized by a flat-spectrum radio
core. So it is “just” a blazar.
The X-ray-to-γ-ray emission of PMN
J0948+0022 is modeled in a framework of
the blazar scenario (compact relativistic
jet close to the SMBH).

EGRET source 3EG J1736-2908 has been
claimed to be associated with radio quiet
Seyfert 1 galaxy GRS 1734-292. This
claim has not been confirmed, however.
Still, any meaningful upper limits in the
GeV photon energy range, offering robust
constraints on the population ofrelativistic particles in the accretion
disks/disk coronae of nearby brightSeyferts, are extremely important.

Foschini/Fermi

γ-ray Loud Blazars
missing BL Lacs?

Most of the detected γ-ray loud AGN
are blazars (FSRQs and BL Lacs).
It may seem that some general
correlations for those have been
already established.
Is it indeed the case? Ce we alreadyassure we are not missing some steepspectrum low-power BL Lacs?

FSRQs
BLLac - ISP
What seems to be a robust finding, is that
we do not miss powerful blazars (FSRQs)
with flat GeV spectra, Γγ < 2. This implies
that the mean electron energies in thosesources are relatively low, <Ee> < GeV.
Is it simply due to the intense circumnuclear
photon field in FSRQs, and therefore theenhanced radiative cooling of jet electrons?
It may be also noted that BL Lacs form very
diverse population with respect to theirbroad-band spectral properties!

missing BL Lacs?

BLLac - LSP
BLLac - HPBs

γ-ray Loud Radio Galaxies
Centaurus A

Perseus A

Finke, Cheung/Fermi

Virgo A

Cheung/Fermi

The γ-ray emission detected byFermi/LAT from Virgo A, Centaurus A,
and Perseus A radio galaxies is modeled
Kataoka/Fermi
in a framework of the blazar scenario(compact jet close to the SMBHs) withmoderate beaming.
All three radio galaxies are unusual FR Is: nearby, bright and moderately
beamed sources with different circumnuclear environment, different
large-scale environment, and complex large-scale radio morphologies dueto the recurrent jet activity.
We need a larger sample to address common properties of γ-ray loud RGs!

II. Cosmological Context
AGN are detected up to the highest cosmologicaldistances corresponding to redshifts up to z = 6
and beyond, probing thus uniquely and directly the
Universe which was less than Gyr-old (< 10% of its
present age). Unfortunately, huge diversity in theemission properties of active galaxies hampers
using them as standard candles. Nevertheless, if
sufficiently understood, such distant objects
should reveal several fundamental aspects of anearly Universe.
The other issue is the role of accreting SMBHs, andin particular of the jets/outflows formed bythese, in formation of the structures in the
Universe. It is already established that thegrowth of SMBHs is strictly connected with thegrowth of galaxies, and that this connection ishighly non-linear, with accreting SMBHs
influencing substantially the global structure of
the forming system via radiative and mechanicalfeedback. Yet the physics involved remains vague.

Di Matteo et al.

AGN & Cosmology
GENERAL OPEN QUESTIONS:
What is the physics behind the feedback process?
How do AGN jets/outflows interact with the interstellar and intergalactic medium?
Are AGN jets/outflows powerful enough to quench starformation in elliptical galaxies and toheat intracluster gas in cooling flows?
Recent findings:• First quasars forms together
with first galaxies.
• AGN activity is linked to the onset ofand quenching of the starformation
in merging systems.
• AGN activity dominates productionof cosmic background radiation atleast in X-ray frequencies.

Ferrarese & Merritt

OPEN QUESTIONS FOR FERMI (II):What are the γ-ray properties of high-redshift AGN?
Can we probe the evolution of extragalactic background light at optical/UV frequencies withthe γ-ray emission of distant AGN?
Can we explain the extragalactic γ-ray background with the known classes of γ-ray loud AGN?

RL AGN at High Redshifts
z = 4.3

z = 3.89

z = 3.69

z = 2.1

z = 3.6

z = 4.715

z = 3.82

Cheung et al.

z ≈ 0.2–2 (filled circles)z ≈ 2–4 (open circles)z > 4 (large symbols)z > 4 blazars (stars)

All the detected radio loud AGN at high redshifts
are very similar in the spectral properties of theircores or in the large-scale jet morphologies totheir low-redshift analogs.
This is already an exciting finding!
It also suggests that radio loud AGN at highredshifts may be powerful γ-ray emitters.

Bassett et al.

TeV Absorption on EBLPhoton-photon annihilation:〈σγγ〉~ 0.2 σT δ(εε0 - 1)

Aharonian et al/HESS

Still, modern Cherenkov Telescopes probes only narrow(NIR) segment of the extragalactic background light (EBL:
0.1-1000 μm), and relatively nearby Universe (z < 0.2).
Direct measurements of EBL are difficult due to strong
foreground emission. EBL spectral shape reflects star anddust formation history, and therefore probes galaxy
evolution models. Even small uncertainties in the absorbing
photon number density, n0(ε0)∝ τγγ , translate to large
uncertainties in the γ-ray attenuation:
Sobs(ε) = Sint(ε) × exp[τγγ(ε)]

HESS, Magic, andVERITASobservations of
distant BL Lacs
established that the
Universe issurprisingly
transparent for TeV
photons.

GeV Absorption on Evolving EBL?
In principle, with Fermi/LAT we can probe very
high-redshift Universe via absorption of 10-100
GeV photons on the evolving extragalactic
background light at UV/optical frequencies.
Kneiske et al.

giant isotropic pair halos aroundmisaligned high-z blazars?

Broad-Band Cosmic BackgroundRadio background due toall types of AGN andstarforming galaxies
CMB(missing 70% of sources)

IR-UV background due
to stars and dust
(relatively uncertain)

X-ray background due toall types of AGN(missing 60% of sourcesin the hard X-ray band)

γ-ray background due to…???
Dermer?
?
We should aim for a self-consistent interpretation of the recently characterized
broad-band extragalactic background light (in agreement with the structureformation and AGN unification models!).

Extragalactic γ-ray BackgroundAckermann/Fermi• Are we close to resolving all the extragalactic γray background?
• Is this background produced predominantly by
blazars?
• Is the blazar contribution to the measured γ-ray
background consistent with the AGN unification
models (keeping in mind the observed level of theradio and X-ray backgrounds)?

Population Studies with Fermi
Fermi FSRQs:
strong evolution

Ajello/Fermi

Fermi sources: 30%-100% contribution to
the measured γ-ray background
Singal et al.

Fermi BL Lacs:
no evolution

RQ AGN& galaxies

RL AGN

Known radio sources: 30% contribution to themeasured radio backgroundIII. Elusive Relativistic Jets
1021-1024 [cm]matter dominated?

Global large-scale
structure of extragalactic
relativistic jets
Rj ~ 1021 - 1024 [cm]can be modeled with
MHD simulations
Non-thermal processrg ~ Ee/eB~ 109 - 1021 [cm]can be studied with
MC simulations
Microphysics ofcollisionless magnetizedplasma1014-1018 [cm]magnetic dominated?
only non-thermal emission!
(no standard plasma diagnostics…)

λe ~ c/ωe~ 108-109 [cm]can be investigated with
PIC simulations

Jet Physics
GENERAL OPEN QUESTIONS:
How are relativistic jets launched from the vicinities of SMBHs?
What is the jet content? Is the jet composition changing along the outflow?
What are the main processes controlling energy dissipation and particle
acceleration processes in relativistic jets?

McKinney et al.

Merging agreement:• Jets are launched as Poynting flux-dominated outflows
from the ergospheres of SMBHs and/or the innermost
parts of their accretion disks.
• Homogeneous one-zone blazar models require the jets
to be dynamically dominated by (cold) protons.
• The observed non-thermal broad-band jet emission is
predominantly leptonic in origin (implying the presenceof 1-100 TeV energy electrons).

OPEN QUESTIONS FOR FERMI (III):
What is the location and structure of γ-ray-emitting regions in AGN jets?
What are the γ-ray spectra of different types of AGN jets?
What are the underlying electron energy spectra and the particle acceleration processes
involved?
What is the γ-ray duty cycle of blazars? What controls γ-ray variability of blazar sources?

Blazar Emission Zone**

In a framework of the “standard” leptonic blazar scenario,
one-zone homogeneous emission zone is assumed (Maraschi
et al. 91, Dermer & Schlickeiser 93, Sikora et al. 94). This
simple model is relatively successful in explaining several
blazar properties established so far. If this is the caseindeed, the question to be asked is why there is only one,well defined and very compact region of the enhancedenergy dissipation within the outflow, instead of asuperposition of different emission zones (Blandford &Levinson 95)? Also, is it a moving blob or a stationary
feature within the jet? With Fermi/LAT observations
accompanied by truly simultaneous broad-band campaigns,
we can finally confront different model predictions withthe observations, and try to answer these questions!

One zone or multiple zones?
Homogeneous region or
stratified region?

Where Is Blazar Emission Zone?
We can use the following constraints:
1.
Characteristic (PSD) and shortest(flux doubling) variability timescales
2. Opacity for the observed γ-ray
photons (both internal and external to
10 eV 10 eV
the emission zone)
100 keV
3. Luminosity ratios between synchrotron
and inverse-Compton components
0.01 eV 100 keV
4. Spectral position of the peak
0.1 eVfrequencies for the synchrotron andinverse-Compton components
5. Presence and position of different
spectral breaks (radiative coolingbreaks, Klein-Nishina breaks, etc.)
6. Lack of bulk-Compton and absorption
Detailed modeling of the broad-band blazar
features in broad-band spectra
7. Correlations between variable fluxes spectra performed so far typically suggests thatthe blazar emission zone in FSRQs is locatedin different frequency ranges
relatively far from SMBHs, ~ 1018 cm ~ 104 Rg.
8. Correlations between high-energy
Still, distances outside of this range are notflares with morphological changes of
excluded. In particular, for BL Lacs the blazar
resolved radio structures
emission zone seems to be located much closer to
9. Correlation between high-energy
SMBHs. Fermi/LAT data will enable to criticallyflares with changes in radio/opticalre-examine all the discussed constraints!
polarization

Structure of the Blazar Zone

Spine - shear layer scenario (Ghisellini et al.)Multi-blob scenario (Lenain et al.)Decelerating jet scenario (Georganopoulos et al.)Jet-in-jet scenario (Giannios et al.)Complex (confusing!) pattern of a broad-band rapid variability established for several sources,
as well as the detections of radio galaxies at high energy γ-ray photon energy range, let to theemergence of stratified models for the blazar emission zone.
More truly simultaneous broad-band data for a larger sample of objects are needed to verify
the proposed scenarios, and to understand the true structure of the emission region.

High Energy Blazar Spectra
3C 454.3

Kataoka et al.
EGRET

Fermi

Madejski, Lott/Fermi

Bright FSRQs reveal repeatedly very flat X-ray
spectra of power-law forms with photon indicesΓX ≤ 1.5, and steep γ-ray spectra of broken
power-law form with photon indices Γγ > 2 withbreaks ΔΓ > 0.5. Such high-energy (inverseCompton) spectra deviate substantially fromthe ones expected in a framework of a“standard” scenario (a low-energy Γlow = 1.5
continuum modified at high energies byradiative cooling to give Γhigh = 2.0). The highenergy spectral breaks observed by Fermi/LAT
seem to be due to intrinsic breaks in theunderlying electron energy distribution.

Underlying Electron Spectra
The most surprising finding is that therevealed photon (and therefore electron)
spectra do not depend on the activity
state of a source! This, again, is not what
we have expected to observe….
FSRQ
3C 454.3
Madejski, Lott/Fermi

In FSRQs, the energy distribution of the radiating
electrons seem to be of the form ne(E) ∝
E-s1
E-s2

for
for

Emin < E < Ebr
E > Ebrwiths1 ≤ 2, s2 > 2, Emin ~ MeV, Ebr ~ 0.1-1 GeVSuch electron spectra differ substantially fromthe ones expected in the case of a diffusive shock
acceleration (non-relativistic test-particle limit).

At the moment, the theory of particle
acceleration in relativistic regime is notquantitative enough to make robust
predictions regarding emerging particle
spectra. Fermi observations will help todevelop theoretical models of particle
acceleration in relativistic plasma!

“Supercritical” Phenomena?

BL LacMrk 421
Paneque/Fermi

For example, “photon breeding” model by Stern & Poutanen.

What about “supercritical”
phenomena, which are atsome level inevitable forrelativistic outflows in adense radiativeenvironment? In general,
what about absorption
effects and emission of
secondary particles? Can
we find any spectral
signatures for such?
Excellent Fermi data will
enable to look for these.

Looking Forward• This is an exciting time for AGN research, since for the very first time
truly multiwavelength and simultaneous high-quality data can begathered for a large number of sources.
• Despite intense investigations in the past, the physics of AGN and theirrelativistic outflows still remains elusive. On the other hand, during thelast years a substantial progress in this field has been made, mostly due
to the development in numerical modeling and observational techniques.
• Fermi/LAT will definitely help to answer several open questions
regarding the physics of AGN in a near future, for example:
1. What are the γ-ray properties of different types of AGN?
2. Are radio quiet AGN γ-ray emitters at some level?
3. Is the γ-ray emission of RL AGN shaped by the jet properties (on small and large scales) and/or by theproperties of the accreting matter?
4. What are the γ-ray properties of high-redshift AGN?
5. Can we probe the evolution of extragalactic background light at optical/UV frequencies with the γ-ray
emission of distant AGN?
6. Can we explain the extragalactic γ-ray background with the known classes of γ-ray laud AGN?
7. What is the location and structure of γ-ray-emitting regions in AGN jets?
8. What are the γ-ray spectra of different types of AGN jets?
9. What are the underlying electron energy spectra and the particle acceleration processes involved?
10. What is the γ-ray duty cycle of blazars? What controls γ-ray variability of blazar sources?