Greg Madejski (KIPAC/SLAC)

With Masaaki Hayashida, Marek Sikora, Jun Kataoka, Lukasz Stawarz

for the Fermi-LAT collaboration

Fermi results on AGN:

Insights from multi-band observations

Fermi sky

after two years: Projection

in Galactic coordinates

As expected, Fermi is detecting many blazars

A number (all?) of those are showing rapid variability, flare-like behaviorA number of papers on individual sources is in the press

First list of gamma-ray bright AGN is in press; spectral and variability papers published

--------------------------Blazars are sources of radiation in nearly all measured bands: many are known for their

strong TeV gamma-ray emission

What is the nature/structure of blazars? Approach to their study is like peeling an onion: study the broad-band spectrum and variability, understand the radiation process, deduce the process that energizes the radiating particles, infer the ultimate energy source

-> relativistic jet pointing close to our line of sight powered by accretion of matter onto a black hole in the center of the host galaxy

• Multi-band observations are essential: variability! • Participating observatories : more than 20 instruments

Multi-band observations

* Includes optical polarization datafrom Kanata (in Japan) and KVA (in La Palma)* TeV observatories

3

Gamma-ray: > 200 MeV

RXTE-PCA

X-ray

Fermi-LAT (uninterrupted monitoring)

Swift-XRT

Consensus regarding the emission processes in all blazars

General spectral behavior: two broad peaks: - one in radio – to optical bands - another in X-ray to gamma-ray bands

Best picture for the emission processes:

- Low energy peak emission is via synchrotron process- High energy peak is produced by inverse Compton process

-Big questions: what is the structure of the jet? Location of radiation region?

Pre-Fermi broad-bandspectrum of blazar 3C454.3(more on it later!)

Population properties: spectral diversity

<- High luminosity sources low luminosity sources -> soft spectra hard spectra

• Clear correlation of -ray spectrum with blazar sub-class• This has strong implications on contribution of blazars to the diffuse extragalactic gamma-ray background: still work in progress, but we expect both types would contribute at some level

Gamma-ray spectra in the Fermi band are quite diverse (based on the 3-month “LAT Bright AGN Survey” (LBAS): Abdo et al. 2009)

Fermi LAT spectra of blazars in the context of broad-band spectra - Clear spectral diversity!

all

BL Lacs

FSRQs

<>= 2.33+/-0.01

<>=1.99+/-0.01

Photon index

Nu

mb

er

of

so

urc

es

FSRQs

Lo

g

F

Log

BLLacs

Those are commonly detected as TeV sources

Origin of spectral diversity of blazars / implication on their structure

• Fermi+broad-band data confirm the “blazar sequence”

higher luminosity sources have respective synchrotron and Compton peaks at lower energies and vice versa

• What does it tell us about radiating particles?

• Electrons producing spectral peaks of HBL blazars have higher energy than those in FSRQ blazars

• One hint:

* HBLs have relatively lower luminosity than FSRQs

* Both classes have high black hole mass, ~ 109 Mo

-> HBLs have lower luminosity in Eddington units -> lower accretion rate in Eddington units

“Blazar sequence” (Fossatti et al. 1998)

Origin of spectral diversity of blazars / connection to the accretion properties

• If HBLs have indeed lower luminosity in Eddington units -thus lower accretion rate in Eddington units – this implies:

* Luminous, quasar-type, FSRQ blazars have cold, luminous accretion disks * Low-luminosity, HBL-type blazars have hot, advective, underluminous accretion flows

• This scenario can explain the difference in electron energies in the two classes:

* Maximum electron energy is determined by the competition of acceleration and cooling of electrons

* In FSRQs, luminous accretion disk provides ample external photons that are in turn seeds for Compton-upscattering by energetic electrons -> Maximum electron energy is limited

* In HBL blazars, such external photon field is much weaker – cooling less effective - -> electrons can attain higher energies

Gamma-ray spectra of BL Lac type blazars

• Examples of broad-band -ray spectra of HBL-type blazars: Mkn 421, PKS 1553

* Those are often TeV sources

• Spectra hard, roughly connect to that measured by TeV instruments – but a sharp break! - highly variable - need to measure spectra simultaneously

Gamma-ray spectrum of PKS 1553 (above)and gamma-ray spectrum & MAXI light curve of Mkn 421 (right)

Gamma-ray variability properties of blazars

Flares generally symmetrical, occasionally faster rise time than decay time

Nearly-continuous, well-sampled -ray light curves allow a robust measurement of Power Density Spectra of blazars

PDS well-described as a power lawwith an index 1.5 +/- 0.2

PKS 1502+106

Mis-aligned blazars are also -ray emitters

• NGC 1275, a.k.a. 3C84 or Perseus A, is a nearby radio galaxy containing a flat-spectrum, compact (VLBI-scale) blazar-like, variable radio source

• It has been detected in the Fermi LAT data, at a much higher level than the upper limit from EGRET -> variable (on ~ year time scales)

• It is located in the Perseus cluster of galaxies, but variability clearly excludes the association of -rays with the cluster

• Jet is not pointing close to the line of sight

• Seyfert-like characteristics of the nucleus

-ray emission also seen from “mis-aligned” jets

• Fermi discovered several more, both FR-I and FR-II• Luminosity generally low, ~1045 erg/s

The jet associated with the active galaxy is most likely responsible for inflating the “cavities’’ seen in the

Chandra images of the Perseus cluster

Variability of NGC 1275 in the LAT bandFermi spectra of FR-II Luminosity distribution of gamma-ray emitting AGN

FSRQ 3C 279 (z = 0.536)

• One of the EGRET-brightest AGN– Apparent luminosity: as high as 1048 erg/s– Can vary in -ray flux by roughly two orders of

magnitude– > 100 GeV emission detected by MAGIC – Black hole mass: ~ 6 x 108Msolar

© NRAO / AUI / NSF

Radio VLBI image shows superluminal motion: jet ~ 15, angle to line of sight ~ 2o

Adopted EGRET-era emission model radio- optical : synchrotron X-ray : SSC Gamma-ray : EC (disk+BEL)

1996 flare

Wehrle et al. 2001

12

-ray

X-ray

-ray (LAT)

-ray photon index (LAT)

X-ray

optical-UV

optical polarization degree (PD)

optical polarizationangle (EVPA)

Near-Infrared

Radio

Lack of mm variability, via synchrotron self-absorption, constrains the size of the /optical emission region :Rb < 5 x 1016 cm 13

Light curves covering ~ a year starting July 2008 (Abdo et al. 2010, Nature 463, 919;more recent data in a few VG)

Gamma-ray flare with polarization change

• flux vs. polarization degree (PD)

-ray flare coincides with the drop of PD– fluxes decrease, followed by a

temporary recovery of PD

clear correlation:

- a highly ordered magnetic field

• polarization angle (PA or EVPA)

– during the low PD, it gradually decreases by 208 deg (~12 deg/day)

– -> non-axisymmetric structure of jet

The event lasts for ~ 20 days

opt.

PD

EVPA

14

20 days

The rotation of optical polarization angle seems to be a common feature of blazars (R. Itoh, M. Uemura posters) – conclusions might be more general than just 3C279

Location of the dissipation region

-ray flare correlated with polarization change : duration ~ 20 days-> constrains on the location of the ray/optical emission region

1. helical magnetic field model(Marscher et al. Nature 2008)

2. curved jet model

3. “flow-through” (“quivering jet”) scenario(emission pattern may move much

slower than the bulk speed of the jet)

emission region can be located closer (x 2jet) to BH :

revent ~ ctevent ~ 103 gravitational radii

location of the emission region : ~ 105 gravitational radii

jet

jet

15

Nu

me

rica

l sim

ula

tion

s b

y

Mc

Kin

ne

y +

Bla

nd

ford

2

00

8)

Best model for the jet structure?

• Comparison to previous EVPA observations

(Larionov et al. 2008) (our results)

(2) curved jet model(1) helical magnetic field (3) flow-through

Scenarios 1 and 3 imply the rotation of EVPA should always rotate the same direction-> But, not in the observations: needs further confirmation

16

jet

jet

© Nature, vol436, 887 (2010) A.Young

Favored (with caution)

Spectral Energy Distribution

1.70+/-0.13

2.38+/-0.08

2.57+/-0.10

1.62+/-0.10

-ray emission is dominant and most variable -

several x 1047 erg/s at the peak• comparable photon indices between two states• With emission region “far,” Comptonization of

the dust-originating IR radiation is favored

(Sikora+ 2009)

Red Blue

(number: photon index)

17

Isolated X-ray flare

• a significant, symmetrical flare at 54950 MJD (~60 days after the -ray flare)

– the optical and -ray emission is in a lower state – the X-ray spectrum is much harder than the optical spectrum

• likely to be generated by IC scattering of low energy electrons– duration ~ 20 days; similar profile to the -ray flare

• X-ray flare cannot be a simple delayed version of the -ray flare (due to, e.g, particle cooling)

– Still, the -ray emission is dominant; 5 times higher the energy flux even during the isolated X-ray flare event

challenge to the simple, one-zone emission models

-ray

X-ray

18

So – what’s been happening to 3C279 lately?

• Relatively less active than last year – but some activity in -rays and X-ray band

- It is too close to the Sun for optical monitoring, but that is restarting

3C454.3 with LAT

• Shows rapid flares, with the risetime on a scale of ~3 days - compact emission region

• Can’t be optically thick to the escape of -rays via e+/e- pair production -> another way to determine Doppler factor

• Here, > 6 – consistent with the VLBI-measured jet geometry

Vital statistics:

* Well-known radio, EGRET gamma-ray source - an OVV quasar at z = 0.859

* Good multi-epoch VLBI data, superluminal exp., = 25, jet ~15, ~0.8o

* Very active (bright, variable) since 2000

Snapshot of the broad-band spectrum for Aug 7-12 2008

3C454.3 with LAT: -ray spectrum

• Fermi LAT data: • -ray spectrum not a simple power law

• It steepens to higher energy – can be described as a broken power law with a break, 1 ~ 2.3 to ~3.5 at Ebr ~ 2 GeV

* Origin of the spectral break? Not a simple “cooling break” (expected =0.5) – more likely a signature of intrinsic break in the electron distribution, with the of electrons radiating at the break of ~ 3000

• Similar breaks are seen in many of the high luminosity (FSRQ) blazars

• The cooling time scales are quite short, much shorter than the source crossing time - imply distributed acceleration throughout the jet volume

Progress on the modelling front

• Gamma-ray spectra of many bright blazars show relatively sharp spectral break (by ~ 1), similar to 3C454.3 (Abdo et al. 2010, paper on blazar spectra)

• This has been interpreted as possibly due to Klein-Nishina effect via drop of cross-section for Compton scattering (Finke & Dermer 2010)

• Here the IC “seed” photons are BLR

• Detailed modelling (applied to 3C454.3 flare) indicates that the break produced by

KN is via upscattering of BLR photons not sufficiently sharp

(Abdo et al. 2010)

• Break in the energy spectrum of the radiating particles is still the best scenario

3C454.3 -ray spectrum with KN model in red (Abdo et al. 2010)

Late-breaking news on the 3C454.3 front

• 3C454.3 has been flaring again in mid-November

* In the Fermi LAT band it was the most luminous source in the sky (besides gamma-ray bursts): isotropic luminosity of almost 1050 erg/s

* Active in other bands: currently probably at the historical high in X-rays (Swift)

Recent light curves of 3C454.3 in the Fermi band (left) and Swift band (right)Should be detectable by MAXI!

Summary

• Fermi sensitivity allows detailed studies of jet-dominated AGN: blazars, mis-oriented jets (radio galaxies)

• Clear differences between luminous FSRQs and low-luminosity HBLs:

differences probably governed by the accretion rate

• For FSRQ objects as bright as 3C279 or 3C454.3,

Fermi-LAT can resolve the -ray activity on daily scales

In the gamma-ray emitting FSRQs,-ray band is energetically dominant

* 3C279: -ray flare associated with optical polarization change lasting ~ 20 days

– evidence for the presence of highly ordered magnetic filed

– non-axisymmetric trajectory of the emission pattern

– constraints on the location of the dissipation region

• Isolated X-ray flare challenges the simple,

one-zone emission models

• Spectral break seen in the -ray band not due to KN effects

• 3C454.3 – current “record holder” for most luminous

source in the Universe

Long-term multi-band observations including Fermi-LAT and optical polarization are essential for investigating the structure of quasar jets 24