Markus BӧttcherOhio University

Athens, OH

VHE Gamma-Ray Induced Pair Cascades in Blazars

and Radio Galaxies

Leptonic Blazar ModelsRelativistic jet outflow with G ≈ 10

Injection, acceleration of ultrarelativistic

electrons

Qe (g,

t)

g

Synchrotron emission

nFn

n

Compton emission

nFn

n

g2g1

g-q

Radiative cooling ↔ escape =>

Seed photons:

Synchrotron (within same region [SSC] or slower/faster earlier/later emission regions [decel. jet]), Accr. Disk, BLR, dust torus (EC)

Qe (g,

t)

gg2g1

g-(q+1)

gb

g-q or g-2

g2gb g1

gb: tcool(gb) = tesc

Spectral modeling results along the Blazar Sequence: Leptonic Models

High-frequency peaked BL Lac (HBL):

No dense circumnuclear material → No strong external

photon field

Synchrotron

SSC

Low B fields (~ 0.1 G);

High electron energies (up to TeV);

Large bulk Lorentz factors (G > 10)

The “classical” picture

Spectral modeling results along the Blazar Sequence: Leptonic Models

Radio Quasar (FSRQ)

Plenty of circumnuclear material → Strong external photon field

SynchrotronExternal Compton

High magnetic fields (~ a few G);

Lower electron energies (up to GeV);

Lower bulk Lorentz factors (G ~ 10)

Spectral modeling with pure SSC would require extreme parameters(far sub-equipartition B-field)

3C66A October 2008

Intermediate BL Lac Objects

Including External-Compton on an IR radiation field allows formore natural parameters and near-equipartition B-fields

(Abdo et al. 2011) (Acciari et al. 2009)

gg-Absorption Features?

Şentürk et al., in prep.

Absorption trough from Lya expected at

Eabs~ 25/(1+z) GeV.

CAVEAT: • Non-simultaneous data• Vastly different integration times

gg-Absorption Features?

M. Errando - Şentürk et al., in prep.

gg-Absorption Features?

M. Errando - Şentürk et al., in prep.

VHE g-ray production within dense external radiation fields

HE g-ray Detections of Radio Galaxies:M 87 , NGC 1275, 3C 120, 3C 270, 3C 3803C 111, NGC 6251, Cen A

Fermi EGRET + Fermi

VHE Gamma-Ray Induced Pair Cascades

gg absorptionPair cascades Deflection by B-fields

The trajectories of the particles are followed in full

three-dimensional geometry.

Power-law spectrum of HE – VHE g-rays from the inner jet

Arbitrary ext. photon spectrum

a) Monoenergetic optical/UV (BLR)b) Thermal IR (torus)

Model Setup

In AGN env.: Inverse Compton

Scattering dominates

Deflection, isotropization

Compton supported cascades

Compton vs. Synchrotron

General Considerations

• Electron/positron escape:

• Energy-dependent Isotropization -> High-energy spectral turnover:

qdefl(lIC) = qobs

tesc(g) = tcool(g) -> Eesc ~ Es g2

Viewing Angle B = 1 mGqB = 5o

uext = 10-5 erg cm-3

Rext = 1018 cmTBB = 1000 K

m= cosqobs

External Radiation Field, uext

Magnetic Field B

q ~ 60o B = 1 mG

For moderate/large inclination angles, isotropization becomes very efficient!

U ext = 5* 10 -2 erg/cm -3

R ext = 10 16 cm

B= 1 mG, q B = 80

300 < q < 430

D = 74 Mpc LBLR = 1.6x1042 erg/sq ≈ 30o – 55o

L BLR = 4 p R2ext c uext

Incident (forward) g-ray spectrum normalized to a moderately bright Fermi blazar

Application to NGC 1275

(Roustazadeh & Bӧttcher 2010)

D = 3.7 MpcViewing angle ~ 50o – 80o

T = 2300 K → peak frequency at K-band

Fit to the broad-band SED (Boettcher & Chiang 2002)Cascade Emission

Sum of both contributions

U ext = 1.5* 10 -3 erg/cm -3

R ext =3* 10 16 cm

B= 1 mG, q B = 40

670 < q < 730

L BLR = 4 p R2ext c uext

= 5*1041 ergs-1

nLn ~ 6X1041 erg/sR ~ 6x1017 cm

Leptonic fit by Abdo et al. (2010) required gmax = 108!

Application to Cen A

(Roustazadeh & Bӧttcher 2011)

1- Leptonic models prefer external-Compton over SSC in non-HBL blazars -> VHE g-ray emission in intense external radiation fields -> gg absorption (detectable by Fermi+CTA)-> Compton supported pair cascades.

2 - VHE gamma-ray induced cascades are effectively isotropized even in weak perpendicular (By) magnetic fields (By ~ mG) -> MeV- GeV gamma-ray flux in directions misaligned with respect to the jet axis.

3 - Fermi detections of radio galaxies (NGC 1275, Cen A) can be modeled as off-axis VHE gamma-ray induced pair cascade emission.

Summary

Blazar Classification

Quasars:

Low-frequency component from radio to optical/UV,

nsy ≤ 1014 Hz

High-frequency component from X-rays to g-rays, often dominating

total power

(Hartman et al. 2000)

High-frequency peaked BL Lacs (HBLs):

Low-frequency component from radio to UV/X-rays,

nsy > 1015 Hz

often dominating the total power

High-frequency component from hard X-rays to high-energy

gamma-rays

Low-frequency peaked / Intermediate BL Lacs (LBLs/IBLs):

Peak frequencies at IR/Optical and GeV gamma-rays,

1014 Hz < nsy ≤ 1015 Hz

Intermediate overall luminosity

Sometimes g-ray dominated

(Abdo et al. 2011)

3C66A

(Acciari et al. 2009)

TeV g-Ray BlazarsName Class z DateMrk421 HBL 0.031 08/1992Mrk501 HBL 0.034 01/19961ES 2344+514 HBL 0.044 07/1998Mrk 180 HBL 0.045 09/20061ES1959+650 HBL 0.048 08/1999AP Lib LBL 0.049 07/2010PKS 0548-322 HBL 0.069 07/2007BL Lacertae LBL 0.069 05/2005PKS 2005-489 HBL 0.071 06/2005RGBJ0152+017 HBL 0.08 02/2008SHBL J001355.9… HBL 0.095 09/2010W Comae IBL 0.102 08/20081ES 1312-423 HBL 0.108 12/2010PKS 2155-304 HBL 0.116 06/1999RGB J0710+591 HBL 0.125 03/20091H 1426+428 HBL 0.129 02/20021ES 1215+303 LBL 0.13 01/2011

Name Class z Date1ES 0806+524 HBL 0.138 02/20081ES 0229+200 HBL 0.14 02/20061RXS J101015.9… HBL 0.142 12/20101H 2356-309 HBL 0.165 04/20061ES 1218+304 HBL 0.182 05/20061ES 1101-232 HBL 0.186 04/20061ES 0347-121 HBL 0.188 08/2007RBS 0413 HBL 0.19 10/2009PKS 0447+439 HBL 0.20 12/20091ES 1011+496 HBL 0.212 09/20071ES 0414+009 HBL 0.287 11/2009S5 0716+714 LBL 0.318 04/20081ES 0502+675 HBL 0.341 11/2009PG1553+114 HBL 0.35 03/2006PKS 1510-089 FSRQ 0.36 03/20104C+21.35 FSRQ 0.432 06/20103C66A IBL 0.444 ? 03/20083C279 FSRQ 0.536 06/2008PKS1424+240 IBL ??? 06/2009