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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
nF n
n
Compton emission
nF n
n
g2
g1
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)
gg2
g1
g-(q+1)
gb
g-q or g-2
g2
gb
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
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
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 < < 43q 0
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 < < 73q 0
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 08/1992
Mrk501 HBL 0.034 01/1996
1ES 2344+514 HBL 0.044 07/1998
Mrk 180 HBL 0.045 09/2006
1ES1959+650 HBL 0.048 08/1999
AP Lib LBL 0.049 07/2010
PKS 0548-322 HBL 0.069 07/2007
BL Lacertae LBL 0.069 05/2005
PKS 2005-489 HBL 0.071 06/2005
RGBJ0152+017 HBL 0.08 02/2008
SHBL J001355.9… HBL 0.095 09/2010
W Comae IBL 0.102 08/2008
1ES 1312-423 HBL 0.108 12/2010
PKS 2155-304 HBL 0.116 06/1999
RGB J0710+591 HBL 0.125 03/2009
1H 1426+428 HBL 0.129 02/2002
1ES 1215+303 LBL 0.13 01/2011
Name Class z Date1ES 0806+524 HBL 0.138 02/2008
1ES 0229+200 HBL 0.14 02/2006
1RXS J101015.9… HBL 0.142 12/2010
1H 2356-309 HBL 0.165 04/2006
1ES 1218+304 HBL 0.182 05/2006
1ES 1101-232 HBL 0.186 04/2006
1ES 0347-121 HBL 0.188 08/2007
RBS 0413 HBL 0.19 10/2009
PKS 0447+439 HBL 0.20 12/2009
1ES 1011+496 HBL 0.212 09/2007
1ES 0414+009 HBL 0.287 11/2009
S5 0716+714 LBL 0.318 04/2008
1ES 0502+675 HBL 0.341 11/2009
PG1553+114 HBL 0.35 03/2006
PKS 1510-089 FSRQ 0.36 03/2010
4C+21.35 FSRQ 0.432 06/2010
3C66A IBL 0.444 ? 03/2008
3C279 FSRQ 0.536 06/2008
PKS1424+240 IBL ??? 06/2009