models for non-hbl vhe gamma-ray blazars
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Models for non-HBL VHE Gamma-Ray Blazars
Markus Böttcher
Ohio University, Athens, OH, USA
“TeV Particle Astrophysics”SLAC, Menlo Park, CA, July 13 – 17, 2009
Until a few years ago, all TeV blazars were high-frequency-peaked BLLac objects (HBLs).
Recently, the Intermediate BL Lac objects W Comae and 3C66A (VERITAS), the low-frequency-peaked BL Lac object (LBL) BL Lacertae, and even the FSRQ 3C279
(MAGIC) and were detected in TeV -rays.
Similar in physical parameters to other TeV blazars (HBLs)? (→ SSC dominated?)
Or more similar to LBLs? (→ EC required?)
Motivation
Leptonic Blazar ModelRelativistic jet outflow with ≈ 10
Injection, acceleration of ultrarelativistic
electrons
Qe (,
t)
Synchrotron emission
F
Compton emission
F
-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 (,
t)
-(q+1)
b
-q or -2
b 1
b: cool(b) = esc
Spectral modeling results along the Blazar Sequence: Leptonic Models
High-frequency peaked BL Lac (HBL):
No dense circumnuclear material → No strong external
photon field
SynchrotronSSC
Low magnetic fields (~ 0.1 G);
High electron energies (up to TeV);
Large bulk Lorentz factors ( > 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 ( ~ 10)
The Quasar 3C279 on Feb. 23, 2006
Feb. 23:
• High optical flux
• Steep optical spectrum ( = 1.7 -> p = 4.4)
• High X-ray flux
• Soft X-ray spectrum
sy ~ 5x1013 Hz => sy ~ 4x10-7 ~ 1025 Hz =>
~ 105
Fsy ~ 1013 Jy Hz
F ~ 5x1013 Jy Hz
Accretion disk: LD ~ 2x1045 erg/s; D ~ 10-5
Parameter Estimates: SSC
• Optical index = 1.7 => p = 4.4 => cooling break (3.4 -> 4.4) would not produce a F peak => peak must be related to low-energy cutoff, p = 1
• Separation of synchrotron and gamma-ray peak
=> p = (/sy)1/2 ~ 1.6x105
• sy = 4.2x106 p2 BG D/(1+z) Hz
=> BG D1 ~ 7x10-5
Parameter Estimates: External Compton• External photons of s ~ 10-5 can be Thomson scattered up
to ~ 105 => Accretion disk photons can be source photon field.
• Location of gamma-ray peak
=> p = (/[2s])1/2 ~ 104 1-1
• sy = 4.2x106 p2 BG D/(1+z) Hz
=> BG ~ 1.8x10-2 12 D1
-1
• Relate synchrotron flux level to electron energy density, eB = u’B/u’e
=> eB ~ 10-8 17 R16
3
a) ~ 15, B ~ 0.03 G, eB ~ 10-7
b) eB ~ 1, B ~ 0.25 G, ~ 140 R16-3/7
X-rays severely underproduced!
Attempted leptonic one-zone model fit, EC dominated
(Bӧttcher, Reimer & Marscher 2009)
Requires far sub-equipartition magnetic fields!
Alternative: Multi-zone leptonic model
Linj = 2.3*1049 erg/s
min = 104
max = 106
q = 2.3
B = 0.2 G
= D = 20
RB = 6*1015 cm
u’B/u’e = 2.5*10-4
X-ray through gamma-ray spectrum reproduced by SSC; optical spectrum has to be produced in a different part of the jet.
(Bӧttcher, Reimer & Marscher 2009)
• Optical and -ray spectral index can be decoupled
• X-rays filled in by electromagnetic cascades
• However: Requires very large jet luminosities, Lj ~ 1049 erg/s
Hadronic Model Fits
(Bӧttcher, Reimer & Marscher 2009)
W Comae• Detected by VERITAS in March 2008 (big flare on March 14)
• One-zone SSC model requires extreme parameters:
Acciari et al. (2008) Linj = 2.8*1045 erg/s
min = 450
max = 4.5*105
q = 2.2
B = 0.007 G
= D = 30
RB = 1017 cmWide peak separation and low X-ray flux
require unusually low magnetic field!
LB/Le = 5.7*10-2
W Comae• Much more natural parameters for EC model
Linj = 2*1044 erg/s
min = 700
max = 105
q = 2.3
RB = 1.8*1015 cm
B = 0.25 G
-> Equipartition!
= D = 30
tvar ~ 35 min. allowed with external IR photon field
• For Compton scattering in Thomson regime, external photons must have E ~ (mec2)2/EVHE ~ 0.1 – 1 eV => IR
(Acciari et al. 2008)
W ComaeMajor VHE -ray flare detected by VERITAS in June 2008.
Similar modeling conclusions to March 2008:
SSC fit:
B= 0.24 G
LB/Le = 2.3*10-3
EC fit:
B = 0.35 G
LB/Le = 0.32
High flux state on MJD 54624
(Acciari et al. 2009, in prep.)
3C66AMajor VHE -ray flare detected by VERITAS in October 2008
Pure SSC fit requires far sub-equipartition magnetic field:
B = 0.1 G
LB/Le = 8.0*10-3
= D = 30
RB = 3*1016 cm
=> tvar,min = 13 hr
3C66AFit with external IR radiation field (ext = 1.5*1014 Hz)
yields more natural parameters:
B = 0.3 G
LB/Le = 0.1
= D = 30
RB = 2*1016 cm
=> tvar,min = 8.9 hr
Summary1. The MAGIC detection of 3C279 poses severe
problems for leptonic models. Hadronic models provide a viable alternative, but require a very large jet power.
2. Recent VHE gamma-ray detections of inermediate- and low-frequency peaked BL Lac objects extends the TeV blazar source list towards new classes of blazars.
3. IBLs appear to require source parameters truly intermediate between HBLs and LBLs: In leptonic models, a non-negligible contribution from external Compton on an external IR radiation field yields more natural parameters than a pure SSC interpretation.
Blazar Classification
Quasars:
Low-frequency component from radio to optical/UV
High-frequency component from X-rays to -rays, often dominating total power
Peak frequencies lower than in BL Lac objects
(Hartman et al. 2000)
High-frequency peaked BL Lacs (HBLs):
Low-frequency component from radio to UV/X-rays, often
dominating the total power
High-frequency component from hard X-rays to high-
energy gamma-rays
Intermediate objects:
Low-frequency peaked BL Lacs (LBLs):
Peak frequencies at IR/Optical and GeV gamma-rays
Intermediate overall luminosity
Sometimes -ray dominated
(Boettcher & Reimer 2004)
Estimates from the SED:
F (sy)
F (C)
F (C) / F (sy) ~ u’rad / u’B
Constraints from Observations
C/sy = p2
sy C
sy = 3.4*106 (B/G) (D/(1+z)) pHz
→ Estimate u’rad
→ Estimate peak of electron spectrum, p
If -rays are from SSC:
If -rays are from EC (BLR or IR):
C ~ ext p2
From synchrotron spectral index:
Electron sp. Index p
F ~
If -rays are Compton emission in Thomson regime:
F (sy)
F (C)
F (C) / F (sy) ~ (dE/dt)T / (dE/dt)sy = u’rad / u’B
u’rad =
’ = in the co-moving frame of the emission region
u’sy
u’BLR ≈
u’IR ≈
u’disk ≈
u’jet ≈
SSC
EC(disk)
EC(BLR)
EC(IR)
EC(jet)
LD
4r22cLD BLR 2
4rBLR2c
2 uIR
rel2 u’jet
Constraints from Observations
W ComaeLow-flux state around MJD 54626 is poorly constrained
because of lack of -ray detections
SSC fit:
B= 1.0 G
LB/Le = 0.40
(can easily be ruled out by any -ray-detection!)
EC fit:
B = 0.35 G
LB/Le = 0.35
Low-flux state on MJD 54626
W ComaeIntermediate state around MJD 54631.5 (XMM-Newton
ToO) ; also poorly constrained -ray spectrum
SSC fit:
B= 0.7 G
LB/Le = 0.10
EC fit:
B = 0.45 G
LB/Le = 0.78
Intermediate-flux state on MJD 54631.5
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