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 Model Relativistic jet outflow with  ≈ 10 Injection, acceleration of ultrarelativistic electrons Q e ( ,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) Q e ( ,t)     -(q+1) bb  -q or  -2  bb 1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 Synchrotron SSC 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 Synchrotron External 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 ~ 5x10 13 Hz =>  sy ~ 4x10 -7  ~ Hz =>   ~ 10 5 F  sy ~ Jy Hz F  ~ 5x10 13 Jy Hz Accretion disk: L D ~ 2x10 45 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 ~ 1.6x10 5 sy = 4.2x10 6  p 2 B G D/(1+z) Hz => B G D 1 ~ 7x10 -5

Parameter Estimates: External Compton External photons of  s ~ can be Thomson scattered up to   ~ 10 5 => Accretion disk photons can be source photon field. Location of gamma-ray peak =>  p = (   /[  2  s ]) 1/2 ~ 10 4  1 -1 sy = 4.2x10 6  p 2 B G D/(1+z) Hz => B G ~ 1.8x10 -2  1 2 D 1 -1 Relate synchrotron flux level to electron energy density, e B = u’ B /u’ e => e B ~  1 7 R 16 3 a)  ~ 15, B ~ 0.03 G, e B ~ b) e B ~ 1, B ~ 0.25 G,  ~ 140 R 16 -3/7

X-rays severely underproduced! Attempted leptonic one-zone model fit, EC dominated (B ӧ ttcher, Reimer & Marscher 2009)

Requires far sub-equipartition magnetic fields! Alternative: Multi-zone leptonic model L inj = 2.3*10 49 erg/s  min = 10 4  max = 10 6 q = 2.3 B = 0.2 G  = D = 20  R B = 6*10 15 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. (B ӧ ttcher, Reimer & Marscher 2009)

Optical and  -ray spectral index can be decoupled X-rays filled in by electromagnetic cascades However: Requires very large jet luminosities, L j ~ erg/s Hadronic Model Fits (B ӧ ttcher, 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) L inj = 2.8*10 45 erg/s  min = 450  max = 4.5*10 5 q = 2.2 B = G  = D = 30  R B = cm Wide peak separation and low X-ray flux require unusually low magnetic field! L B /L e = 5.7*10 -2

W Comae Much more natural parameters for EC model L inj = 2*10 44 erg/s  min = 700  max = 10 5 q = 2.3 R B = 1.8*10 15 cm B = 0.25 G -> Equipartition!  = D = 30   t var ~ 35 min. allowed with external IR photon field For Compton scattering in Thomson regime, external photons must have E ~ (m e c 2 ) 2 /E VHE ~ 0.1 – 1 eV => IR (Acciari et al. 2008)

W Comae Major VHE  -ray flare detected by VERITAS in June Similar modeling conclusions to March 2008: SSC fit:  B  = 0.24 G L B /L e = 2.3*10 -3 EC fit: B = 0.35 G  L B /L e = 0.32 High flux state on MJD (Acciari et al. 2009, in prep.)

3C66A Major VHE  -ray flare detected by VERITAS in October 2008 Pure SSC fit requires far sub-equipartition magnetic field: B = 0.1 G  L B /L e = 8.0*10 -3  = D = 30 R B = 3*10 16 cm =>  t var,min = 13 hr

3C66A Fit with external IR radiation field ( ext = 1.5*10 14 Hz) yields more natural parameters: B = 0.3 G  L B /L e = 0.1  = D = 30 R B = 2*10 16 cm =>  t var,min = 8.9 hr

Summary 1.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 =  p 2 sy C sy = 3.4*10 6 (B/G) (D/(1+z))  p  Hz → Estimate u’ rad → Estimate peak of electron spectrum,  p If  -rays are from SSC: If  -rays are from EC (BLR or IR): C ~    ext  p 2 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) LDLD 4r22c4r22c L D  BLR  2 4  r BLR 2 c  2 u IR  rel 2 u’ jet Constraints from Observations

W Comae Low-flux state around MJD is poorly constrained because of lack of  -ray detections SSC fit:  B  = 1.0 G L B /L e = 0.40 (can easily be ruled out by any  -ray-detection!) EC fit: B = 0.35 G  L B /L e = 0.35 Low-flux state on MJD 54626

W Comae Intermediate state around MJD (XMM-Newton ToO) ; also poorly constrained  -ray spectrum SSC fit:  B  = 0.7 G L B /L e = 0.10 EC fit: B = 0.45 G  L B /L e = 0.78 Intermediate-flux state on MJD