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Models for non-HBL VHE Gamma-Ray Blazars

0. Models for non-HBL VHE Gamma-Ray Blazars. Markus B öttcher Ohio University, Athens, OH, USA. “TeV Particle Astrophysics” SLAC, Menlo Park, CA, July 13 – 17, 2009. Motivation. Until a few years ago, all TeV blazars were high-frequency-peaked BLLac objects (HBLs).

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Models for non-HBL VHE Gamma-Ray Blazars

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  1. 0 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

  2. Motivation 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 g-rays. Similar in physical parameters to other TeV blazars (HBLs)? (→ SSC dominated?) Or more similar to LBLs? (→ EC required?)

  3. 0 Leptonic Blazar Model Synchrotron emission Injection, acceleration of ultrarelativistic electrons Relativistic jet outflow with G ≈ 10 nFn g-q Qe (g,t) n Compton emission g1 g2 g Radiative cooling ↔ escape => nFn g-q or g-2 n Qe (g,t) g-(q+1) Seed photons: Synchrotron (within same region [SSC] or slower/faster earlier/later emission regions [decel. jet]), Accr. Disk, BLR, dust torus (EC) g1 gb g2 g gb: tcool(gb) = tesc gb g1 g2

  4. 0 Spectral modeling results along the Blazar Sequence: Leptonic Models High-frequency peaked BL Lac (HBL): Low magnetic fields (~ 0.1 G); High electron energies (up to TeV); Large bulk Lorentz factors (G > 10) The “classical” picture Synchrotron No dense circumnuclear material → No strong external photon field SSC

  5. 0 Spectral modeling results along the Blazar Sequence: Leptonic Models Radio Quasar (FSRQ) High magnetic fields (~ a few G); Lower electron energies (up to GeV); Lower bulk Lorentz factors (G ~ 10) External Compton Plenty of circumnuclear material → Strong external photon field Synchrotron

  6. The Quasar 3C279 on Feb. 23, 2006 0 nFn, g ~ 5x1013 Jy Hz nFn,sy ~ 1013 Jy Hz • Feb. 23: • High optical flux • Steep optical spectrum (a = 1.7 -> p = 4.4) • High X-ray flux • Soft X-ray spectrum nsy ~ 5x1013 Hz => esy ~ 4x10-7 ng ~ 1025 Hz => eg ~ 105 Accretion disk: LD ~ 2x1045 erg/s; eD ~ 10-5

  7. Parameter Estimates: SSC • Optical index a = 1.7 => p = 4.4 => cooling break (3.4 -> 4.4) would not produce a nFn peak => peak must be related to low-energy cutoff, gp = g1 • Separation of synchrotron and gamma-ray peak => gp = (eg/esy)1/2 ~ 1.6x105 • nsy = 4.2x106gp2 BG D/(1+z) Hz => BG D1 ~ 7x10-5

  8. Parameter Estimates: External Compton • External photons of es ~ 10-5 can be Thomson scattered up to eg ~ 105 => Accretion disk photons can be source photon field. • Location of gamma-ray peak => gp = (eg/[G2es])1/2 ~ 104G1-1 • nsy = 4.2x106gp2 BG D/(1+z) Hz => BG ~ 1.8x10-2G12 D1-1 • Relate synchrotron flux level to electron energy density, eB = u’B/u’e => eB ~ 10-8G17 R163 a) G ~ 15, B ~ 0.03 G, eB ~ 10-7 b) eB ~ 1, B ~ 0.25 G, G ~ 140 R16-3/7

  9. Attempted leptonic one-zone model fit, EC dominated X-rays severely underproduced! (Bӧttcher, Reimer & Marscher 2009)

  10. Alternative: Multi-zone leptonic model X-ray through gamma-ray spectrum reproduced by SSC; optical spectrum has to be produced in a different part of the jet. • Linj = 2.3*1049 erg/s • gmin = 104 • gmax = 106 • q = 2.3 • B = 0.2 G • G = D = 20 • RB = 6*1015 cm • u’B/u’e = 2.5*10-4 (Bӧttcher, Reimer & Marscher 2009) Requires far sub-equipartition magnetic fields!

  11. Hadronic Model Fits (Bӧttcher, Reimer & Marscher 2009) • Optical and g-ray spectral index can be decoupled • X-rays filled in by electromagnetic cascades • However: Requires very large jet luminosities, Lj ~ 1049 erg/s

  12. W Comae • Detected by VERITAS in March 2008 (big flare on March 14) • One-zone SSC model requires extreme parameters: • Linj = 2.8*1045 erg/s • gmin = 450 • gmax = 4.5*105 • q = 2.2 • B = 0.007 G • G = D = 30 • RB = 1017 cm Acciari et al. (2008) LB/Le = 5.7*10-2 Wide peak separation and low X-ray flux require unusually low magnetic field!

  13. W Comae • Much more natural parameters for EC model • For Compton scattering in Thomson regime, external photons must have E ~ (mec2)2/EVHE ~ 0.1 – 1 eV => IR • Linj = 2*1044 erg/s • gmin = 700 • gmax = 105 • q = 2.3 • RB = 1.8*1015 cm • B = 0.25 G • -> Equipartition! • G = D = 30 Dtvar ~ 35 min. allowed with external IR photon field (Acciari et al. 2008)

  14. W Comae Major VHE g-ray flare detected by VERITAS in June 2008. Similar modeling conclusions to March 2008: High flux state on MJD 54624 • SSC fit: • B= 0.24 G • LB/Le = 2.3*10-3 • EC fit: • B = 0.35 G • LB/Le = 0.32 (Acciari et al. 2009, in prep.)

  15. 3C66A Major VHE g-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 • G = D = 30 • RB = 3*1016 cm • => dtvar,min = 13 hr

  16. 3C66A Fit with external IR radiation field (next = 1.5*1014 Hz) yields more natural parameters: • B = 0.3 G • LB/Le = 0.1 • G = D = 30 • RB = 2*1016 cm • => dtvar,min = 8.9 hr

  17. 0 Summary The MAGIC detection of 3C279 poses severe problems for leptonic models. Hadronic models provide a viable alternative, but require a very large jet power. 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. 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.

  18. 0 Blazar Classification Intermediate objects: Low-frequency peaked BL Lacs (LBLs): Peak frequencies at IR/Optical and GeV gamma-rays Intermediate overall luminosity Sometimes g-ray dominated (Hartman et al. 2000) Quasars: Low-frequency component from radio to optical/UV High-frequency component from X-rays to g-rays, often dominating total power Peak frequencies lower than in BL Lac objects 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 (Boettcher & Reimer 2004)

  19. Constraints from Observations Estimates from the SED: nFn (C) / nFn (sy) ~ u’rad / u’B → Estimate u’rad nFn (C) nsy = 3.4*106 (B/G) (D/(1+z)) gp2Hz → Estimate peak of electron spectrum, gp nFn (sy) Fn ~ n-a If g-rays are from SSC: nC/nsy = gp2 If g-rays are from EC (BLR or IR): nC ~ G2eextgp2 From synchrotron spectral index a: nsy nC Electron sp. Index p = 2a + 1

  20. Constraints from Observations If g-rays are Compton emission in Thomson regime: u’sy SSC LD nFn (C) u’disk ≈ EC(disk) 4pr2G2c LD tBLRG2 u’BLR ≈ nFn (sy) EC(BLR) u’rad = 4prBLR2c u’IR ≈ G2 uIR EC(IR) EC(jet) u’jet ≈ Grel2u’jet ’ = in the co-moving frame of the emission region nFn (C) / nFn (sy) ~ (dE/dt)T / (dE/dt)sy = u’rad / u’B

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

  22. W Comae Intermediate state around MJD 54631.5 (XMM-Newton ToO) ; also poorly constrained g-ray spectrum Intermediate-flux state on MJD 54631.5 • SSC fit: • B= 0.7 G • LB/Le = 0.10 • EC fit: • B = 0.45 G • LB/Le = 0.78

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