Diagnostics of Leptonic vs. Hadronic Emission Models for Blazars Prospects for H.E.S.S.-II and CTA Markus Böttcher North-West University Potchefstroom South Africa
Q e (g,t) Injection, acceleration of ultrarelativistic electrons Q e (g,t) g -q Leptonic Blazar Model Relativistic jet outflow with G 10 nf n Synchrotron emission n g 1 g 2 Radiative cooling escape => g 1 g -q or g -2 g b g -(q+1) g 2 g 2 g g t cool (g b ) = t esc g b g 1 g b : nf n Seed photons: Compton emission Synchrotron (within same region [SSC] or slower/faster earlier/later emission regions [decel. jet]), Accr. Disk, BLR, dust torus (EC) n
Sources of External Photons ( Location of the Blazar Zone) Direct accretion disk emission (Dermer et al. 1992, Dermer & Schlickeiser 1994) d < few 100 1000 R s Optical-UV Emission from the BLR (Sikora et al. 1994) d < ~ pc Infrared Radiation from the Obscuring Torus (Blazejowski et al. 2000) d ~ 1 10s of pc Synchrotron emission from slower/faster regions of the jet (Georganopoulos & Kazanas 2003) d ~ pc - kpc Spine Sheath Interaction (Ghisellini & Tavecchio 2008) d ~ pc - kpc
Diagnosing the Location of the Blazar Zone Energy dependence of cooling times: Distinguish between EC on IR (torus Thomson) and optical/uv lines (BLR Klein-Nishina) If EC(BLR) dominates: Blazar zone should be inside BLR gg absorption on BLR photons GeV spectral breaks (Poutanen & Stern 2010) (Dotson et al. 2012) No VHE g-rays expected! VHE g-rays from FSRQs must be from outside the BLR (e.g., Barnacka et al. 2013)
g-g Absorption in the Extragalactic Background Light e - High-Energy g-rays are absorbed in intergalactic space by g-g pair production Absorption optical depth t gg Franceschini et al. (2008) F g obs = F g em e -tgg
Relativistic Particle Acceleration Very hard s < 2 electron spectra needed for EBLcorrected spectra of several blazars n(g) ~ g -s (Finke et al. 2010) (Summerlin & Baring 2012) CTA + Fermi will measure detailed GeV TeV spectra of blazars isolate EBL effect to measure intrinsic spectra.
Q e,p (g,t) Hadronic Blazar Models Injection, acceleration of ultrarelativistic electrons and protons Relativistic jet outflow with G 10 Proton-induced radiation mechanisms: nf n nf n g 1 g -q g 2 g Synchrotron emission of primary e - n (Mannheim & Biermann 1992; Aharonian 2000; Mücke et al. 2000; Mücke et al. 2003) Proton synchrotron pg pp 0 p 0 2g n pg np + ; p + m + n m m + e + n e n m secondary m-, e-synchrotron Cascades
Requirements for lepto-hadronic models To exceed p-g pion production threshold on interactions with synchrotron (optical) photons: E p > 7x10 16 E -1 ph,ev ev For proton synchrotron emission at multi-gev energies: E p up to ~ 10 19 ev (=> UHECR) Require Larmor radius r L ~ 3x10 16 E 19 /B G cm a few x 10 15 cm => B 10 G (Also: to suppress leptonic SSC component below synchrotron) Typically require jet powers of L p ~ 10 44 erg/s (HBLs) up to > 10 48 erg/s (FSRQs)
Leptonic and Hadronic Model Fits along the Blazar Sequence Accretion Disk Red = Leptonic Green = Hadronic External Compton of direct accretion disk photons (ECD) Synchrotron Electron synchrotron Synchrotron self- Compton (SSC) Proton synchrotron External Compton of emission from BLR clouds (ECC) (Bӧttcher, Reimer et al. 2013)
Leptonic and Hadronic Model Fits along the Blazar Sequence Hadronic models can more easily produce VHE emission Synchrotron through cascade self- synchrotron Spectral Compton upturns (SSC) (PKS 1424+240) Red = Leptonic Green = Hadronic Proton synchrotron + Cascade synchrotron Proton synchrotron Synchrotron Electron synchrotron Electron SSC External Compton of emission from BLR clouds (ECC) (Bӧttcher, Reimer et al. 2013)
Leptonic and Hadronic Model Fits Along the Blazar Sequence 3C66A (IBL) Red = leptonic Green = lepto-hadronic
Lepto-Hadronic Model Fits Along the Blazar Sequence (HBL) Red = leptonic Green = lepto-hadronic In many cases, leptonic and hadronic models can produce equally good fits to the SEDs. Possible Diagnostics to distinguish: Neutrinos Variability Polarization(?)
Distinguishing Diagnostic: Variability Time-dependent leptonic one-zone models produce correlated synchrotron + gamma-ray variability (Mastichiadis & Kirk 1997, Li & Kusunose 2000, Bӧttcher & Chiang 2002, Moderski et al. 2003) Time-dependent leptonic one-zone model for Mrk 421
Rapid Gamma-Ray Variability Rapid variability is problematic for hadronic models (long proton cooling times) PKS 2155-304 CTA can measure VHE g-ray variability on sub-minute time scales (if present) (Aharonian et al. 2007)
Distinguishing Diagnostic: Variability Time-dependent hadronic models can produce uncorrelated variability / orphan flares (Dimitrakoudis et al. 2012, Mastichiadis et al. 2013, Weidinger & Spanier 2013) (M. Weidinger)
Possible Distinguishing Diagnostic: X-Ray and Gamma-Ray Polarization Upper limits on high-energy polarization, assuming perfectly ordered magnetic field perpendicular to the line of sight Synchrotron polarization: Standard Rybicki & Lightman description SSC Polarization: Bonometto & Saggion (1974) for Compton scattering in Thomson regime (Zhang & Bӧttcher, 2013)
X-Ray and Gamma-Ray Polarization: FSRQs Hadronic model: Synchrotron dominated => High P, generally increasing with energy (SSC contrib. in X-rays). Leptonic model: X-rays SSC dominated: P ~ 20 40 %; g-rays EC dominated => Negligible P. (Zhang & Bӧttcher, 2013)
Observational Strategy Results shown here are upper limits (perfectly ordered magnetic field perpendicular to line of sight) Scale results to actual B-field configuration from known synchrotron polarization (e.g., optical for FSRQs/LBLs) => Expect 10-20 % X-ray and g-ray polarization in hadronic models! X-ray and g-ray polarization values substantially below synchrotron polarization will favor leptonic models, measurable g-ray polarization clearly favors hadronic models!
Summary 1. Both leptonic and hadronic models can generally fit blazar SEDs well. 2. Distinguishing diagnostics: Variability, Polarization, Neutrinos? 3. Time-dependent hadronic models are able to predict uncorrelated synchrotron vs. gamma-ray variability 4. X-Ray / Gamma-Ray polarimetry as potential diagnostic? Hadronic Models predict high degree of X/gamma polarization
X-Ray and Gamma-Ray Polarization: LBLs (Zhang & Bӧttcher, 2013) Hadronic model: Mostly synchrotron dominated => High P, except for X-rays, where SSC may dominate. Leptonic model: X-rays = transition from sy. to SSC: P rapidly decreasing with energy; g-rays EC dominated => Negligible P.
The Cherenkov Telescope Array (CTA) Low-energy array: A few (~4 5) 23 m telescopes (~ 10 GeV threshold) Core array: Several (~ 15 25) 10 12 m telescopes (~0.1 10 TeV) High-energy array: Many (~ 100) 4-6 m telescopes; 10 km 2 area Multi-TeV energies (only for CTA-South) Plan: Two arrays: CTA-North / CTA-South for full sky coverage
CTA Performance: Sensitivity Factor ~10 improved sensitivity in core energy range (100 GeV 10 TeV) Lower low-energy threshold to ~ 10 GeV (factor ~ 5 lower than current) Higher high-energy threshold to ~ 100 TeV (factor ~ 10 higher than current)
CTA Performance: Angular resolution Factor ~10 better angular resolution than current instruments H.E.S.S. view of the Galactic Plane CTA (Simulation)
The CTA Consortium Almost 1200 scientists 177 institutes 28 countries
USA (Arizona) CTA Candidate Sites Canary Islands Chile Argentina Namibia
USA (Arizona) CTA Candidate Sites Canary Islands Chile Argentina Namibia Site Decision: October 2014 Start Construction: 2015 First Science Operations: 2016-2017
X-Ray and Gamma-Ray Polarization: IBLs Hadronic model: Synchrotron dominated => High P, throughout X-rays and g-rays (Zhang & Bӧttcher, 2013) Leptonic model: X-rays sy. Dominated => High P, rapidly decreasing with energy; g-rays SSC/EC dominated => Small P.
X-Ray and Gamma-Ray Polarization: HBLs Hadronic model: Synchrotron dominated => High P Leptonic model: X-rays sy. Dominated => High P, rapidly decreasing with energy; g-rays SSC/EC dominated => Small P. (Zhang & Bӧttcher, 2013)