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Dimitrios Giannios “High Energy Messengers” Workshop, June 10th 2014
Sironi L. and Giannios D. 2014, ApJ, 787, 49; arXiv:1312.4538
Is the IGM heated by TeV blazars?Is the IGM heated by TeV blazars?
TeV blazarsTeV blazars
Cerenkov Telescopes:Blazars dominate the extragalactic TeV sky
(Ghisellini et al 11)
(credit: TEVCat)
The blazar “sequence”:• a continuous sequence LBL - IBL - HBL• TeV blazars are dim
TeV photons are absorbed in the IGMTeV photons are absorbed in the IGM
TeV photons from blazars pair-produce in the IGM by interacting with ~ eV EBL photons.• mean free path is ~100 Mpc
The beam of electron-positron pairs has:
Lorentz factor γ and density ratio α (wrt the IGM plasma)
These pairs should IC scatter off the CMB, producing ~ GeV photons. • mean free path is ~ 100 kpc (IC cooling length)
No excess GeV emission from blazarsNo excess GeV emission from blazarsEvery TeV blazar should have a GeV halo of reprocessed light. However, not seen!
(Neronov & Vovk 10)
IGM fields or plasma instabilities?IGM fields or plasma instabilities?
2) The pair energy is deposited into the IGM by plasma instabilities (Broderick, Chang, Pfrommer 12, 13)
1) IGM magnetic fields deflect the streaming pairs (Neronov & Vovk 10 …)
Every TeV blazar should have a GeV halo of reprocessed light. However, not seen!Two possibilities:
(Tavecchio et al. 11)
Fermi upper limits
reprocessed GeV emission from pairs deflected by IGM fields
intrinsic TeV spectrum
absorbed TeV spectrum
Plasma instabilities in the IGMPlasma instabilities in the IGMInterpenetrating beams of charged particles are unstable (beam-plasma instabilities)
Blazar-induced relativistic pairs
IGM plasma
Two-stream (bump on tail) instability
energy from waves to particles: → damping
energy from particles to waves: → instability
microscopic scales!
Oblique instability
beam
(Sironi & Giannios 14)
collective limit
Beam-plasma linear evolutionBeam-plasma linear evolutionLinear analysis: the oblique instability grows 10-100 times faster than the IC cooling time.
The non-linear evolution of the beam-plasma system requires PIC simulations...
(Broderick et al. 12)
IF the instability grows until all the beam
energy is deposited into the IGM:
• No reprocessed blazar GeV emission
• IGM field estimates are invalid
• IGM heating from blazars will have
cosmological implications
(Chang et al. 12)
What happens to TeV photons?
8
?
The PIC methodThe PIC method
Particle-in-Cell (PIC) method:
1. Particle currents deposited on a grid
• Electromagnetic fields solved on the grid via
Maxwell’s equations
• Lorentz force interpolated to particle locations
No approximations, plasma physics at a fundamental level
Tiny length and time scales need to be resolved huge simulations,
limited time coverage
• Relativistic 3D e.m. PIC code TRISTAN-MP (Buneman ‘93, Spitkovsky ‘05)
Yee mesh
Cold beam: non-linear evolutionCold beam: non-linear evolution
The oblique instability grows fast, but it is quenched by self-heating of the beam
heating fraction
Exponential phase
Blazar-induced beams: Lorentz factor γ and density ratio α
COLD beam withγ and α
In the end, the beam longitudinal dispersion
~0.2 γ, and the plasma heating fraction ~10%
Relaxation phase
heating fraction
B energy
E energy
10% in heat, 90% in GeV emission10% in heat, 90% in GeV emissionBlazar-induced beams: Lorentz factor γ and density ratio α
Numerically tractable: Lorentz factor γ and density ratio α
COLD beams:
• Regardless of the beam γor α, the beam longitudinal dispersion reaches ~0.2 γ, and the IGM heating fraction ~10%.• Only 10% of the beam energy is deposited into the IGM, 90% is still available to power the reprocessed GeV emission.
IGM
hea
ting
fract
ion
(LS & Giannios 14)
10% in heat: a generous upper limit10% in heat: a generous upper limit
The heating fraction can be 10%:≪
•if the initial longitudinal beam
dispersion is already > 0.2 γ
•if pre-existing magnetic fields are dispersing the beam sideways.
(Miniati et al 13)
suppressed• in the presence of density inhomogeneities in the IGM.
→ suppression
Blazar beams are not coldBlazar beams are not cold
The heating fraction can be 10%:≪
•if the initial longitudinal beam
dispersion is already > 0.2 γ IGM
hea
ting
fract
ion
(Sironi & Giannios 14)
Blazar beams are born warm:
• the pair production cross section peaks at ~ few mec2. • the TeV blazar spectrum and the EBL spectrum are broad.
distance
Is the IGM heated by TeV blazars?Is the IGM heated by TeV blazars?
Not much.Not much.
• At the end of the relaxation phase, the beam-plasma system is still highly anisotropic, so still unstable (to the Weibel instability).• Blazar-induced pair beams might be a potential mechanism for generating small-scale (~ c/ωp ~ 108 cm) magnetic fields in cosmic voids?
Beam-aligned electric field Magnetic energy
beam beam
z [c/ω]
y [c/ω]
x [c/ω]
z [c/ω]
y [c/ω]
x [c/ω]
Long term beam-plasma evolutionLong term beam-plasma evolution
(Sironi & Giannios, in prep.)
• TeV photons from blazars will pair-produce in the IGM. The resulting
electron-positron beam is unstable to the excitation of plasma instabilities.
• Electrostatic plasma instabilities deposit 10% of the beam energy into ≪the IGM. Most of the beam energy will result in GeV emission by IC
scattering off the CMB.
• After the saturation of electrostatic plasma instabilities, the beam is still
anisotropic, and it might generate magnetic fields from scratch via the
Weibel instability.
Summary Summary
Beam distribution functionBeam distribution function
The complete evolutionThe complete evolution
The complete evolutionThe complete evolution
The complete evolutionThe complete evolution
Dependence on the beam propertiesDependence on the beam properties
Dependence on the beam temperatureDependence on the beam temperature