Is the IGM heated by TeV blazars?
Dimitrios Giannios
Purdue Workshop, May 12th 2014
Sironi L. and Giannios D. 2014, ApJ in press, arXiv:1312.4538
TeV blazars
Cerenkov Telescopes:
Blazars dominate the
extragalactic TeV sky
(credit: TEVCat)
The blazar sequence:
• a continuous sequence
LBL - IBL - HBL
• TeV blazars are dim
(Ghisellini et al 11)
TeV 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 6-7 and density ratio  -5--8 (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 blazars
Every TeV blazar should have a GeV halo of reprocessed light. However, not seen!
(Neronov & Vovk 10)
IGM fields or plasma instabilities?
Every TeV blazar should have a GeV halo of reprocessed light. However, not seen!
Two possibilities:
1) IGM magnetic fields deflect the streaming pairs
(Neronov & Vovk 10, Tavecchio et al. 11)
Fermi upper limits
reprocessed GeV emission
from pairs deflected by
IGM fields
intrinsic TeV
spectrum
absorbed TeV
spectrum
(Tavecchio et al. 11)
2) The pair energy is deposited into the IGM by plasma instabilities
(Broderick, Chang, Pfrommer 12, 13)
Plasma instabilities in the IGM
Interpenetrating beams of charged particles are unstable (beam-plasma instabilities)
microscopic scales!
Blazar-induced
relativistic pairs
IGM plasma
Two-stream (bump on tail) instability
Oblique instability
beam
energy from particles to waves:
→ instability
energy from waves to particles:
→ damping
(Sironi & Giannios 14)
Beam-plasma linear evolution
Linear analysis: the oblique instability
grows 10-100 times faster than the IC
cooling time.
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
(Broderick et al. 12)
(Chang et al. 12)
The non-linear evolution of the beam-plasma system requires PIC simulations...
The 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 evolution
Blazar-induced beams: Lorentz factor 6-7 and density ratio  -5--8
COLD beam with  and  -2
Exponential phase
Relaxation phase
heating fraction
heating fraction
B energy
E energy
The oblique instability grows fast, but it is
quenched by self-heating of the beam
In the end, the beam longitudinal dispersion
~0.2 , and the plasma heating fraction ~10%
10% in heat, 90% in GeV emission
Blazar-induced beams: Lorentz factor 6-7 and density ratio  -5--8
IGM heating fraction
Numerically tractable: Lorentz factor - and density ratio  ---
(LS & Giannios 14)
COLD beams:
• Regardless of the beam or  , the beam longitudinal dispersion reaches ~0.2 ,
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.
Blazar beams are not cold
Blazar beams are born warm:
distance
• the pair production cross section
peaks at ~ few mec2.
• the TeV blazar spectrum and the
EBL spectrum are broad.
• if the initial longitudinal beam
dispersion is already > 0.2 .
IGM heating fraction
The heating fraction can be ≪10%:
(Miniati et al 13)
(Sironi & Giannios 14)
Is the IGM heated by TeV blazars?
Not much.
Long term beam-plasma evolution
Beam-aligned electric field
Magnetic energy
beam
beam
z [c/ p]
z [c/ p]
x [c/ p]
y [c/ p]
x [c/ p]
y [c/ p]
(Sironi & Giannios, in prep.)
• 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?
Summary
• 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 can generate magnetic fields from scratch via the
Weibel instability.
10% in heat: a generous upper limit
The heating fraction can be ≪10%:
• if the initial longitudinal beam
dispersion is already > 0.2 .
→ suppression
• if pre-existing magnetic fields are
dispersing the beam sideways.
• in the presence of density
inhomogeneities in the IGM.
(Miniati et al 13)
Beam distribution function
The complete evolution
The complete evolution
The complete evolution
Dependence on the beam properties
Dependence on the beam temperature