Dimitrios Giannios Purdue Workshop, May 12th 2014

Sironi L. and Giannios D. 2014, ApJ in press, 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, Tavecchio et al. 11)

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)

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)

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)

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)

(Miniati et al 13)

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 can generate magnetic fields from scratch via the

Weibel instability.

Summary Summary

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

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