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Dimitrios Giannios Purdue Workshop, May 12th 2014

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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 γ=106-107 and density ratio α=10-15-10-18(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)

1) IGM magnetic fields deflect the streaming pairs (Neronov & Vovk 10, Tavecchio et al. 11)

intrinsic TeV spectrum

Fermi upper limits

absorbed TeV spectrum

reprocessed GeV emission from pairs deflected by IGM fields

(Tavecchio et al. 11)

IGM fields or plasma instabilities?

Every TeV blazar should have a GeV halo of reprocessed light. However, not seen!

Two possibilities:

2) The pair energy is deposited into the IGM by plasma instabilities (Broderick, Chang, Pfrommer 12, 13)

Two-stream (bump on tail) instability

Oblique instability

beam

energy from particles to waves: → instability

energy from waves to particles: → damping

(Sironi & Giannios 14)

Plasma instabilities in the IGM

Interpenetrating beams of charged particles are unstable (beam-plasma instabilities)

microscopic scales!

Blazar-induced relativistic pairs

IGM plasma

- 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)

Beam-plasma linear evolution

Linear analysis: the oblique instability grows 10-100 times faster than the IC cooling time.

(Broderick et al. 12)

The non-linear evolution of the beam-plasma system requires PIC simulations...

Is the instability growing at the linear rate until it deposits all the beam energy into the IGM?

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)

The PIC method

- Particle-in-Cell (PIC) method:
- Particle currents deposited on a grid
- Electromagnetic fields solved on the grid via Maxwell’s equations
- Lorentz force interpolated to particle locations

Yee mesh

Relaxation phase

heating fraction

B energy

E energy

In the end, the beam longitudinal dispersion ~0.2 γ, and the plasma heating fraction ~10%

Cold beam: non-linear evolution

Blazar-induced beams: Lorentz factor γ=106-107 and density ratio α=10-15-10-18

COLD beam with γ=300 and α=10-2

Exponential phase

heating fraction

The oblique instability grows fast, but it is quenched by self-heating of the beam

IGM heating fraction

(LS & Giannios 14)

- COLD beams:
- Regardless of the beam γ or 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.

10% in heat, 90% in GeV emission

Blazar-induced beams: Lorentz factor γ=106-107 and density ratio α=10-15-10-18

Numerically tractable: Lorentz factor γ=101-103 and density ratio α=10-1-10-3

- The heating fraction can be ≪10%:
- if the initial longitudinal beam dispersion is already > 0.2 γ.

IGM heating fraction

(Sironi & Giannios 14)

Blazar beams are not cold

- 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

(Miniati et al 13)

Is the IGM heated by TeV blazars?

Not much.

Long term beam-plasma evolution

Beam-aligned electric field

Magnetic energy

beam

beam

z [c/ωp]

z [c/ωp]

x [c/ωp]

x [c/ωp]

y [c/ωp]

y [c/ωp]

(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.

- in the presence of density inhomogeneities in the IGM.

suppressed

(Miniati et al 13)

10% 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.

→ suppression

Beam distribution function

The complete evolution

The complete evolution

The complete evolution

Dependence on the beam properties

Dependence on the beam temperature