Probing the Intergalactic Magnetic Field Using Intensity Fluctuations in the Extragalactic Gamma-ray Background* Tonia Venters Astrophysics Science Division NASA Goddard Space Flight Center *based on Venters & Pavlidou 2013, MNRAS, 432, 3485 Wednesday, August 28, 13
The Problem... UHECR Source Pierre Auger (concept) Intergalactic B-field weak deflection (IGMF)? strong deflection halo B? Milky Way Telescope Array (concept) Wednesday, August 28, 13
Our Allies Gamma-ray Satellites Neutrino Expts. Air Shower Arrays IACTs Wednesday, August 28, 13
The Gamma-ray Sky Resolved Point Sources Inverse Compton π 0 -decay Bremsstrahlung Galactic diffuse emission Isotropic diffuse emission (CR interactions with the interstellar medium) (presumably extragalactic) Wednesday, August 28, 13
Gamma-ray Sky (after subtraction) Isotropic diffuse emission (presumably extragalactic) Wednesday, August 28, 13
The Spectrum of the EGB Isotropic diffuse emission (presumably extragalactic) Wednesday, August 28, 13
Components of the EGB Known players: Star-forming galaxies Active galaxies (blazars, and maybe some from other types of radio galaxies) Inverse Compton π 0 -decay Resolved Suspected contributors: • Truly diffuse emission - Bremsstrahlung gamma rays produced in EM Galactic diffuse emission (CR interactions with the interstellar medium) cascades of highly energetic particles Players about which we like to speculate: • Exotic physics (e.g., dark Isotropic diffuse emission (presumably extragalactic) matter annihilation?) Wednesday, August 28, 13
VHE Gamma Rays in the EBL Blazar Extragalactic background light (EBL) consists of: Emission from starlight at NIR/Opt./UV wavelengths Reradiated thermal dust emission at FIR wavelengths γ γ γ e - e - Gilmore et al. 2009 Cascades - e + e - pair production e + e + inverse Compton scattering of cascade electrons γ γ γ See also D. Williams’ talk (tomorrow) Venters 2009 Wednesday, August 28, 13
Magnetic Deflection of Cascades • Gamma-rays initially EM Cascade emitted off observer’s line-of-sight initiate TeV γ cascades that are deflected in direction of GeV γ AGN observer. • Deflected emission Halo image credit: Ando 2009 makes a halo around source. See also W. Essey’s talk (tomorrow) Wednesday, August 28, 13
Anisotropy Studies Isotropic diffuse emission (presumably extragalactic) Star-forming Galaxies l ∞ ∂ I � � a lm Y lm I = l =1 m = − l Blazars � a lm a ∗ l � m � � = C l C l tot ( E ) = f 12 ( E ) C l (1) + f 22 ( E ) C l (2) + cross terms, where f n ( E ) = I n ( E )/ I tot ( E ) Deflected Cascade Wednesday, August 28, 13
Anisotropy Studies Isotropic diffuse emission (presumably extragalactic) ~ 1 GeV l ∞ ∂ I � � a lm Y lm I = l =1 m = − l ~ 10 GeV � a lm a ∗ l � m � � = C l C l tot ( E ) = f 12 ( E ) C l (1) + f 22 ( E ) C l (2) + cross terms, where f n ( E ) = I n ( E )/ I tot ( E ) ~100 GeV Wednesday, August 28, 13
Anisotropy as a Function of Energy T o t a l -6 10 -1 ] Blazars -1 sr -2 s s e i x 2 I E (E) [GeV cm a l a G F S ) 0 = ( B e a d s c C a -7 10 E ) G -14 0 1 x 5 = B ( e d a c s a C -1 0 1 2 10 10 10 10 Energy [GeV] 2 ( E ) C gal C tot 2 ( E ) C bl ( E ) = f bl l + f gal l l Wednesday, August 28, 13
Impact of Cascades (null B) Isotropic diffuse emission (presumably extragalactic) Star-forming Galaxies l ∞ ∂ I � � a lm Y lm I = l =1 m = − l Blazars � a lm a ∗ l � m � � = C l C l tot ( E ) = f 12 ( E ) C l (1) + f 22 ( E ) C l (2) + cross terms, where f n ( E ) = I n ( E )/ I tot ( E ) Cascades from Blazars Wednesday, August 28, 13
The Impact of Cascades (zero B) -6 10 -1 ] Total -1 sr B l a z a r s -2 s SF Galaxies 2 I E (E) [GeV cm -7 10 E ) Cascade (B = 0) G -14 0 1 x 5 = B ( e d a c s a C -1 0 1 2 10 10 10 10 Energy [GeV] C tot ( E ) =( f bl ( E ) + f cas ( E )) 2 C bl l l Wednesday, August 28, 13
Impact of Cascades (intermediate B) Isotropic diffuse emission (presumably extragalactic) Star-forming Galaxies l ∞ ∂ I � � a lm Y lm I = l =1 m = − l Blazars � a lm a ∗ l � m � � = C l C l tot ( E ) = f 12 ( E ) C l (1) + f 22 ( E ) C l (2) + cross terms, where f n ( E ) = I n ( E )/ I tot ( E ) Cascades from Blazars Wednesday, August 28, 13
Impact of Cascades (non-zero B) Isotropic diffuse emission (presumably extragalactic) Star-forming Galaxies l ∞ ∂ I � � a lm Y lm I = l =1 m = − l Blazars � a lm a ∗ l � m � � = C l C l tot ( E ) = f 12 ( E ) C l (1) + f 22 ( E ) C l (2) + cross terms, where f n ( E ) = I n ( E )/ I tot ( E ) Cascades from Blazars Wednesday, August 28, 13
The Impact of Cascades (non-zero B) -6 10 B l a z a r s -1 ] T o t a l -1 sr -2 s SF Galaxies 2 I E (E) [GeV cm -7 10 E G) ) 0 = 4 B e ( 1 a d c a s - C Cascade (B = 5 x10 -8 10 -1 0 1 2 10 10 10 10 Energy [GeV] C tot 2 ( E ) C bl 2 ( E ) C cas + cross terms ( E ) = f bl l + f cas l l Wednesday, August 28, 13
Cascades and EGB Anisotropy -4 10 -1 ] -6 10 Total -1 sr Blazars -2 s SF Galaxies 2 I E (E) [GeV cm C l [sr] -5 10 -7 10 Cascade (B = 0) E G) -14 Cascade (B = 5 x10 -8 10 -6 -1 0 1 2 10 10 10 10 10 2 0 1 2 10 10 10 Energy [GeV] Energy [GeV] Venters & Pavlidou 2013 Wednesday, August 28, 13
Interactions behind CR Propagation EBL CMB Gilmore et al. 2009 e.g., p ± γ → p ± e − e + Protons and Nuclei Bethe-Heitler pair production e.g., n N γ → n N ∗ → n − 1 Np Photodisintegration (nuclei only) e.g., p γ → ∆ (1232) → p π 0 Photomeson Production Synchrotron e.g., p ˜ γ → p γ e ± γ → e ± γ Electrons Inverse Compton e ± γ → e ± e + e − Triple Pair Production e ± ˜ γ → e ± γ Synchrotron γγ → e + e − Photons Pair Production γγ → e + e − e + e − Double Pair Production e.g., µ ± → ¯ ν e ), π 0 → γγ ν µ ( ν µ ) e ± ν e (¯ Mesons & Muons Decay µ ± ˜ γ → µ ± γ Synchrotron 2 Wednesday, August 28, 13
UHECR Propagation in Action Stecker 2006 Decerprit & Allard 2011 Wednesday, August 28, 13
UHECR Propagation in Action Decerprit & Allard 2011 Wednesday, August 28, 13
UHECR Source Stay Tuned! Intergalactic B-field weak (IGMF)? deflection strong halo B? deflection -6 10 B l a Milky z a r s -1 ] T o t a l -1 sr Way -2 s SF Galaxies 2 I E (E) [GeV cm -7 10 E ) G = 0 ) B -14 d e ( c a C a s 0 1 x 5 = B ( e d a c s a C -8 10 -1 0 1 2 10 10 10 10 Energy [GeV] C tot 2 ( E ) C bl 2 ( E ) C cas ( E ) = f bl l + f cas + cross terms l l Wednesday, August 28, 13
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