Probing the Intergalactic Magnetic Field Using Intensity - - PowerPoint PPT Presentation

Tonia Venters

Astrophysics Science Division NASA Goddard Space Flight Center

Probing the Intergalactic Magnetic Field Using Intensity Fluctuations in the Extragalactic Gamma-ray Background*

*based on Venters & Pavlidou 2013, MNRAS, 432, 3485

Wednesday, August 28, 13

The Problem...

Pierre Auger (concept)

Milky Way halo B? Intergalactic B-field (IGMF)?

weak deflection strong deflection

UHECR Source

Telescope Array (concept)

Wednesday, August 28, 13

Gamma-ray Satellites Air Shower Arrays IACTs

Our Allies

Neutrino Expts.

Wednesday, August 28, 13

The Gamma-ray Sky

Inverse Compton π0-decay Bremsstrahlung

Galactic diffuse emission

(CR interactions with the interstellar medium)

Resolved Point Sources Isotropic diffuse emission

(presumably extragalactic)

Wednesday, August 28, 13

Isotropic diffuse emission

(presumably extragalactic)

Gamma-ray Sky (after subtraction)

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

• f radio galaxies)

Suspected contributors:

• Truly diffuse emission -

gamma rays produced in EM cascades of highly energetic particles Players about which we like to speculate:

• Exotic physics (e.g., dark

matter annihilation?)

Inverse Compton π0-decay Bremsstrahlung

Galactic diffuse emission

(CR interactions with the interstellar medium)

Isotropic diffuse emission

(presumably extragalactic)

Resolved

Wednesday, August 28, 13

Extragalactic background light (EBL) consists of:

Emission from starlight at NIR/Opt./UV

wavelengths

Reradiated thermal dust emission at FIR

wavelengths

Gilmore et al. 2009

Cascades -

e+e- pair production inverse Compton scattering of cascade electrons

γ γ e+ e- γ γ γ γ e- e+

VHE Gamma Rays in the EBL

Venters 2009

Blazar See also D. Williams’ talk (tomorrow)

Wednesday, August 28, 13

Magnetic Deflection of Cascades

Halo AGN EM Cascade TeV γ GeV γ

image credit: Ando 2009

• Gamma-rays initially

emitted off observer’s line-of-sight initiate cascades that are deflected in direction of

• bserver.
• Deflected emission

makes a halo around source.

See also W. Essey’s talk (tomorrow)

Wednesday, August 28, 13

Anisotropy Studies

Isotropic diffuse emission

(presumably extragalactic)

alma∗

lm = Cl

∂I I =

∞

• l=1

l

• m=−l

almYlm

Cltot(E) = f12(E)Cl(1) + f22(E)Cl(2) + cross terms, where fn(E) = In(E)/Itot(E)

Star-forming Galaxies Blazars Deflected Cascade

Wednesday, August 28, 13

Anisotropy Studies

Isotropic diffuse emission

(presumably extragalactic)

alma∗

lm = Cl

∂I I =

∞

• l=1

l

• m=−l

almYlm

Cltot(E) = f12(E)Cl(1) + f22(E)Cl(2) + cross terms, where fn(E) = In(E)/Itot(E)

~ 1 GeV ~ 10 GeV ~100 GeV

Wednesday, August 28, 13

10

• 1

10 10

1

10

2

Energy [GeV] 10

• 7

10

• 6

E

2IE(E) [GeV cm

• 2 s
• 1 sr
• 1]

T

• t

a l C a s c a d e ( B = ) C a s c a d e ( B = 5 x 1

• 14

G ) Blazars S F G a l a x i e s

Anisotropy as a Function of Energy

Ctot

l

(E) =fbl

2(E)Cbl l +fgal 2(E)Cgal l

Wednesday, August 28, 13

Impact of Cascades (null B)

Isotropic diffuse emission

(presumably extragalactic)

alma∗

lm = Cl

∂I I =

∞

• l=1

l

• m=−l

almYlm

Cltot(E) = f12(E)Cl(1) + f22(E)Cl(2) + cross terms, where fn(E) = In(E)/Itot(E)

Star-forming Galaxies Blazars Cascades from Blazars

Wednesday, August 28, 13

10

• 1

10 10

1

10

2

Energy [GeV] 10

• 7

10

• 6

E

2IE(E) [GeV cm

• 2 s
• 1 sr
• 1]

Total Cascade (B = 0) C a s c a d e ( B = 5 x 1

• 14

G ) B l a z a r s SF Galaxies

The Impact of Cascades (zero B)

Ctot

l

(E) =(fbl(E) + fcas(E))2Cbl

l

Wednesday, August 28, 13

Impact of Cascades (intermediate B)

Isotropic diffuse emission

(presumably extragalactic)

alma∗

lm = Cl

∂I I =

∞

• l=1

l

• m=−l

almYlm

Cltot(E) = f12(E)Cl(1) + f22(E)Cl(2) + cross terms, where fn(E) = In(E)/Itot(E)

Star-forming Galaxies Blazars Cascades from Blazars

Wednesday, August 28, 13

Impact of Cascades (non-zero B)

Isotropic diffuse emission

(presumably extragalactic)

alma∗

lm = Cl

∂I I =

∞

• l=1

l

• m=−l

almYlm

Cltot(E) = f12(E)Cl(1) + f22(E)Cl(2) + cross terms, where fn(E) = In(E)/Itot(E)

Star-forming Galaxies Blazars Cascades from Blazars

Wednesday, August 28, 13

10

• 1

10 10

1

10

2

Energy [GeV] 10

• 8

10

• 7

10

• 6

E

2IE(E) [GeV cm

• 2 s
• 1 sr
• 1]

T

• t

a l C a s c a d e ( B = ) Cascade (B = 5 x10

• 1

4

G) B l a z a r s SF Galaxies

The Impact of Cascades (non-zero B)

Ctot

l

(E) =fbl

2(E)Cbl l +fcas 2(E)Ccas l

+ cross terms

Wednesday, August 28, 13

Cascades and EGB Anisotropy

10

• 1

10 10

1

10

2

Energy [GeV] 10

• 8

10

• 7

10

• 6

E

2IE(E) [GeV cm

• 2 s
• 1 sr
• 1]

Total Cascade (B = 0) Cascade (B = 5 x10

• 14

G) Blazars SF Galaxies

2

10 10

1

10

2

Energy [GeV] 10

• 6

10

• 5

10

• 4

Cl [sr]

Venters & Pavlidou 2013

Wednesday, August 28, 13

Interactions behind CR Propagation

Gilmore et al. 2009

Protons and Nuclei Bethe-Heitler pair production e.g., p±γ → p±e−e+ Photodisintegration (nuclei only) e.g., nNγ → nN ∗ → n−1Np Photomeson Production e.g., pγ → ∆(1232) → pπ0 Synchrotron e.g., p˜ γ → pγ Electrons Inverse Compton e±γ → e±γ Triple Pair Production e±γ → e±e+e− Synchrotron e±˜ γ → e±γ Photons Pair Production γγ → e+e− Double Pair Production γγ → e+e−e+e− Mesons & Muons Decay e.g., µ± → ¯ νµ (νµ) e±νe (¯ νe), π0 → γγ Synchrotron µ±˜ γ → µ±γ

2

CMB EBL

Wednesday, August 28, 13

Decerprit & Allard 2011

UHECR Propagation in Action

Stecker 2006

Wednesday, August 28, 13

Decerprit & Allard 2011

UHECR Propagation in Action

Wednesday, August 28, 13

Stay Tuned!

10

• 1

10 10

10

Energy [GeV] 10

• 8

10

• 7

10

• 6

E

• 2 s
• 1 sr
• 1]

• t

• 14

Ctot

l

(E) =fbl

2(E)Cbl l +fcas 2(E)Ccas l

+ cross terms Milky Way halo B? Intergalactic B-field (IGMF)? weak deflection strong deflection UHECR Source

Wednesday, August 28, 13