Axion-like particles in the high energy universe
Denis Wouters
Denis Wouters
ALPs in the HE universe
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Which ALPs ?
A. Ringwald, 2012, arXiv: 1210.5081
Low mass ALPs
« Transparency hint »
Denis Wouters
ALPs in the HE universe
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Gould & Schréder, 1967, Phys. Rev.
Stecker, de Jager, Salamon, 1992, ApJ Lett.
Transparency hint?
•  Universe opaque at very high energies (VHE, E ~ 1 TeV)
Background photons
2
2
.5
3
2.5
3
Extragalactic background light (EBL)
3
2
5
1.5
2.
1
.5
2
5
1
1.
5
0.
0
1
.5
0.
.5
-0
1
0
5
0
.5
-0
-1
1
0
0
.5
0.5
0
1
0
.5
-1
-1 0
.5
-0
0
.5
0
1
-1
0
0
-0.5
1
1
.5
0.5
1
e+ e-
2
1
0.5
2
.5
0
-0
.5
-1
0
3
0.5
1
TeV γ-rays
1.5
2
2.5
3
HESS 2, 2012
φ
φ
at the source
at the source
at the Earth
1 TeV
Denis Wouters
1 TeV
E
ALPs in the HE universe
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The Extragalactic Background Light:
•  Various models,
uncertainties
Dole et al. 2006, A&A
•  Optical background (COB)
•  hard to measure
(foreground emission from Milky Way)
TeV γ-ray absorption
Kneiske & Dole, 2010, A&A
Denis Wouters
ALPs in the HE universe
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Universe too transparent?
too much γ
detected at VHE
EBL corrected
φ
at the source
at the Earth
1 TeV
E
HESS Collaboration, 2006, Nature
•  2006: H.E.S.S. observations of 2 sources at z = 0.165, 0.186
•  2008: MAGIC observation of 3C279 at z = 0.536
MAGIC Collaboration, 2008, Science
The universe is more transparent than expected!
Denis Wouters
ALPs in the HE universe
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ALPs can explain this effect:
•  γ-ALP oscillations in intergalactic magnetic field (IGMF)
•  ALPs not absorbed by EBL
Background photons
2
2
.5
3
2.5
3
Extragalactic background light (EBL)
3
2
5
1.5
2.
.5
2
1
5
1
1.
5
0.
0
1
.5
0.
.5
-0
1
0
5
0
.5
-0
-1
1
0
0
.5
0.5
0
1
0
.5
-1
-1 0
.5
-0
0
.5
0
1
-1
0
0
-0.5
1
1
.5
0.5
1
ALPs
2
1
0.5
2
.5
0
-0
.5
-1
0
3
0.5
TeV γ-rays
1
1.5
2
2.5
3
EBL without ALPs corrected
φ
at the source
De Angelis et al. 2007, PRD
Simet et al. 2008, PRD
Sanchez Conde et al. 2009, PRD
EBL with ALPs corrected
at the Earth
1 TeV
Denis Wouters
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ALPs in the HE universe
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γ-ALP mixing:
•  Two photon polarization states, one ALP state
•  Optical depth of photons on the EBL
•  No plasma correction, Faraday rotation, Birefringence
Projection on
polarization axis
Mixing matrix:
EBL absorption
γ-ALP coupling with
transverse component of B
Denis Wouters
ALPs in the HE universe
ALP mass
Csaki et al. 2005, JCAP
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Coherent magnetic field:
•  Magnetic field: Uniform orientation and strength
•  Turn-off EBL absorption (for the moment)
•  Conversion in domain of size s:
δ
•  Effect is energy dependent. Critical energy
–  E << Ec : No conversion
–  E ~Ec : spatial oscillations, energy dependent
–  E >> Ec : spatial oscillations, energy independent
Hochmuth & Sigl, 2007, PRD
•  δ : strength of the coupling ó Hillas criterion (max.
)
Hooper & Serpico, 2007, PRL
Denis Wouters
ALPs in the HE universe
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Coherent magnetic field:
•  Example for different values of δ:
1 domain, g = 8 × 10-11 GeV-1, ma = 2 neV, Ethr = 1 TeV
Wouters & Brun, 2012, PRD
Pγ → γ = 1 - Pγ → a
1
0.9
0.8
0.7
δ=5
δ = 10
δ = 15
0.6
0.5
10-1
1
10
102
Energy (TeV)
•  Unpredictable attenuation at high energies
Denis Wouters
ALPs in the HE universe
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Astrophysical magnetic fields:
•  Various magnetic fields:
–  Milky-way
–  Intergalactic Magnetic Field (IGMF)
–  Cluster of galaxies
•  Not coherent: turbulence
•  Naive representation of the turbulence:
γ
γ, a
Coherence length
Magnetic field coherent within one domain
Denis Wouters
ALPs in the HE universe
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Averaged behavior:
•  Behavior for average over all possible realizations of B
•  Two effects in the spectrum:
–  Drop of 1/3 at Ec
–  Boost at high energies due to non-EBL absorption of ALPs
Sanchez Conde et al. 2009, PRD
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ALPs in the HE universe
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For one realization:
•  Average behavior
Average over many sources
•  Not available at TeV energies: ~ 10 sources.
•  ALP mixing random process: variance of the prediction?
universe can be more transparent or more opaque!
Variance for
prediction
EBL model only
Average prediction
Mirizzi et Montanino, 2009, JCAP
•  Large population studies required for average
Denis Wouters
ALPs in the HE universe
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Population studies:
•  Look for anomaly in spectra of extragalactic sources
•  2012: 24 extragalactic sources discovered
Horns & Meyer, 2012, JCAP
•  Claim for unexpected boost at VHE (4.2σ effect)
•  Not enough sources… prospect for CTA
Denis Wouters
ALPs in the HE universe
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Signature for single source:
•  Boost at VHE not valid for observations of one source
•  Behavior around critical energy?
-  Source at redshift 0.1
-  g = 8 10-11 GeV-1 , ma = 2 neV, B = 1 nG , L = 1 Mpc
0.8
Without EBL
absorption
P
0.6
0.4
0.2
1
« Opacity effect »
P
10-1
-2
10
With EBL
absorption
Wouters & Brun, 2012, PRD
1
EBL without ALPs
10-1
1
10
102
Energy (TeV)
Smooth
Denis Wouters
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Noisy
ALPs in the HE universe
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Smooth
behavior
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Signature for single source:
•  Turbulence of magnetic field
irregularities in spectrum
•  Irregularities around critical energy
•  Can be detected by Cherenkov telescopes
Wouters & Brun, 2012, PRD
0.6
10-4
0.4
-6
10
Relative fit residuals
Flux (photons m-2 s-1 TeV-1 )
-5
10
10-7
-8
10
-9
10
-10
10
10-11
10-12
Unbinned spectrum
(EBL × log-parabola) fit
0.2
0
-0.2
With ALPs
-0.4
Binned spectrum
Without ALPs
-13
10
-0.6
10-1
1
10
Energy (TeV)
10-1
1
Energy (TeV)
•  Proposed for quasars observations in optical
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ALPs in the HE universe
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Application to HESS:
•  Look for irregularities in spectrum of one bright source
Denis Wouters
ALPs in the HE universe
H.E.S.S. Collaboration, 2007, ApJ Lett.
•  Brightest extragalactic source: PKS 2155-304
•  Redshift z = 0.116 (d = 478 Mpc)
•  Big flare in July 2006, ~50*base flux
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Conclusion:
•  ALPs relevant for γ-ray astronomy at very low mass ( < µeV)
•  Unexpected transparency of the universe:
– 
– 
– 
– 
Anomaly weakens with recent EBL models
can be explained by ALPs
can be explained by other exotic models
cannot be used to put constraints on ALP models
•  Irregularities at critical energy:
–  independent from « transparency hint »
–  signature for potential detection
–  can be used to put constraints with HESS
Denis Wouters
ALPs in the HE universe
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