Download Constraints on Axion-Like Particles from HESS observations of PKS

Constraints on Axion-Like Particles from
H.E.S.S. observations of PKS 2155-304
D. Wouters* and P. Brun* for the H.E.S.S. Collabora9on * CEA, Irfu, Centre de Saclay, F-­‐91191 Gif-­‐sur-­‐YveGe, France Rencontres de Moriond 2013
Axion-­‐like par9cles •  1977: U(1) Peccei-­‐Quinn symmetry Possible explana9on for strong CP problem •  par9cle associated to the breaking of the symmetry: Axion •  Coupling with two photons: γ
a gγa Axions: gγa ∝ m γ
•  Axion-­‐like par9cles (ALPs): gγa and m unrelated but same phenomenology •  Possible explana9on for dark maGer (for specific m, gγa) •  CAST (solar ALPs) : gγa < 8*10-­‐11 GeV-­‐1 for m < 0.01 eV H.E.S.S. Collabora9on
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Axion-­‐like par9cles in astrophysics •  γ/ALP oscilla9ons in external magne9c field
B Mixing for a γ
•  Possibly modify opacity of the universe to TeV γ-­‐rays Background photons Extragalac9c background light (EBL) TeV γ-­‐rays ALPs Sanchez Conde et al. 2009, PRD De Angelis et al. 2007, PRD Horns et al. 2012, PRD Simet et al. 2008, PRD … H.E.S.S. Collabora9on
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Phenomenology in turbulent magne9c field around 1
Pγ → γ
0.8
0.6
0.4
0.2
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Pγ → γ
Wouters and Brun, 2012, PRD •  Magne9c fields in astrophysics usually turbulent •  ALPs ⇒ Strong irregulari9es in energy spectra 0.1
1
10
100
1
10
102
10-1
10-2
With ALPs
Without ALPs
10-1
Energy (TeV)
B = 1 nG Coherence length s = 1 Mpc gγa = 8 10-­‐11 GeV-­‐1 m = 2 neV ⇒ Ec = 1 TeV •  Amplitude of irregulari9es driven by gγa •  Posi9on in energy driven by m •  Signature detectable in TeV spectra H.E.S.S. Collabora9on
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Choice of the source •  Brightest AGN observed by H.E.S.S. : PKS 2155-­‐304 •  Strong flare in July 2006: ∼ 7 Crab flux above 200 GeV Large sta9s9cs: accurate spectrum and sensi9vity to irregulari9es •  Galaxy cluster observed around PKS 2155-­‐304 Magne9c field in the cluster •  Redshij z = 0.116 Magne9c field in intergalac9c medium H.E.S.S. Collabora9on
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Modeling of magne9c fields •  Magne9c fields in galaxy clusters –  Measurement by Faraday rota9on –  B > 1 µG –  Kolmogorov power spectrum on scales up to 10 kpc •  PKS 2155-­‐304 cluster – 
– 
– 
– 
Size of the cluster : 370 kpc No measurement of B and turbulence power spectrum Assumes B = 1 µG, Kolmogorov power spectrum on scales 1→10 kpc Conserva9ve descrip9on •  Intergalac9c magne9c field –  10-16 G < Β < 10-9 G –  Assumes turbulence on one scale, 1 Mpc H.E.S.S. Collabora9on
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PKS 2155-­‐304 observa9ons with H.E.S.S. Observa9ons with 4 telescopes Dataset from 2006 flare: background free Energy resolu9on ∼ 15%, threshold of 250 GeV 45505 reconstructed γ-­‐rays Unfolded spectrum: Well –modeled with log-­‐parabola with EBL absorp9on (from Franceschini et al. 2008, A&A) dN/dE [ cm-2 s-1 TeV -1 ]
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• 
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• 
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10-10
10-11
10-12
10-13
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Energy [ TeV ]
H.E.S.S. Collabora9on
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χ
-  α = 3.18 ± 0.03
-  β = 0.32 ± 0.02
-  F( E > 200 GeV) = 8.38 ± 0.43 10-10 cm-­‐2s-­‐1 0
-5
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Energy [ TeV ]
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Method for the constraints (1) •  Intrinsic spectral shape unknown •  Es9mate irregulari9es in spectrum without spectral shape assump9on: i
Energy
•  Look for anomalous devia9ons in triplet of successive bins •  Es9mator of irregulari9es in spectrum •  Assump9on: intrinsic spectrum log-­‐linear on scales of 3 bins H.E.S.S. Collabora9on
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Method for the constraints (2) •  Template of irregulari9es depends on exact magne9c field structure (unknown) •  Simula9ons of spectra for given (m, gγa) with various magne9c field structure, in same observa9on condi9ons •  Spectral shape: fiGed on data * ALP irregularity template •  Reconstruc9on with same analysis chain P.D.F. of the irregularity es9mator for given (m, gγa) •  Compare with es9mator value measured on data H.E.S.S. Collabora9on
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Method for the constraints (3) •  Exclusion on a sta9s9cal basis: 1000
Measured value
g a = 0.5 10-11 GeV-1
Entries
g a = 10-10 GeV-1
500
0
10
1
10
I
•  Measured value slightly depends on the binning 2
Es9mate fluctua9ons with different bin sizes, take upper value H.E.S.S. Collabora9on
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Constraints (1) •  Constraints expressed in parameters making them independent of B and L (size of conversion domain) 10
H.E.S.S. exclusions at 95% C.L.
1
Galaxy cluster Magnetic Field
Intergalactic Magnetic Field
10-3
10-4
E
H.E.S.S. Collabora9on
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Constraints (2) L → size of conversion domain s → coherence length •  Transla9on to gγa and m with: B L L/s Cluster magne9c field 1 µG 370 kpc 37 IGMF 1 nG 478 Mpc 505 gγ a [ 10-11 GeV-1 ]
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H.E.S.S. exclusions at 95 % C.L.:
Intergalactic Magnetic Field (optimistic)
Galaxy Cluster magnetic field (conservative)
1
10-1
CAST limit
1
10
102
m [ neV ]
H.E.S.S. Collabora9on
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Conclusion •  ALPs usually considered in γ-­‐ray astronomy for «transparency hint » •  ALPs signature: irregulari9es in γ-­‐ray spectra •  Mixing considered for: –  Galaxy cluster magne9c field (conserva9ve) –  Intergalac9c magne9c field (op9mis9c) •  Constraints in limited mass range •  First exclusion from TeV γ-­‐ray astronomy H.E.S.S. Collabora9on
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