Cosmic-ray iron and electron
detection with H.E.S.S.
Rolf Bühler •
ACKS Seminar
Astrophysics Colloquium
• 28 of January, Stanford
Outline
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Introduction to cosmic rays
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The H.E.S.S. telescopes
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Measuring the iron spectrum
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Measuring the electron spectrum
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Summary and Outlook
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Cosmic Ray Discovery
Discovered (beyond doubt) by
Victor Hess
“The result of these observations
seems best explained by a
radiation of great penetrating power
entering our atmosphere from
above..”
Phys. Zeitschriften 1912
High energy particles reaching
Earth at a rate of ≈1000 s-1m-2
3
Energy Spectrum
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●
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Remarkably featureless
energy spectrum
Well described by powerlaw with softening at
≈4 PeV (the “knee”)
γ ≈ 2.7
“knee”
~4 PeV
Confined to the galaxy
below the knee
Total energy density
≈1 eV cm-3
Nuclei (98%)
Electrons (2%)
γ
≈ 3.0
4
Composition
Similar to solar but:
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Enhancement below C-NO and Fe
→ Spallation, traversed
≈40 g cm-2 at 1 GeV
C-N-O
Si
Fe
Engelmann et al. 1990
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Radioactive “clocks”
→ confinement of ≈10 Myrs
at 1 GeV
Yanasak et al. 2001
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Less H and He
→ Less high ionization energy
or high volatility elements
Normalized to Silicon
At 1 TeV
Meyer et al. 1997
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Composition
Index independent of
element
→ Hints at common origin
Spallation elements
have softer spectrum
→ Energy dependent
escape from galaxy
Swordy et al. 1990
Tracer & CRN Ave et al. 2008
CREAM II Ahn et al. 2009
Compilation Wieble Sooth 1998
6
Where do they come from?
Isotropic flux, deflected by magnetic
fields, no directional information left
Options:
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Measure spectrum and composition
and model source/propagation
Use neutral tracers (photons,
neutrinos)
→ Everything points to Super
Novae Remnants (below the knee)
7
Why Super Novae Remnants?
1) Photon observations:
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Non-thermal spectrum, consistent
with origin from pion decays at
high energies
Aharonian et al. 2004, Abdo et al. 2010,
Ellison et al. 2010
RXJ 1713 above 200 GeV
2) Cosmic-ray spectrum:
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Power law of index 2 result from Fermi I acceleration. Index of
2.7 from propagation effects Bell 1978
Knee could correspond to maximum particle energy (gradually
light to heavy nuclei break away) Hoerandel 2004
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Why Super Novae Remnants?
3) Energetics:
Vϱ
P≈
≈1041 erg s−1

≈ 107 years
(from spallation and
radioactive isotopes)
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1051 erg
P supernovae ≈
≈10 42 erg s−1
30 years
Assume local cosmic ray
density in galaxy
Supernovae rate from
similar galaxies
They do efficiently release energy into CR
Helder et al. 2009
The sources of cosmic-ray electrons:
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Are not constrained by (1), could also be pulsars, which also fulfill
arguments (2), (3)
Should be local ( ≈1kpc) for ≈1 TeV electrons due to fast energy loss
Kobayashi et al. 2004
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H.E.S.S. Telescopes
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Located in Namibia (1800 a.s.l.)
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Sensitive between ~0.1 to 100 TeV
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Field of View of 5º diameter
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Gamma-ray detection
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Image shower Cherenkov light
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High cosmic-ray background
Rejection of ~99%, hadron showers
are wider
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γ-ray
≈ 30 km
Remaining background from
regions off the source
S
EA
ht
-lig
Not possible for diffuse signal
11
Shower reconstruction
Resolution:
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Direction 0.1°
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Core position 20m
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Energy 15%
≈ 30 km
ht
-lig
er
ow
Sh
≈ 2º
Shower direction
γ-ray
Energy from total
intensity and core distance
≈ 100 m
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Iron detection
Z
Detection of Cherenkov Light
before first interaction
DC-light
ht
ht
-lig
er DC-lig
ow
Sh
≈ 2º
Shower direction
Shower-light
≈ 100 m
13
DC-Light detection
Fe
Shower outshines
Cherenkov
threshold
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DC-light ~ Z2
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Shower intensity ~ E
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Iron detection >13 TeV
(high Z and flux)
Kieda et al. 1999
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Dataset & Charge Reconstruction
Effective exposure of ≈107
m2 sr s
1.5 < lg( E/TeV ) <1.7
→ In total 1899 events with
DC-light in 2 telescopes
(background-free)
Charge reconstruction over
DC-light intensity.
Z =k  , E   I DC
Fit iron fraction in five
energy bins
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Iron Spectrum
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Good agreement with
other experiments
Hadronic model
≈20% on
normalizarion
(smaller than at higher
energies)
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Power-law Index
γQGSJET= 2.62 +- 0.11
γSIBYLL= 2.76 +- 0.11
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Among most precise
Proof of principle
Aharonian et al. 2007
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Electron Detection
Electrons (positrons) induce narrow
EM-showers
Analysis done by
Kathrin Eggberts
No off-source region
→ background from simulations
(SIBYLL 2.1 and QGSJET II)
“Electron likeness” ζ from random
forest resulting in 10-4 hadron
rejection in ζ > 0.9
Large effective exposure of
≈2·107 m sr s
Data
Electron excess
Simulated
background
ζ
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Electron Detection
Fit electron contribution in energy bands in >0.6 region
(contribution of heavier elements negligible)
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Gamma-ray Background?
Only extra-galactic sky off
sources considered, still similar
showers, so diffuse gammas?
Gammas interact 7/9 rad. length
lower. Fit of Xmax distribution
→ gamma-rays less than 50%
Low level of gamma-ray
background expected due to
pair creation on photon
background
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Electron Spectrum
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Spectral softening at ≈1 TeV
( γ ≈ 3→4.1 )
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Extends up to 4 TeV
→ source within ≈1 kpc
ATIC peak disfavoured
(yet not excluded)
Fermi & HESS spectrum can
be modelled including KleinNishina effect and source
cutoff
→ No “exotic physics” required.
Stawarz et al. 2009, Schlickeiser et al. 2009
Aharonian et al. 2008, Acero et al. 2009
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Atmosphere Uncertainties
Error on energy scale of 15% from:
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Uncertainty of atmospheric
density profile (showers could be
closer/nearer, ≈3 g cm2 at Xmax)
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Uncertainty in dust and ozon
absorbtion
No temporal variations
considered
Optical efficiency of detector and
opacity low atmosphere known
though muons.
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Hadronic-model Uncertainties
SIBYLL and QGSJET results in ≈20% difference in flux
normalization and ≈0.2 in index, comes from:
p
Electrons
How often does a proton look like an electron?
π0
γ
Iron
At which depth does the nuclei interact?
Fe
N
Which particles are created?
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Conclusions
Iron measurement
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One of the most precise between 13-200 TeV
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Agreement with independent technique
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Proof of Principle for DC-light detection
Electron measurement
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Extension of spectral measurements to 4 TeV
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Spectral cutoff around 1 TeV
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ATIC-peak disfavored
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Proof of principle of ground based detection
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Outlook
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AGIS / CTA increase in
exposure by ~30 with respect
to H.E.S.S.
CTA / AGIS (~2014)
→ Iron spectrum to ~PeV
→ Electron spectrum ~15 TeV
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Lower energy threshold of
~100 GeV for electrons.
Maybe already with
H.E.S.S. II or MAGIC II
MAGIC II (2009)
H.E.S.S. II (~2011)
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Outlook
Improvement of systematics
Hadronic Models
Will be highly constrained by LHC experiments testing forward
direction reactions (LHCf, TOTEM) Dova et al. 2007
Will reach lab energies of few PeV
(Already sufficient: ~10 TeV p on N → ECM~50 GeV)
Atmospheric
Future instruments will have atmospheric
monitoring
→ Great prospects for cosmic-rays measurements
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Backup slides..
Dataset & Background
Simulated flux assumes composition of
Hoerandel et al. 2003
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Charge reconstruction
Z * =k  , E   I DC
1.3 < lg( E / TeV ) < 1.5
DC-intensity depends on:
- first interaction height
- energy (const > Ethreshold)
→ Allows measurement of the iron fraction in the data.
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