Cosmic-ray iron and electron detection with H.E.S.S. Rolf Bühler • ACKS Seminar • 28 of January, Stanford Astrophysics Colloquium

Outline Introduction to cosmic rays The H.E.S.S. telescopes Measuring the iron spectrum Measuring the electron spectrum Summary and Outlook

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

Energy Spectrum Remarkably featureless energy spectrum Well described by power-law with softening at ≈4 PeV (the “knee”) Confined to the galaxy below the knee Total energy density ≈1 eV cm-3 γ ≈ 2.7 “knee” ~4 PeV Nuclei (98%) Electrons (2%) γ ≈ 3.0

Composition Similar to solar but: Enhancement below C-N-O and Fe → Spallation, traversed ≈40 g cm-2 at 1 GeV C-N-O Si Fe Engelmann et al. 1990 Radioactive “clocks” → confinement of ≈10 Myrs at 1 GeV Yanasak et al. 2001 Meyer et al. 1997 Less H and He → Less high ionization energy or high volatility elements Normalized to Silicon At 1 TeV

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

Where do they come from? Isotropic flux, deflected by magnetic fields, no directional information left Options: Measure spectrum and composition and model source/propagation Use neutral tracers (photons, neutrinos) Everything points to Super Novae Remnants (below the knee) →

Why Super Novae Remnants? 1) Photon observations: 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: Power law of index 2 result from Fermi I acceleration. Index of 2.7 from propagation effects Knee could correspond to maximum particle energy (gradually light to heavy nuclei break away) Bell 1978 Hoerandel 2004

Why Super Novae Remnants? 𝑃≈ 𝑉ϱ τ ≈ 10 41 𝑒𝑟𝑔 𝑠 −1 Assume local cosmic ray density in galaxy ≈ 107 years (from spallation and radioactive isotopes) 𝑃 𝑠𝑢𝑝𝑒𝑟𝑛𝑜𝑣𝑎𝑒 ≈ 10 51 𝑒𝑟𝑔 30𝑦𝑒𝑎𝑟𝑠 ≈ 10 42 𝑒𝑟𝑔 𝑠 −1 Supernovae rate from similar galaxies 3) Energetics: Helder et al. 2009 They do efficiently release energy into CR The sources of cosmic-ray electrons: 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

H.E.S.S. Telescopes Located in Namibia (1800 a.s.l.) Sensitive between ~0.1 to 100 TeV Field of View of 5º diameter

Gamma-ray detection Image shower Cherenkov light High cosmic-ray background Rejection of ~99%, hadron showers are wider Remaining background from regions off the source ≈ 30 km EAS-light Not possible for diffuse signal

Shower reconstruction Shower-light ≈ 30 km γ-ray Resolution: Direction 0.1° Core position 20m Energy 15% ≈ 2º Shower direction Energy from total intensity and core distance ≈ 100 m

Iron detection Detection of Cherenkov Light before first interaction Z DC-light Shower-light ≈ 2º DC-light Shower direction Shower-light ≈ 100 m

DC-Light detection DC-light ~ Z2 Shower intensity ~ E Fe Shower outshines DC-light ~ Z2 Shower intensity ~ E Iron detection >13 TeV (high Z and flux) Cherenkov threshold Kieda et al. 1999

Dataset & Charge Reconstruction Effective exposure of ≈107 m2 sr s → In total 1899 events with DC-light in 2 telescopes (background-free) Charge reconstruction over DC-light intensity. Fit iron fraction in five energy bins 1.5 < lg( E/TeV ) <1.7 𝑍=𝑘 θ,𝐸 𝐼 𝐷𝐶

Iron Spectrum Good agreement with other experiments Hadronic model ≈20% on normalizarion (smaller than at higher energies) Power-law Index γQGSJET= 2.62 +- 0.11 γSIBYLL= 2.76 +- 0.11 Among most precise Proof of principle Aharonian et al. 2007

Electron Detection Electrons (positrons) induce narrow EM-showers Analysis done by Kathrin Eggberts Electrons (positrons) induce narrow EM-showers No off-source region → background from simulations (SIBYLL 2.1 and QGSJET II) “Electron likeness”  from random forest resulting in 10-4 hadron rejection in  > 0.9 Large effective exposure of ≈2·107 m sr s  Simulated background Data Electron excess

Electron Detection Fit electron contribution in energy bands in >0.6 region (contribution of heavier elements negligible)

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

Electron Spectrum Spectral softening at ≈1 TeV ( γ ≈ 3→4.1 ) Extends up to 4 TeV → source within ≈1 kpc ATIC peak disfavoured (yet not excluded) Fermi & HESS spectrum can be modelled including Klein- Nishina effect and source cutoff → No “exotic physics” required. Aharonian et al. 2008, Acero et al. 2009 Stawarz et al. 2009, Schlickeiser et al. 2009

Atmosphere Uncertainties Error on energy scale of 15% from: Uncertainty of atmospheric density profile (showers could be closer/nearer, ≈3 g cm2 at Xmax) Uncertainty in dust and ozon absorbtion No temporal variations considered Optical efficiency of detector and opacity low atmosphere known though muons.

Hadronic-model Uncertainties SIBYLL and QGSJET results in ≈20% difference in flux normalization and ≈0.2 in index, comes from: Electrons How often does a proton look like an electron? Iron At which depth does the nuclei interact? Which particles are created? p π0 γ Fe N

Conclusions Iron measurement One of the most precise between 13-200 TeV Agreement with independent technique Proof of Principle for DC-light detection Electron measurement Extension of spectral measurements to 4 TeV Spectral cutoff around 1 TeV ATIC-peak disfavored Proof of principle of ground based detection

Outlook AGIS / CTA increase in exposure by ~30 with respect to H.E.S.S. → Iron spectrum to ~PeV → Electron spectrum ~15 TeV Lower energy threshold of ~100 GeV for electrons. Maybe already with H.E.S.S. II or MAGIC II CTA / AGIS (~2014) MAGIC II (2009) H.E.S.S. II (~2011)

Outlook Improvement of systematics Hadronic Models Atmospheric → Will be highly constrained by LHC experiments testing forward direction reactions (LHCf, TOTEM) Will reach lab energies of few PeV (Already sufficient: ~10 TeV p on N → ECM~50 GeV) Atmospheric Future instruments will have atmospheric monitoring Dova et al. 2007 → Great prospects for cosmic-rays measurements

Backup slides..

Simulated flux assumes composition of Dataset & Background Simulated flux assumes composition of Hoerandel et al. 2003 Effective exposure of ≈107 m2 sr s → In total 1899 events with DC-light in 2 telescopes (background-free) DC and shower light yield prediction depends on hadronic model → Estimate error from using QGSJET 01 / SIBYLL 2.1

Charge reconstruction 𝑍=𝑘 θ,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. Reconstructed charge: (k(E,θ) normalizes Z* to iron)