Cosmic Ray Electrons and GC Observations with H.E.S.S. Christopher van Eldik (for the H.E.S.S. Collaboration) MPI für Kernphysik, Heidelberg, Germany TeVPA '09, SLAC, July 2009
The Centre of the Milky Way
The Centre of the Milky Way H.E.S.S. 2004 (55 hours) H.E.S.S. J1745-290 H.E.S.S. J1745-290 G 0.9+0.1 38 sigma (55h) point-like < 1.2' (95% CL) Aharonian et al. (2006)
The Centre of the Milky Way H.E.S.S. 2004 (55 hours) H.E.S.S. J1745-290 point-like < 1.2' (95% CL) G 0.9+0.1 Diffuse emission Aharonian et al. (2006)
Molecular Cloud Association Lack of γ-rays for l > 1 Injection of protons at GC Assume k = ~3 kpc2 Myr-1 for TeV protons injection 104 years ago Fits age of Sgr A East
Diffuse Emission Spectrum Not just passive illumination - enhanced flux for > 1 TeV - photon index ~2.3 Similar index as HESS 1745-290 everywhere in the region
Possible Counterparts? VLA 300'' SNR Sgr A East? Chandra SMBH Sgr A*? DM? 10'' PWN G359.95-0.04?
Possible Counterparts? Position? Variability? Energy spectrum? VLA 300'' SNR Sgr A East? Chandra SMBH Sgr A*? DM? 10'' PWN G359.95-0.04?
Position: Sgr A East ruled out Best Fit HESS J1745-290 (Aharonian et al. 2004) Best Fit HESS J1745-290 (van Eldik et al. 2007) - preliminary0.04 deg Sgr A* Sgr A East Dedicated data set using optical guiding telescopes 6'' systematic pointing error Lack of association with Sgr A East Chance probability 10-4... 10-11 VLA 90cm image CvE et al., Proc. ICRC (2007)
Variability studies Sgr A* highly variable at other wavelengths Quasi-periodic oscillation Expect correlated VHE variability if emission produced close to BH surface No obvious variability in VHE lighcurve observed based on 93 hours of data HESS J1745-290, 28 min flux points Aharonian et al. (2009), arxiv:0906.1247
Flare Sensitivity Maximum needed lightcurve amplification for 3σ flare detection (as usual) statistics limited Aharonian et al. (2009) Aharonian et al. (2009)
Variability studies Simultaneous HESS and Chandra observations X-ray flare detected - 1700s duration - 9x quiescent level No increase of gamma flux 100% flux increase discarded at 99% CL Aharonian et al. (2008)
Search for QPOs (small time scales) Quasi-periodic oscillations observed in X-rays and IR X-ray periodicity 100 s, 219 s, 700 s, 1150 s, 2250 s Related to accretion disk? Rayleigh test for continuous 28 min observations (2004-2006 averaged) no hint for QPOs < 1150 s in VHE data Aharonian et al. (2009)
Search for QPOs (large time scales) Lomb-Scargle periodogram averaged over 2004-2006 Power spectrum compatible with noise No indication for QPOs on 600 s 1.5 h time scales Aharonian et al. (2009)
Spectrum a bit of history Hard spectrum: Γ = 2.25 ± 0.04 ± 0.10 10% Crab above 1 TeV No cut-off: EC > 9 TeV (95% CL)
Spectrum HESS J1745-290 2004-2006 data 93 h live time 4185 γ-rays (61 σ) 160 GeV < E < 70 TeV Exponential cut-off Γ = 2.10±0.04±0.10 Ec = 15.7±3.5±2.5 TeV χ²/d.o.f. = 23/26 Aharonian et al. (2009) A&A accepted arxiv:0906.1247
Spectrum HESS J1745-290 2004-2006 data 93 h live time 4185 γ-rays (61 σ) 160 GeV < E < 70 TeV Exponential cut-off Γ = 2.10±0.04±0.10 Ec = 15.7±3.5±2.5 TeV χ²/d.o.f. = 23/26 Aharonian et al. (2009) A&A accepted arxiv:0906.1247 Broken powerlaw Γ1 = 2.02±0.08±0.10 Γ2 = 2.63±0.14±0.10 EB = 2.57±0.19±0.44 χ²/d.o.f. = 20/19
HESS J1745: a pulsar wind nebula? Hinton + Aharonian (2007) G359.95 Sgr A* 10'' Wang et al. (2005) Dense radiation fields Comparably low magnetic field IC dominant plausible candidate
Sgr A* Emission Models pp interactions in accretion disk Aharonian & Neronov (2005) All models viable with current statistics CTA/AGIS will help LAT? electron scenario curvature + IC
Cosmic Ray Electrons Suffer severely from synchrotron and inverse Compton losses steep GeV spectrum ~E-3.3 steepening at TeV energies ~E-3.9 TeV electrons must come from local sources Compatible with lower-energy measurements: Г = 3.1 with cut-off at 2.1 TeV H.E.S.S. can measure electrons at TeV energies electrons are gamma-like large detection area Large backgrounds - Cosmic ray showers - Galactic diffuse emission - extragalactic diffuse emission Aharonian et al. (2008)
Berge et al. (2007) Standard Background Modelling
Electrons: Background Modelling data simulated background Random Forest: train machine learning algorithm on shower image parameters needs electron simulations and cosmic background for training For each shower, RF determines electron likeness parameter ζ ε [0;1] For ζ>0.9, total background suppression is 10-6 Signal extraction Fit ζ-distribution with combination of electron/proton simulations depends on hadronic interaction model (Sybill/QGSJet)
Gamma-ray contamination Extrapolation of gamma-ray flux to VHE energies suggests small contribution only FERMI preliminary extragalactic diffuse gamma flux softer than EGRET Test with first interaction height (only poorly reconstructed) At most 50% gamma contamination Aharonian et al. (2008)
Energy Spectrum Separate fits in energy bands Two complementary analyses: - high energies: 600 GeV 5 TeV (hard cuts for best reconstruction) Aharonian et al. (2008) - low energies: 340 GeV 700 GeV (looser cuts on image intensity, 2004/2005 data only) Aharonian et al. (2009) arxiv:0905.0105
Low Energy Analysis No indication of feature similar to ATIC Aharonian et al. 2009 Break in spectrum: Г1 = 3.0±0.1±0.3 Г2 = 4.1±0.3±0.3 EB = 0.9±0.1 TeV Compatible to FERMI within energy shift uncertainty Aharonian et al. (2009) arxiv:0905.0105
Putting Electrons and GC together Meade et al. (2009) arxiv:0905.0480
Summary Solid detection of the GC point source Sgr A East excluded as a source After 100 hours of observation, spectrum shows significant deviation simple power-law No indication for variability No indication for QPOs Measurement of CR electrons (+ extragalactic diffuse gammas) Implies existence of nearby sources Energy range 340 GeV 5 TeV Consistent with FERMI No indication for ATIC spectral feature Significant spectral steepening beyond 1 TeV Thanks!