Cosmic Ray Electrons and GC Observations with H.E.S.S.

Transcription

1 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

2 The Centre of the Milky Way

3 The Centre of the Milky Way H.E.S.S (55 hours) H.E.S.S. J H.E.S.S. J G sigma (55h) point-like < 1.2' (95% CL) Aharonian et al. (2006)

4 The Centre of the Milky Way H.E.S.S (55 hours) H.E.S.S. J point-like < 1.2' (95% CL) G Diffuse emission Aharonian et al. (2006)

5 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

6 Diffuse Emission Spectrum Not just passive illumination - enhanced flux for > 1 TeV - photon index ~2.3 Similar index as HESS everywhere in the region

7 Possible Counterparts? VLA 300'' SNR Sgr A East? Chandra SMBH Sgr A*? DM? 10'' PWN G ?

8 Possible Counterparts? Position? Variability? Energy spectrum? VLA 300'' SNR Sgr A East? Chandra SMBH Sgr A*? DM? 10'' PWN G ?

9 Position: Sgr A East ruled out Best Fit HESS J (Aharonian et al. 2004) Best Fit HESS J (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 VLA 90cm image CvE et al., Proc. ICRC (2007)

10 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 J , 28 min flux points Aharonian et al. (2009), arxiv:

11 Flare Sensitivity Maximum needed lightcurve amplification for 3σ flare detection (as usual) statistics limited Aharonian et al. (2009) Aharonian et al. (2009)

12 Variability studies Simultaneous HESS and Chandra observations X-ray flare detected s duration - 9x quiescent level No increase of gamma flux 100% flux increase discarded at 99% CL Aharonian et al. (2008)

13 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 ( averaged) no hint for QPOs < 1150 s in VHE data Aharonian et al. (2009)

14 Search for QPOs (large time scales) Lomb-Scargle periodogram averaged over Power spectrum compatible with noise No indication for QPOs on 600 s 1.5 h time scales Aharonian et al. (2009)

15 Spectrum a bit of history Hard spectrum: Γ = 2.25 ± 0.04 ± % Crab above 1 TeV No cut-off: EC > 9 TeV (95% CL)

16 Spectrum HESS J 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:

17 Spectrum HESS J 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: 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

18 HESS J1745: a pulsar wind nebula? Hinton + Aharonian (2007) G Sgr A* 10'' Wang et al. (2005) Dense radiation fields Comparably low magnetic field IC dominant plausible candidate

19 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

20 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)

21 Berge et al. (2007) Standard Background Modelling

22 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)

23 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)

24 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:

25 Low Energy Analysis No indication of feature similar to ATIC Aharonian et al 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:

26 Putting Electrons and GC together Meade et al. (2009) arxiv:

27 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!