The Galactic diffuse emission

1. The Galactic diffuse emission Sabrina Casanova, MPIK Heidelberg XXth RENCONTRES DE BLOIS 18th - 23rd May 2008, Blois

2. Outline • Motivations : Sources of cosmic rays and galactic diffuse gamma emission • GeV excess measured by EGRET • Pinpointing the sources of cosmic rays • TeV observations with HESS and Milagro • ‘’Truly’’ diffuse emission and unresolved sources • Goals of future observations and theoretical speculations

3. Do g-rays answer the open questions concerning the origin of cosmic rays ? • What are the cosmic ray accelerators and the primary spectra ? g-rays are produced by cosmic-ray interactions in their sources and point back to the production locations • How high in energy can galactic sources produce particles? • The highest energy particles produce highest energy g-rays. • Are the accelerators of hadrons different from electrons? • Hadronic and leptonic mechanisms produce different energy spectra and g-ray time variability • How do cosmic rays propagate in the Galaxy ? What is the rate of production of relativistic particles ? • Gamma ray spectrum and spatial distribution provide spectra and density of hadrons and leptons in different regions of the Galaxy

4. Production mechanisms :Hadronic Processes p + p -> po +…->g +… p + p -> p± +…-> e ± + n +… p p p p e+ p n μ+ n γ π+ π0 n p p γ

5. Electromagnetic Processes : • Synchrotron Losses ( not important as g emission mechanism but important for the electron cooling ) • E g a (Ee/mec2)2 B • Inverse Compton Scattering (dominant leptonic emission mechanism at GeV-TeV) • E f ~ (Ee/mec2)2 E I • Bremsstrahlung (important emission mechanism at MeV energies) • E g ~ 0.5 E

6. c q or gg or Zg q lines? c “Exotic” Gamma-Ray Production • Particle-Antiparticle Annihilation • WIMP called neutralino, c, is postulated by SUSY • 50 GeV< mc< few TeV • Primordial Black Hole Evaporation • As mass decreases due to Hawking radiation, temperature increases causing the mass to evaporate faster • Eventually temperature is high enough to create a quark-gluon plasma and hence a flash of gamma-rays

7. The conventional model for the Galactic diffuse g ray flux. Electron and proton flux measured locally Matter distribution Low energy photon density Electron flux measured locally

8. GeV excess measured by EGRET Hard nucleon injection spectrum (Gralewicz et al. 1997; Aharonian & Atoyan, 2000 )‏ Hard electron injection spectrum (Porter & Protheroe 1997, Strong & Moskalenko, 2000 )‏ Physics of 0 production ( Kamae et al, 2004 )‏ Unresolved - ray sources Exotic: dark matter (DeBoer et al, 2005) Instrumental – EGRET response (Stecker et al, 2007 & Moskalenko et al, 2007)‏ GeV Excess Hunter et al. ApJ (1997)‏

9. Model of cosmic-ray production & propagation in the Galaxy: optimized GALPROP model Uses antiproton & gamma data to fix the nucleon and electron spectra • Uses antiprotons to fix the intensity of CR nucleons @ HE • Uses gammas to adjust • the nucleon spectrum at LE • the intensity of the CR electrons • Uses EGRET data up to 100 GeV EGRET COMPTEL o inverse Compton TOTAL brems- strahlung extragalactic background Strong,Moskalenko & Reimer,2004

10. CONVENTIONAL MODEL: the electron and proton spectra locally measured are representative of the Galactic cosmic ray spectrum everywhere in the Galaxy (Bertsch et al, 1993). • OPTIMIZED GALPROP MODEL: the proton and electron densities are allowed to vary roughly of a factor 2 and 4 in order to match the EGRET data (Strong, Moskalenko & Strong, 2004). • Cosmic ray injection is a stochastic process : • The cosmic ray spectra close to injection sources vary in both spectral index and normalization with respect to the so called ‘’sea’’ of cosmic rays due to energy dependent diffusion processes . • The cosmic ray spectra close to sources are time dependent due to injection and diffusion history .

11. The cosmic ray flux close to a source varies in spectral index and intensity. Aharonian & Atoyan, 1996 CR sea 1= 102yr 2 =103yr 3=104yr 4=105yr Do = 1026 cm2/s Do = 1028 cm2/s at 10 GeV

12. Detection of Passive clouds • Maybe some of EGRET unidentified sources • At energies < 1 GeV GLAST can detect close clouds if M5 /d2 > 0.1 • At energies >> 1 GeV GLAST can detect clouds only if M5 /d2 >10

13. Detection of clouds with an accelerator Typical CLOUD : n = 130 cm-3 , radius = 20 pc, mass = 105 solar masses Impulsive source Continuous source 102 105 103 104 103 105 104 102 Agile sensitivity at 1 GeV : 4 X 10-8 GeV cm-2 s-1

14. Looking for pevatrons: the emission from a SNR and from a cloud close to the SNR Gabici & Aharonian 07 at 1 Kpc 400 yr 2000 yr 8000 yr (104 solar masses)‏ 2000 yr 8000 yr 32000 yr CR spectrum inside the SNR shell extends to PeV energies mainly during the Sedov phase

15. SNR stochasticity and electron spectrum Swordy, ICRC 2003 100 TeV Electron 1 GeV Electron B = 3mG and CMB photons for 100 TeV electrons te = 103 years Rdiff= 100 pc Bremsstrahlung E(dE/dt)-1,yr Ionization Coulomb IC, synchrotron 107 yr 106 yr Ekin, GeV

16. TeV observations of diffuse sources

17. TeV Diffuse Emission from the Galactic Center as a Probe for Cosmic Ray Sources Spectral index 2.29 ± 0.07 ± 0.20 implies harder CR spectrum than in solar neighborhood  Proximity of accelerator and target (Aharonian et al, 2006)‏

18.  Interaction of CRs with molecular cloud material Correlation with molecular clouds -0.2° < b < 0.2° molecular clouds 150 pc at 8 kpc, 0.2° ~ 30 pc at 8 kpc, 0.2° ~ 30 pc

19. 2 x GP Milagro Galactic Longitude Profile Inner Galaxy (Abdo et al, 2008)‏ -2 <b<2 Optimized GALPROP model Cygnus Region The CygnusRegion shows an excess with respect to the optimized GALPROP model. The emission from the inner Galaxy is consistent with the GALPROP optimized model.

20. Column densities from Milagro inner Galaxy and from the Cygnus Region. 30° Milagro inner Galaxy 65° Cygnus Region 85°

21. Galactic Latitude Profile of Milagro Observations (Abdo et al, 2008)‏ total IC p0 The narrow data distribution seems to favour a hadronic mechanism b0=0 and s= 0.9 (for the inner Galaxy) and 2.0 (for the Cygnus region)

22. Strong & Moskalenko GALPROP model of Cygnus Region Pin standard optimized bremsstrahlung Inverse Compton TeV Diffuse Emission from the Cygnus Region probe the cosmic ray distribution Abdo et al, 2007 100 pc Cygnus Region: Matter Density Contours overlaying Milagro Obs.

23. “Truly” diffuse emission or unresolved sources ?

24. Milagro emission from the inner Galaxy • CR spectrum 1: • CR spectrum 2 : hard spectrum due to a population of CR sources E2 dN/dE TeV cm-2 s-1sr-1 1 2 TeV Consequences for diffuse neutrino fluxes for km3net

25. Population of unresolved sources? Aharonian et al., ApJ 636, 2006

26. Diffuse emission due to unresolved sources Number-intensity relation for HESS source population • 11 of 15 new HESS sources detected above 6 per cent Crab flux are included in the logN-logS plot • TeV sources (PWNe and SNRs) distributed like radio pulsars in the Galaxy • A significant part of the Milagro diffuse emission is due to unresolved sources Casanova & Dingus, 2008

27. Cosmic ray injection is a stochastic process : • The cosmic ray spectra close to injection sources vary in both spectral index and normalization with respect to the so called ‘’sea’’ of cosmic rays due to energy dependent diffusion processes . • The cosmic ray spectra close to sources are time dependent due to injection and diffusion history .

28. Goals of Observations of Diffuse Sources • Image spectrum + spatial distribution of large scale Galactic diffuse emission • Determine level of small scale emission that is clumpy (clouds)‏ • Compare morphology of diffuse emission at the resolution of H2 and H1 survey • Compare images + spectra with those from other wavelengths • Observe all possible photons energy fluxes

29. Fluctuations in the cosmic ray flux produce significant fluctuations in the gamma ray flux if the region around the cosmic ray source contains enough target material ! CR sea Aharonian & Atoyan, 1996 CR sea Impulsive source Continuous source