Presentation on theme: "The Galactic diffuse emission Sabrina Casanova, MPIK Heidelberg XXth RENCONTRES DE BLOIS 18th - 23rd May 2008, Blois."— Presentation transcript:
The Galactic diffuse emission Sabrina Casanova, MPIK Heidelberg XXth RENCONTRES DE BLOIS 18th - 23rd May 2008, Blois
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
What are the cosmic ray accelerators and the primary spectra ? -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 -rays. Are the accelerators of hadrons different from electrons? Hadronic and leptonic mechanisms produce different energy spectra and -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 Do -rays answer the open questions concerning the origin of cosmic rays ?
p + p o +… +… p + p ± +… e ± + +… Production mechanisms :Hadronic Processes p p p p π0π0 γ γ p p n p π+π+ μ+μ+ e+e+
Synchrotron Losses ( not important as emission mechanism but important for the electron cooling ) –E (E e /m e c 2 ) 2 B Inverse Compton Scattering (dominant leptonic emission mechanism at GeV-TeV) –E f ~ (E e /m e c 2 ) 2 E I Bremsstrahlung (important emission mechanism at MeV energies) –E ~ 0.5 E Electromagnetic Processes :
“Exotic” Gamma-Ray Production Particle-Antiparticle Annihilation WIMP called neutralino, is postulated by SUSY 50 GeV< m < 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 q q or or Z lines?
The conventional model for the Galactic diffuse ray flux. Electron and proton flux measured locally Electron flux measured locally Matter distribution Low energy photon density
Hunter et al. ApJ (1997) GeV Excess 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 measured by EGRET
EGRET COMPTEL extragalactic background inverse Compton brems- strahlung oo TOTAL 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 HE Uses gammas to adjust the nucleon spectrum at LE the intensity of the CR electrons Uses EGRET data up to 100 GeV Strong,Moskalenko & Reimer,2004
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.
The cosmic ray flux close to a source varies in spectral index and intensity. Aharonian & Atoyan, = 10 2 yr 2 =10 3 yr 3=10 4 yr 4=10 5 yr CR sea D o = cm 2 /s at 10 GeV D o = cm 2 /s
Detection of Passive clouds Maybe some of EGRET unidentified sources At energies 0.1 At energies >> 1 GeV GLAST can detect clouds only if M 5 /d 2 >10
Detection of clouds with an accelerator Impulsive source Continuous source Agile sensitivity at 1 GeV : 4 X GeV cm -2 s Typical CLOUD : n = 130 cm -3, radius = 20 pc, mass = 10 5 solar masses
Looking for pevatrons: the emission from a SNR and from a cloud close to the SNR Gabici & Aharonian yr 2000 yr 8000 yr yr (10 4 solar masses) at 1 Kpc 8000 yr 2000 yr CR spectrum inside the SNR shell extends to PeV energies mainly during the Sedov phase
SNR stochasticity and electron spectrum 10 7 yr 10 6 yr Bremsstrahlung Ionization Coulomb IC, synchrotron Ekin, GeV E(dE/dt) -1,yr B = 3 G and CMB photons for 100 TeV electrons t e = 10 3 years R diff = 100 pc 1 GeV Electron 100 TeV Electron Swordy, ICRC 2003
TeV observations of diffuse sources
(Aharonian et al, 2006) Spectral index 2.29 ± 0.07 ± 0.20 implies harder CR spectrum than in solar neighborhood Proximity of accelerator and target TeV Diffuse Emission from the Galactic Center as a Probe for Cosmic Ray Sources
Correlation with molecular clouds Interaction of CRs with molecular cloud material 150 pc molecular clouds -0.2° < b < 0.2° at 8 kpc, 0.2° ~ 30 pc at 8 kpc, 0.2° ~ 30 pc
The Cygnus Region shows an excess with respect to the optimized GALPROP model. The emission from the inner Galaxy is consistent with the GALPROP optimized model. Milagro Galactic Longitude Profile 2 x GP Cygnus Region Optimized GALPROP model Inner Galaxy (Abdo et al, 2008) -2
Column densities from Milagro inner Galaxy and from the Cygnus Region. 85° 65° 30° Milagro inner Galaxy Cygnus Region
(Abdo et al, 2008) Galactic Latitude Profile of Milagro Observations The narrow data distribution seems to favour a hadronic mechanism b 0 =0 and = 0.9 (for the inner Galaxy) and 2.0 (for the Cygnus region) IC total
TeV Diffuse Emission from the Cygnus Region probe the cosmic ray distribution Cygnus Region: Matter Density Contours overlaying Milagro Obs. Strong & Moskalenko GALPROP model of Cygnus Region standard optimized Inverse Compton Pin bremsstrahlung Abdo et al, pc
“Truly” diffuse emission or unresolved sources ?
Milagro emission from the inner Galaxy TeV E 2 dN/dE TeV cm -2 s -1 sr -1 CR spectrum 1: CR spectrum 2 : hard spectrum due to a population of CR sources 1 2 Consequences for diffuse neutrino fluxes for km3net
Population of unresolved sources? Aharonian et al., ApJ 636, 2006 Aharonian et al., ApJ 636, 2006
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 Diffuse emission due to unresolved sources
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.
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
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 ! Impulsive source Continuous source Aharonian & Atoyan, 1996 CR sea