Fermi 衛星でみた拡散ガンマ線放射と銀河宇宙線 Tsunefumi Mizuno Hiroshima Univ. June 15, 2009

1. Fermi衛星でみた拡散ガンマ線放射と銀河宇宙線Fermi衛星でみた拡散ガンマ線放射と銀河宇宙線 Tsunefumi Mizuno Hiroshima Univ. June 15, 2009 "Fermi Large Area Telescope Measurements of the Diffuse Gamma-RayEmission at Intermediate Galactic Latitudes":Abdo, A. A et al.Phys. Rev. Lett., 103, 251101 (2009)"Fermi observations of Cassiopeia and Cepheus: diffuse gamma-rayemission in the outer Galaxy"Abdo, A.A. et al.arXiv:0912.3618

2. SNR RX J1713-3946 B HESS π 0 e e π gas gas + + + - - - Introduction Cosmic-Rays and Galactic Diffuse Gamma-Rays (1) HE g-rays are produced via interactions between Galactic cosmic-rays (CRs) and the interstellar medium (or interstellar radiation field) (CR accelerator) (Interstellar space) (Observer) ISM X,γ synchrotron Chandra, Suzaku, Radio telescopes IC ISRF P He CNO diffusion energy losses reacceleration convection etc. bremss Pulsar, m-QSO ACTs and Fermi (see K. Hayashi’s talk) (GMC is one of the best target matter) Pioneering theoretical works by Hayakawa (1952), Morrison (1958), etc. A powerful probe to study CRs in distant locations

3. Introduction Cosmic-Rays and Galactic Diffuse Gamma-Rays (2) • Prediction of Gamma-rays • inverse Compton scattering (photon & CR electron) • p0-decay (matter & CR nucleon) • bremsstrahlung (matter & CR electron) • GeVg-rays probes CR protons (and ISM)

4. Introduction Cosmic-Rays and Galactic Diffuse Gamma-Rays (3) • Prediction of Gamma-rays • inverse Compton scattering (photon & CR electron) • p0-decay (matter & CR nucleon) • bremsstrahlung (matter & CR electron) • GeVg-rays probes CR protons (and ISM) p0 component has a bump around 1 GeV in E2 spectrum Gp=2 Fermi-LAT (Eg ~ 0.1-10 GeV) Gp=2.4 Local Interstellar Spectrum Aharonian 2004

5. Target: Interstellar Medium (Gas) • Gas distribution determined from radio surveys • velocity => distance through a rotation curve 25° • HI density from LAB survey • Opacity correction needed especially close to Gal. plane G.C. Clements(1985) (R0,v0)=(8.5 kpc, 220 km/s) http://www.astro.uni-bonn.de/~webaiub/english/tools_labsurvey.php 30° Dame et al. 2001 0° • H2 density from 2.6 mm CO line • assumptions on Xco=N(H2)/WCO -30° target for producing gamma-rays through p0-decay and electron bremsstrahlung

6. Outstanding Question: EGRET GeV Excess (1) • We can “measure” the CR spectrum in distant locations by observing diffuse g-rays. • EGRET observations showed excess emission > 1 GeV everywhere in the sky when compared with models based on directly measured CR spectra • Potential explanations • Dark Matter • Unexpectedly large variations in cosmic-ray spectra over Galaxy • Unresolved sources (pulsars, SNRs, …) • Instrumental • Fermi-LAT is able to confirm or reject this phenomenon |b|=6°-10° 0.1 1 10 GeV |b|=2°-6° |b|<=2° ~100% difference above 1 GeV Hunter et al. 1997

7. Outstanding Question: EGRET GeV Excess (2) • We can “measure” the CR spectrum in distant locations by observing diffuse g-rays. • EGRET observations showed excess emission > 1 GeV everywhere in the sky when compared with models based on directly measured CR spectra • Potential explanations • Dark Matter • Unexpectedly large variations in cosmic-ray spectra over Galaxy • Unresolved sources (pulsars, SNRs, …) • Instrumental • Fermi-LAT is able to confirm or reject this phenomenon Orion Region (Digel et al. 1999, Aharonian 2001) Data vs. model by E-2.1 spectrum

8. Intermediate Latitude Region seen by LAT (1) |b|=10°-20° EGRET LAT Abdo, A. A et al.Phys. Rev. Lett., 103, 251101 (2009) 0.1 1 10 GeV • |b|=10°-20°: avoid Galactic Plane, high statistics and high S/N ratio (Extragalactic diffuse) • EGRET spectrum extracted for the same region • LAT spectrum is significantly softer and does not confirm the EGRET GeV excess • Strongly constrains the DM interpretation

9. Intermediate Latitude Region seen by LAT (2) Abdo, A. A et al.Phys. Rev. Lett., 103, 251101 (2009) See also Abdo et al. 2009, ApJ 703, 1249 EGRET LAT p0 isotropic 0.1 1 10 GeV IC bremsstrahlung • LAT spectrum is compatible with a prediction based on the LIS • p0 is the dominant component

10. Possible Cause of EGRET/LAT Discrepancy • EGRET also showed significantly harder spectrum for Vela Pulsar (BG negligible). • Could be due to Calibration uncertainty (large correction for backsplash)

11. CR Distribution in Galaxy • CR distribution in our Galaxy is a key for understanding their origin and propagation • Distribution of SNRs not well measured • Fermi-LAT is able to map out CR distributions in the Galaxy • LAT data in the 2nd and 3rd Galactic quadrant provide us with accurate measurement of CR density distributions in the outer Galaxy • Recently accepted article (arXiv:0912.3618) discusses the g-rays in the the 2nd quadrant • Report on the relevant study in the 3rd quadrant is in preparation Gal. Center Inner Galaxy local arm Outer Galaxy Perseus arm

12. Gas Density Distribution • Simple slicing using the rotation curve is not good enough to fully exploit the LAT data • Region boundaries are shifted to the intensity minima • Fit the profile with gaussians and apply spillover correction.

13. Data and Analysis Procedure Gamma-ray flux 2 HI maps Extra galactic diffuse (uniform) R=0-7.5kpc, 7.5-9.5kpc Inverse compton model map (galprop) Excess of E(B-V) map (Grenier et al. 2005) 2 CO maps Gamma-rays are modeled as a linear combination of each component Fit data at each energy bin : “(100~144 MeV), (144~200 MeV), … , (9.05~12.8 GeV)” Gamma-ray spectrum ( ) of each component

14. Local HI (CR) Spectrum • Local HI spectrum (Gould Belt) is well represented by the interaction of CRs and ISM • Absolute intensity is ~50% larger than the galprop model • CR flux uncertainty, heavy nuclei in CRs and ISM

15. Emissivity (CR density Gradient) • Galprop model is based on CR source distribution (traced by pulsars) and conventional CR propagation model (e.g., CR halo of 4 kpc) • Measured gradient is flatter than the model • flatter CR source distribution and/or larger halo than previously thought • detailed discussion in forthcoming paper (3rd quadrant, large-scale diffuse)

16. Emissivity Spectrum in Outer Galaxy • HI spectral ratio to that of Gould Belt • Possible spectral hardening is observed (not seen in the 3rd quadrant) • Systematic uncertainty (unresolved sources, etc.) not ruled out Local arm to Gould Belt Perseus arm to Gould Belt

17. HI vs. CO Emissivities Gould Belt Local arm • HI emissivity vs. CO emissivity of 3 regions • Proportionality supports the idea that CRs penetrate to the core of molecular clouds • Different slope indicate evolution of CO-to-H2 ratio (see next) Perseus arm

18. Xco Evolution • Moderate evolution ov Xco (=N(H2)/Wco) is observed • Could be due to the metallicity gradient • Xco in outer Galaxy is much smaller than that inferred by the EGRET study

19. Summary • Diffuse gamma-rays are powerful probe to study CRs and ISM in our Galaxy • Useful to constrain the CR protons • EGRET GeV excess not confirmed • Strongly constrain the DM interpretation • Local CRs are compatible with those measured at the Earth • Detailed study of the 2nd quadrant • CRs and ISM in the outer Galaxy • Flatter CR gradient than previously assumed • Flatter but significant evolution of CO-to-H2 ratio • Relevant studies of the 3rd quadrant and large-scale analysis in progress

20. HI Emissivity Spectra Gould Belt Local arm Perseus arm

21. InterStellarRadiation Field • CR e+/e- need targets to create g-rays • Interstellar radiation field determined from a realistic model taking into account stellar and dust distribution • Starlight (~ 0.1 mm – 10 mm) • Dust (~ 10 mm – 300 mm) • CMB (>300 mm) ISRF energy density R=0 kpc R=4 kpc R=8 kpc R=12 kpc There are uncertainties associated with gas and ISRF Stellar CMB Dust Porter et al. 2008