Diffuse Gamma-Rays seen by Fermi Gamma-ray Space Telescope Tsunefumi Mizuno Hiroshima Univ. on behalf of the Fermi-LAT Collaboration 27 March 2009, Tokyo, Japan

Short History of Gamma-ray Astronomy Prediction of Gamma-rays Feenberg & Primakoff (1948): inverse Compton scattering (photon & CR electron) Hayakawa (1952): p0-decay (matter & CR nucleon) Hutchinson (1952): bremsstrahlung (matter & CR electron) Morrison (1958) Early Observations OSO-3 (1967-1968): First detection of gamma-rays from Gal. plane SAS-2 (1972-1973) COS-B (1975-1982) EGRET(1991-2000) Diffuse gamma-rays has been one of main topics of gamma-ray astronomy since the beginning of its history map of the Gal. plane study CR and matter distribution Amenomori et al. (2002), Mori (ppt), Pinkau (1996) and references therein A powerful probe to study cosmic-rays (CRs) and the interstellar medium/radiation field in the Milky Way

The Era of the Fermi Gamma-ray Space Telescope LAT GBM Two instruments: Large Area Telescope (LAT) 20 MeV - >300 GeV Gamma-ray Burst Monitor (GBM) 10 keV - 25 MeV (launched on June 11, 2008) sensitivity to point sources Large Effective Area (>=8000 cm2 in 1-10 GeV) Good Angular Resolution (3.5deg@100 MeV and 0.6 deg@1 GeV; 68% contaminant radii) Large FOV (2.4 sr) and uniform exposure Ideal for the study of diffuse gamma-rays Atwood et al. (arXiv:0902.1089)

The LAT 3 Month All-Sky Map The Fermi-LAT has already surpassed the EGRET in many aspects More than 3 dozen pulsars (6 by EGRET) 205 bright sources and 444 above 5 s (271 by EGRET) arXiv:0902.1340 exciting results on individual targets (CTA1, GRB080916C, etc.) LAT all-sky E>200 MeV Mid/High-latitude region (local CR flux and spectrum) Mid-latitude region (so-called GeV excess) 3 month sky map (LAT confluence), start at 200 MeV for Front and 400 MeV for Back. In c/s at exposure at 1 GeV Understand CRs close to the solar system

EGRET GeV Excess EGRET observations showed excess emission > 1 GeV everywhere in the sky when compared with cosmic-ray propagation models based on directly measured cosmic-ray nuclei and electron spectra Variety of possible explanations Variations in cosmic-ray spectra over Galaxy Unresolved sources (pulsars, SNRs, …) Dark matter Instrumental |b|=6°-10° 0.1 1 10 GeV |b|=2°-6° |b|<=2° ~100% difference above 1 GeV Hunter et al. 1997

The Fermi LAT View PRELIMINARY |b|=10°-20° a PRELIMINARY Spectra shown for mid-latitude range => GeV excess in this region of the sky is not confirmed. Sources are not subtracted but are a minor component. LAT errors are dominated by systematic uncertainties and are currently estimated to be ~10% -> this is preliminary Gamma-ray spectra can be explained by cosmic-ray propagation model consistent with directly measured cosmic-ray nuclei and electron spectra.

Study of Local CRs using HI Galactic Center (sigificant contribution from IC) Observed Wco (Dame, et al. 2001) 20° 0° -20° 180° 90° 0° 270° 180° l=200°-260° |b|=22°-60° Galactic Longitude Want to decouple gamma-rays related to the atomic gas from other components Study region away from the Galactic center and the plane: Small contribution from point sources, IC and molecular hydrogen gas Most of HI gas is close to the solar system (<=1 kpc) IC model map @ 1 GeV (Relative) 100 Galactic Longitude 10 1 Galactic Longitude Measurement of HI emissivity spectrum constrains the local CR flux and the spectrum

Correlation with the HI Column Density Mask point sources (1° radius) and subtract IC and residual point source contributions. Correlation btw. g-ray intensity and gas column density in 0.2-10 GeV. The slope gives the g-ray emissivity spectrum of local HI gas produced through interactions with CRs. 200-282 MeV 1.6-2.26 GeV E2 x g-ray Intensity PRELIMINARY (error bars are statistical only) HI column density (1020 cm-2) 400-566 MeV 6.4-9.05 GeV

Emissivity of Local Atomic Hydrogen Gamma-ray emissivity spectrum per H-atom Agree with the model prediction from the local interstellar spectrum consistent with measurements at Earth CR nucleon spectrum in the vicinity of the sloar system is close to that directly measured at Earth (30% sys error is assumed below 1 GeV) PRELIMINARY Calibration below 200 MeV is underway and will allow us to discuss CR electron spectrum nucleon-nucleon electron-bremsstrahlung

Summary Fermi-LAT is a superb instrument for diffuse emission studies Diffuse gamma-ray is a powerful probe to study cosmic-ray spectrum and distribution Fermi-LAT is a superb instrument for diffuse emission studies Large Aeff and good angular resolution uniform and deep coverage of the sky First results on mid latitude Galactic emission show no evidence for EGRET feature > 1 GeV seen in the same region of the sky Mid/high-latitude observation indicates the CR nucleon spectrum in the vicinity of the solar system is close to that directly measured at Earth Work to analyze and understand diffuse emission over the entire sky is in progress.

Mayer-Hasselwander et al. 1998 And More to Come Contributions to ICRC 2009 (http://icrc2009.uni.lodz.pl/) GeV-non-excess Large-scale diffuse The Galactic-Center Extragalactic gamma-ray background Orion/Monoceros molecular clouds LMC Diffuse gamma-rays from Cassiopeia region Orion MC Digel et al. 1999 ? G.C. Mayer-Hasselwander et al. 1998 ? Stay tuned for further results on diffuse gamma-ray emission by Fermi-LAT

Backup Slides

EGRET, the Predecessor Instrument EGRET all-sky (galactic coordinates) E>100 MeV Map from Steve’s talk at AAS 2009 1991-2000, 30 MeV-30 GeV resolved 271 gamma-ray sources (Hartman et al. 1999) detailed study of Galactic diffuse emission (Hunter et al. 1997) and extragalactic diffuse emission (Sreekumar et al. 1998)

EGRET GeV Excess (2) Above 1 GeV, EGRET data are above the GALPROP prediction everywhere in the sky 4a-f From Igor’s talk of galprop (2005). Text is from Troy’s talk at TIT (2009Mar) LAT statistics are already good enough to confirm/refute all-sky nature of this excess Strong, Moskalenko & Reimer ApJ 613, 962 (2004)‏

Gamma-ray Count Maps Count maps in E>=200 MeV, accumulated from Aug. 4 to Oct. 30 PRELIMINARY Count Map (South Region) Count Map (North Region) (green crosses: LAT catalog source positions) The Fermi-LAT has already tripled the number of known gamma-ray sources (29 by LAT three month catalog and 9 in EGRET 3rd catalog). The diffuse spectrum by Fermi-LAT is less affected by unresolved sources than early missions.

Column Density of Atomic Hydrogen Column density maps of HI gas (w/ optical depth correction) N(HI) (South Region) N(HI) (North Region) N(HI) is small, less than 18x1020 cm-2 throughout the region Small uncertainty of the optical depth correction We see a correlation between diffuse gamma-ray counts and N(HI)