1. 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

2. 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 ( ): First detection of gamma-rays from Gal. plane
SAS-2 ( )
COS-B ( )
EGRET( )
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

3. 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 MeV
(launched on June 11, 2008)
sensitivity to point sources
Large Effective Area (>=8000 cm2 in 1-10 GeV)
Good Angular Resolution MeV and 0.6 GeV; 68% contaminant radii)
Large FOV (2.4 sr) and uniform exposure
Ideal for the study of diffuse gamma-rays
Atwood et al. (arXiv: )

4. 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:
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

5. 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°
GeV
|b|=2°-6°
|b|<=2°
~100% difference above 1 GeV
Hunter et al. 1997

6. 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.

7. Study of Local CRs using HI
Galactic Center
(sigificant contribution from IC)
Observed Wco
(Dame, et al. 2001)
20° 0°
-20°
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 1 GeV (Relative)
100
Galactic Longitude 10 1
Galactic Longitude
Measurement of HI emissivity spectrum constrains the local CR flux and the spectrum

8. 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 GeV. The slope gives the g-ray emissivity spectrum of local HI gas produced through interactions with CRs.
MeV
GeV
E2 x g-ray Intensity
PRELIMINARY
(error bars are statistical only)
HI column density (1020 cm-2)
MeV
GeV

9. 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

10. 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.

11. Mayer-Hasselwander et al. 1998
And More to Come
Contributions to ICRC 2009 (
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

12. Backup Slides

13. EGRET, the Predecessor Instrument
EGRET all-sky (galactic coordinates) E>100 MeV
Map from Steve’s talk at AAS 2009
, 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)

14. 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)‏

15. 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.

16. 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)