Fermi Fermi- -LAT Study of LAT Study of Diffuse rays and CRs - - PowerPoint PPT Presentation

Fermi_ICRC2011.ppt

Fermi Fermi-

• LAT Study of

LAT Study of Diffuse Diffuse γ rays and CRs rays and CRs Diffuse Diffuse γ-rays and CRs rays and CRs in the outer Galaxy in the outer Galaxy

• Aug. 11, 2011 @ ICRC2011

T Mizuno L Tibaldo and

• T. Mizuno, L. Tibaldo and
• I. Grenier

On behalf of the Fermi-LAT On behalf of the Fermi-LAT

collaboration

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Fermi_ICRC2011.ppt

Introduction Introduction

• Objective

C i (CR) i i d ti – Cosmic-ray (CR) origin and propagation M th d (and ISM)

• Method

– Diffuse γ-rays seen by Fermi-LAT Probe CRs in distant location Eγ/Ep ~ 1/6 (e.g., Mori 97): CR t t 100 G V – Detailed study of γ-rays in outer Galaxy CR spectrum up to > 100 GeV

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The Outer Galaxy (LAT view) The Outer Galaxy (LAT view)

Abdo+10, ApJ 710, 133 Ackermann+11, ApJ 726, 81 anticenter GC

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NASA/DOE/Fermi-LAT Collab. Eγ > 200 MeV

Fermi_ICRC2011.ppt

The Outer Galaxy (Top view) The Outer Galaxy (Top view)

Galactic Center Sun

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II quad III quad

NASA/JPL-Caltech/R. Hurt

Fermi_ICRC2011.ppt

The Outer Galaxy (Top view) The Outer Galaxy (Top view)

• No distance ambiguity in velocity separation
• f gas
• f gas

– suitable to study CRs associated with arms and interarm regions Sun Local Arm arms and interarm regions

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II quad III quad

NASA/JPL-Caltech/R. Hurt

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CR Spectra CR Spectra

• We found no significant variation of emissivity

spectra with Galactocentric distances spectra with Galactocentric distances

• Agree well with the model for CRs directly measured

at the Earth Γ~2.7 for Ep<=100 GeV

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0.1 1 10 GeV

Fermi_ICRC2011.ppt

Optical Depth Correction for HI gas Optical Depth Correction for HI gas

• Uncertainty is dominated by N(HI) determination,
• ften overlooked in the past
• ften overlooked in the past

II quad q III quad

• No large emissivity

(CR density) gradient

typical range of

• ptical depth correction

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Sun

Fermi_ICRC2011.ppt

CR Density Gradient CR Density Gradient

• CR density profile flatter than expected from

SNR/pulsar distribution and standard CR halo (4kpc) SNR/pulsar distribution and standard CR halo (4kpc)

Emissivity (CR density) profile (CR density) profile SNR/pulsar distribution

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4kpc (solid line)

Fermi_ICRC2011.ppt

Implications Implications

• CR density profile flatter than expected from

SNR/pulsar distribution and standard CR halo (4kpc) SNR/pulsar distribution and standard CR halo (4kpc)

• Implications:

– Large CR halo (>10 kpc)

Emissivity (CR density) profile SNR/pulsar distribution & (CR density) profile

20kpc 15kpc

Different CR halo

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p 10kpc 4kpc (solid line)

Fermi_ICRC2011.ppt

Implications Implications

• CR density profile flatter than expected from

SNR/pulsar distribution and standard CR halo (4kpc) SNR/pulsar distribution and standard CR halo (4kpc)

• Implications:

– Large CR halo (>10 kpc) – Flat CR source distribution

Standard CR halo (4kpc) & modified source distr.

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Implications Implications

• CR density profile flatter than expected from

SNR/pulsar distribution and standard CR halo (4kpc) SNR/pulsar distribution and standard CR halo (4kpc)

• Implications:

– Large CR halo (>10 kpc)

also possible:

• Non-uniform diffusion

– Flat CR source distribution

• Vast amount of missing gas

(e.g., Evoli+08, Papadopoulos+02)

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Summary Summary

• Diffuse γ-ray emission: Powerful probe of CRs in

distant locations distant locations

• Extensive study in the outer Galaxy

– N(HI) is the dominant source of uncertainty – Flat CR density distribution, implying

• Larger CR halo
• More CR sources
• Non-uniform diffusion/missing gas

Thank you for your Attention

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Thank you for your Attention

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Backup Slides Backup Slides Backup Slides Backup Slides

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Data Analysis Data Analysis

γ-rays

“Standard” approach:

• linear combination of ISM maps
• no a-priori assumption of CR spectra
• no a-priori assumption of CR spectra

=

qHI,1X +qHI,2X +qHI,3X +qHI,4X N(HI),1 N(HI),2 N(HI),3 N(HI),4

=

Local arm interarm Perseus arm Outer arm

+qCO 1X +qCO 2X +qCO 3X WCO,1 WCO,2 WCO,3

Local arm interarm Perseus arm Outer arm

+qCO,1X +qCO,2X +qCO,3X E(B V)

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+qEBVX E(B-V)res +IC+Iso+PS

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Data Analysis Data Analysis

Improvement over the EGRET era:

• Energy coverage, statistics
• dust to trace gas unaccounted by HI and Wco

γ-rays

=

• dust to trace gas unaccounted by HI and Wco
• Careful evaluation of uncertainty due to

N(HI) determination

=

qHI,1X +qHI,2X +qHI,3X +qHI,4X N(HI),1 N(HI),2 N(HI),3 N(HI),4

Local arm interarm Perseus arm Outer arm

+qCO 1X +qCO 2X +qCO 3X WCO,1 WCO,2 WCO,3

Local arm interarm Perseus arm Outer arm

+qCO,1X +qCO,2X +qCO,3X E(B V)

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+qEBVX E(B-V)res +IC+Iso+PS

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Summary Spectra Summary Spectra

• Dominated by γ-rays from HI gas

Emissivity ratio to that of the Local arm

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Emissivity Ratio Emissivity Ratio

• Consistent with “No spectral variation”

Emissivity ratio to that of the Local arm

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Optical Depth Correction for HI gas Optical Depth Correction for HI gas

• Uncertainty is dominated by N(HI) determination,
• ften overlooked in the past
• ften overlooked in the past

II quad q III quad

• No large emissivity

(CR density) gradient

• No large arm-interarm

g contrast

Local arm interarm Perseus arm

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Sun