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Fermi_CR_2009Sep.ppt
Galactic CosmicRays Observed by
Fermi-LAT
Tsunefumi Mizuno
Hiroshima Univ.
on behalf of the Fermi-LAT
Collaboration
JPS 2009 Autumn Meeting
September 11, 2009, Kobe, Japan
Tsunefumi Mizuno
1
Fermi_CR_2009Sep.ppt
Plan of the Talk
1. Cosmic-ray overview and Fermi Gammaray Space Telescope
2. Cosmic-ray electrons seen by Fermi-LAT
(direct measurement of CRs)
3. Galactic CRs revealed by diffuse g-ray
emission observed by Fermi-LAT (CRs in
distant location)
Tsunefumi Mizuno
2
Fermi_CR_2009Sep.ppt
Introduction:
Cosmic-Rays and the
Fermi Gamma-ray Space Telescope
Tsunefumi Mizuno
3
Fermi_CR_2009Sep.ppt
Cosmic-Rays Overview
• Discovered by V. Hess in 1912
V.
Hess, 1912
• Globally power-law spectrum with some structures (knee
and ankle)
 hint of the origin
 E<Eknee are (probably) Galactic origin
• Composition:
 e- ~ (1/100 - 1/1000) x p, e+ ~ (1/10) x e-
energy density: ~1 eV cm-3
 comparable to UB and Urad
• Studied by direct and indirect
measurements
Flux (m2 sr s GeV)-1
• Large
1 particle/m2/sec
Galactic
Knee
1 particle /m2/yr
G or EG?
Extragalactic
Ankle
1 particle/km2/yr
Tsunefumi Mizuno
Energy (eV)
4
Introduction (1):
Fermi_CR_2009Sep.ppt
What Can We Learn from HE e-/e+ (and p/p) ?
• Inclusive spectra: e- + e+
 Electrons, unlike protons, lose energy rapidly by Synchrotron
and Inverse Compton: at very high energy they probe the nearby
sources
• Charge composition: e+/(e- + e+) and p/(p + p) ratios
 e+ and p are produced by the interactions of high-energy cosmic
rays with the interstellar matter (secondary production)
 There might be signals from additional (astrophysical or exotic)
sources
• Different measurements provide complementary information of the
origin, acceleration and propagation of cosmic rays
 All available data must be interpreted in a coherent scenario
Study nearby sources (astrophysical or exotic)
Tsunefumi Mizuno
5
Fermi_CR_2009Sep.ppt
Introduction (2):
What Can We Learn from Galactic Diffuse Gamma-Rays?
HE g-rays are produced via interactions between Galactic cosmic-rays
(CRs) and the interstellar medium (or interstellar radiation field)
(CR Accelerator)
SNR
RX J1713-3946
(Interstellar space)
X,γ
(Observer)
ISM
+
e-
Chandra, Suzaku,
Radio telescopes
B
HESS
Pulsar,
m-QSO
P diffusion
He
energy losses
CNO
reacceleration +
convection e
+
etc. π -
IC
ISRF
gas
π0
ACTs,
Fermi
gas
A powerful probe to study CRs in distant locations
Tsunefumi Mizuno
6
Fermi_CR_2009Sep.ppt
Fermi Launch
• Launched from Cape Canaveral Air
Station on June 11, 2008
• Science Operation on Aug 4, 2009
• Orbit: 565 km, 26.5o (low BG)
Tsunefumi Mizuno
7
Fermi_CR_2009Sep.ppt
Fermi Gamma-ray Space Telescope
LAT
Two instruments:
• Large Area Telescope (LAT)
20 MeV - >300 GeV
• Gamma-ray Burst Monitor (GBM)
8 keV - 40 MeV
GBM
Fermi-LAT consists of three subsystems
• ACD: segmented plastic scintillators
 BG rejection
• Tracker: Si-strip detectors & W converters
 ~1.5 R.L. (vertical)
 Identification and direction measurement of g-rays
• Calorimeter: hodoscopic CsI scintillators
 ~8.5 R.L. (vertical)
 Energy measurement
 Also serves as an Imaging Calorimeter
Ideal for the direct and indirect (through g-ray obs.)
measurement of CRs
Tsunefumi Mizuno
8
Fermi_CR_2009Sep.ppt
Fermi-LAT Results (1):
Direct Measurements of Galactic
CR Electrons
Tsunefumi Mizuno
9
Fermi_CR_2009Sep.ppt
Quick Review of
Positron and Antiproton Fraction: 2008-09
PAMELA positron and antiproton
Nature 458, 607 (2009)
PRL 102, 051101 (2009)
1 GeV
10
100
• Antiproton fraction consistent with secondary production
• Anomalous rise in the positron fraction above 10 GeV
• Several different viable interpretations (>200 papers over the last year)
See also Nature 456, 362 (2008) and PRL 101, 261104 (2008) for pre-Fermi CRE spectrum
by ATIC and HESS.
Tsunefumi Mizuno
10
Fermi_CR_2009Sep.ppt
Fermi-LAT Capability for CR Electrons
• Candidate electrons pass through 12.5 X0 on average ( Tracker and
Calorimeter added together)
• Simulated residual hadron contamination (5-21% increasing with the
energy) is deducted from resulting flux of electron candidates
• Effective geometric factor (Gf) exceeds 2.5 [m2 sr] for 30 GeV to 200 GeV,
and decreases to ~1 [m2 sr] at 1 TeV. Gf times live time has already reached
several x 107 [m2 sr s]. (very high statistics)
• Full power of all LAT subsystems is in use: Tracker, Calorimeter and ACD
act together
Geometric
Factor (Gf)
Residual hadron
contamination
Tsunefumi Mizuno
20 GeV
100 GeV
1 TeV
11
Fermi_CR_2009Sep.ppt
FOM for CRE Measurement
Exposure factor (effectively) determines the # of counts
Ef(E) = Gf(E)*Tobs
L. Baldini
• The exposure factor determines the statistics
• Imaging calorimeters (vs. spectrometers) feature larger Gf
• Space (vs. balloon) experiments feature longer Tobs
Tsunefumi Mizuno
12
Fermi_CR_2009Sep.ppt
Fermi-LAT Electron Spectrum
• Abdo et al. Phys. Rev. Let.
102, 181101 (2009)
• statistics for 6 month data
 >4 million electrons
above 20 GeV
 >400 electrons in the
last energy bin
 Harder spectrum (spectral
index: -3.04) than previously
thought
• Pre-Fermi reference model (GALPROP conventional model): --------- conventional source distribution (uniformly distributed distant sources)
 source PL index: g0=2.54
 diffusion coefficient index: d=0.33
Tsunefumi Mizuno
13
Fermi_CR_2009Sep.ppt
Implication from Fermi-LAT CRE (1)
re-Fermi “conventional” CRE Model
g0=2.54
• for detail, see D. Grasso et al.
arXiv:0905.0636 (accepted by
Astroparticle Physics)
• New “conventional” model
New “conventional” CRE models
g0=2.42
g0=2.33
 g0=2.42 (d=0.33, w/
reacceleration)
 g0=2.33 (d=0.6, plain
diffusion)
• Fermi CRE spectrum can be reproduced by the “conventional”
model with harder injection spectral index (-2.42) than in a pre-Fermi
conventional model (-2.54), within our current uncertainties both
statistical and systematic.
Tsunefumi Mizuno
14
Fermi_CR_2009Sep.ppt
Implication from Fermi-LAT CRE (2)
• Now include recent PAMELA result on positron fraction
New “conventional”
CRE models
Old “conventional”
CRE Model
• If the secondary positrons only
 e+/(e- + e+) ~ E^(-gP+g0), gP~2.7 (proton spectral index)
 The hard e+ + e- spectrum found by Fermi-LAT sharpens the
anomaly
Tsunefumi Mizuno
15
Fermi_CR_2009Sep.ppt
Implication from Fermi-LAT CRE (3)
• It is becoming clear that we are dealing with at least 3 distinct origins of HE e-/e+
 Uniformly distributed distant sources, likely SNRs.
 Unavoidable e+e- production by CRs and the ISM
“conventional” sources
 And those that create positron excess at high energies. Nearby (d<1 kpc) and Mature
(104 - 106 yr) pulsars? DM?
• Energy source: rotation energy of the NS
• Electron and positrons are re-accelerated at the pulsar
wind/shock with a power law spectrum with index G~1.5
• e-/e+ are expected to be confined until T~10-100 kyr
after the birth of pulsar. Only mature (10<T<1000 kyr)
pulsars are expected to be relevant
•Ecut~103 TeV for young PWN. It is expected to decrease
with the pulsar age (Ecut~0.1-10 TeV for mature pulsars)
• Fermi data requires an e-/e+ injection spectrum significantly harder than
generally expected for shell-type SNRs
Tsunefumi Mizuno
16
Fermi_CR_2009Sep.ppt
Pulsar Scenario
• An example of the fit to both Fermi and PAMELA data with Monogem and
Geminga with a nominal choice for the e+/e- injection parameter (blue lines).
This particular model assumes:
40% e-/e+ conversion efficiency
G=1.7
Ecut=1 TeV
Delay=60 kyr
(Discrepancy in positron fraction at low energies can be understood as the
charge-sign effect of solar modulation)
Tsunefumi Mizuno
17
Fermi_CR_2009Sep.ppt
Dark Matter Interpretation
sv [cm3/s]
pure e+e- Models
10-22
Like for the case of pulsars, PAMELA and
Fermi data tighten the DM constraints
lepto-philic
10-24
preferred
10-26
100 GeV
1 TeV (DM mass)
Super-heavy DM likely excluded
10-19
sv =3x10-26
10-21
10-23
Tsunefumi Mizuno
Both in the pure e+e- and lepto-philic
models, a DM interpretation is
possible w/ boost factors of 20-100
18
Fermi_CR_2009Sep.ppt
Summary of Fermi-LAT CRE
• Real breakthrough during last 1-1.5 years in CR electrons: ATIC,
HESS, PAMELA and finally Fermi-LAT
• Fermi-LAT provides precise measurements of CR e-/e+ spectrum in
20 GeV-1 TeV
• With the new data more puzzles than was before. Fermi-LAT’s hard
e-/e+ spectrum contradicts with PAMELA’s positron fraction.
• We may be coming close to the first detection of cosmic-ray
sources
• Source nature (astrophysical or exotic) is still unclear but strongly
constrained by data of Fermi-LAT (+ others)
• More results from Fermi-LAT are coming. Extending energy range
to 5 GeV – 2 TeV and searching for the CRE anisotropy at a level of
~1%.
Tsunefumi Mizuno
19
Fermi_CR_2009Sep.ppt
Fermi-LAT Result (2):
Galactic Diffuse Gamma-ray
Emission (Indirect Probe of
Galactic CRs)
Tsunefumi Mizuno
20
Outstanding Question:
Fermi_CR_2009Sep.ppt
EGRET GeV Excess
• 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
 Unexpectedly large variations in
cosmic-ray spectra over Galaxy
 Dark Matter
 Unresolved sources (pulsars,
SNRs, …)
 Instrumental
• Fermi-LAT is able to confirm or reject
this phenomenon
Tsunefumi Mizuno
|b|=6°-10°
0.1
1
10 GeV
|b|=2°-6°
|b|<=2°
~100% difference above 1 GeV
Hunter et al. 1997
21
Fermi_CR_2009Sep.ppt
Intermediate Latitude Region seen by LAT
|b|=10°-20°
EGRET
LAT
Abdo et al. submitted to PRL
Porter et al. 2009 (arXiv:0907.0294)
0.1
1
10 GeV
• |b|=10°-20°: avoid Gal. plane but still have high statistics
• 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
Tsunefumi Mizuno
22
Fermi_CR_2009Sep.ppt
Probing CRs using Gamma-rays from ISM
• Correlation with gas column density reveals the CR spectrum
 Method goes back to SAS-2/COS-B era
• Fermi-LAT’s high performance + CR propagation model (e.g.
GALPROP) to predict IC
 Sensitivity significantly improved
Gamma-ray intensity
(Fermi LAT data)
ISM
(e.g., LAB HI survey)
(http://www.astro.uni-bonn.de/~webaiub/english/tools_labsurvey.php)
Mid/high latitude region:
Detailed study of local CRs (most of the gas is close to the solar system)
Galactic plane:
CR gradient in the Galaxy (need to resolve point sources)
Tsunefumi Mizuno
23
Fermi_CR_2009Sep.ppt
Accurate Measurements of Local CRs (1)
Mid-high lat. region in 3rd quadrant:
• small contamination of IC and
molecular gas
• correlate g-ray intensity and HI
gas column density
E2 x g-ray Intensity
Abdo et al. 2009, accepeted by ApJ
(arXiv:0908.1171) contact author: TM
(error bars are statistical only)
400-560 MeV
HI column density (1020 cm-2)
1.6-2.3 GeV
400-566 MeV
HI column density (1020 cm-2)
Tsunefumi Mizuno
24
Fermi_CR_2009Sep.ppt
Accurate Measurement of Local CRs (2)
• Best quality g-ray emissivity spectrum (per H-atom) in 100 MeV-10 GeV
(Tp = 1-100 GeV)
• Agree with the model prediction from the local interstellar spectrum (LIS)
LAT data
model from the LIS
EGRET (Digel et al. 2001)
nucleon-nucleon
electronbremsstrahlung
102
103 MeV
• Prove that local CR nuclei spectra are close to those directly
measured at the Earth
• Eg<100 MeV constrain the e- spectrum
Tsunefumi Mizuno
25
104
Fermi_CR_2009Sep.ppt
CR Distribution in Galaxy
• CR distribution is a key to understand
their origin and propagation
• Distribution of SNRs not well measured
• Previous Gamma-ray data suggests a
flatter distribution than SNR/pulsar
distributions (e.g., Strong et al. 2004)
Pulsar distribution
(Lorimer 2004)
SNR distribution
(Case & Bhattacharya 1998)
CR source distribution from g-rays
(Strong & Mattox 1996)
15 kpc
sun 10
• Fermi-LAT is able to map out CR
distributions in the Galaxy with
unprecedented accuracy
• Large scale analysis in progress.
(arXiv:0907.0304)
0
Gal.
Center
Inner Galaxy
Outer
Galaxy
Tsunefumi Mizuno
5
• Preliminary analysis of the 3rd
quadrant (outer Galaxy) will be
discussed. See also the relevant
study of the 2nd quadrant
(arXiv:0907.0312)
26
Fermi_CR_2009Sep.ppt
Fermi-LAT View of the 3rd Quadrant
• One of the best studied regions in g-rays
 Vela, Geminga, Crab and Orion A/B
• Galactic plane between Vela and Geminga (green square) is ideal to study diffuse g-rays
and CRs.
 small point source contamination, kinematically well-separated arms (local arm
and Perseus arm)
Geminga
Vela
Crab
Orion A/B
Tsunefumi Mizuno
Count Map (E>100 MeV)
27
Fermi_CR_2009Sep.ppt
Construction of the Model
Local arm
Perseus arm
+
Local arm
+2 HI maps
(profile fitting technique;
arXiv:0907.0312)
• Fit g-ray data with 8 maps
+ 15 point sources (11
month source list)
• CR spectrum (g-ray
emissivity) is assumed to
be uniform in each
Galactocentric ring
+ 1 CO map + excess E(B-V) map (Grenier et al. 2005)
+ IC map (galprop model) + point sources (11 month list)
Tsunefumi Mizuno
Utilize new techniques, understanding
of the ISM and power of the LAT
28
Fermi_CR_2009Sep.ppt
HI Emissivity (CR) Spectra
HI Emissivity Spectrum of each ring
(local arm)
(interarm)
(Perseus arm)
Point sources with
Ts>=100 are included
in the fitting
• Emissivity (CR) spectrum of local arm (R=8.5-10 kpc) is slightly smaller than
that of LIS
• Decreasing emissivity (local arm => interarm => Perseus arm) are consistent
with decreasing CR density across the Galaxy
• Similar CR spectral shape up to R=16 kpc
Tsunefumi Mizuno
29
Fermi_CR_2009Sep.ppt
CR Flux Distribution
LAT “measured” CR density
SNR distribution by radio
survey or traced by pulsar
(NB propagation effect is
not taken account)
• Emissivity gradient traces the CR density. Robust against the thresholds for
point sources included.
• Significantly flatter than the SNR distributions
 may indicate more CR sources than previously thought in the outer Galaxy, large
halo size, etc.
• Comparison with the model prediction is in progress.
Tsunefumi Mizuno
30
Fermi_CR_2009Sep.ppt
Summary
• Fermi-LAT is a powerful instrument to measure CRs either
directly or indirectly
• Fermi-LAT provides largest statistics of high-energy CR e-/e+
spectrum.
 Precise and hard CR electron spectrum by Fermi-LAT and
PAMELA positron fraction require local sources
(astrophysical or exotic)
 Source nature is still unclear but strongly constrained.
• CRs in distant locations can be “observed” by diffuse g-rays.
 EGRET GeV-excess not confirmed.
 Fermi proves that local CR nuclei spectra are close to
those of LIS.
 Flat and large CR density in the outer Galaxy is indicated.
Tsunefumi Mizuno
Thank you for your attention!
31
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