2013年9月4日
宇宙線研究所フェルミ衛星による銀河系
TeVガンマ線天体の研究

内山 泰伸 (立教大学)
On behalf of Fermi-LAT Collaboration

Milky Way in TeV Gamma-ray: H.E.S.S.
H.E.S.S. (imaging air Cherenkov telescopes, ~1 TeV γ-rays)
has revealed various TeV sources along the Milky Way in 2005

PWN
BinarySNRGCFermi Gamma-ray Space Telescope

Launch in 2008

⬅ LAT sensitivity
for various exposures3

Large Area Telescope (LAT)
onboard Fermi
Si Tracker(18 planes)

γFermi satellite (2008-)

Gamma-ray (20 MeV - 300 GeV)

converter: tungsten
tracker: single-side Si strip

Field of view: 2.4 str
Angular resolution:~ 1 deg at 1 GeV~0.1 deg > 10 GeV

AntiCoincidence
Detector

CsICalorimeter
total radiation length 8.6 X0e+e4x4 modular array
3000 kg, 650 W

銀河系内 GeV ガンマ線源：サプライズ“Fermi Bubble”“Crab Flare”
Light curveSEDΓ = 1.26 ± 0.11
εc = 361 ± 26 MeV> (9/4αF)mec2 = 160 MeV

The Crab: Full of Surprises
RXTE

Fermi-GBM
Swift

SNR

The Crab Nebula:year-scale variability of hard X-ray emission

The Crab pulsar: extra TeV emission

Fermi-LATMAGICVERITAS

The Crab Nebula: GeV flares
Fermi-LAT

The Crab Seen by Fermi-LAT
Average gamma-ray spectra
Buehler+(2012)

PulsarNebula:Synchrotron(variable)

Nebula:IC (constant)

Long Term Lightcurve
Buehler+(2012)

Feb 2009
flare

Sep 2010
flare

Apr 2011
flareThe Crab Flares: LAT Images

The April 2011 Flare
Buehler+(2012)~ 9 min time bins

✛ Synchrotron nebula brightened by a factor of ~30✛ Flux doubling time : 4-8 hours✛ No change in pulsar flux and phase

The April 2011 Flare: Spectral Evolution
Buehler+(2012)Γ = 1.26 ± 0.11
εc = 361 ± 26 MeV

Lγ ~ 1036 erg/s~ 1% of Lsd

A New Flare in March 2013
The second strongest flare so far.
Mayer+(2013)

Significant variationsin 4 hours

Γ = 4.4 ± 1.0

Γ = 2.4 ± 0.1

Γ = 3.8 ± 0.3

X-ray Images During the Flare
Weisskopf+(2013) inc. Uchiyama✛ Chandra observations during the April 2011 flare✛ No correlated activities in the inner ring region

Why Puzzling?
✚ Compactness
Doubling time t ~ 4-8 hours ➜ Emission region < ct ~ 3x10-4 pc(Inner ring ~ 0.1 pc)
Large luminosity (~ 1% of spindown power) from a compact region✚ Spectrum
Γ = 1.26 ± 0.11: Flare energy is carried by the highest energy electrons
εc = 361 ± 26 MeV: Appears to violate the radiation reaction limit

Balance between acceleration (E<B) and synchrotron cooling➜ Cutoff of synchrotron spectrum must be:εc < (9/4αF)mec2 = 160 MeV
At least, relativistic beaming is necessary ( δ ~ a few or more)(But HST/Chandra images show only a mildly relativistic flow of ~0.5c)

Prospect for CTA
Counterpart in the TeV bandpass (IC) would be detectablewith CTA
Kohri, Ohira, Ioka (2012)

Highly Relativistic Post-shock Flow?
✚ Komissarov & Lyutikov (2012)- RMHD simulations suggest highly relativistic flows neartermination shock (Komissarov & Lyubarsky 04).
- High resolution simulations suggest variability of termination shock(Camus+09).
Lorentz factor (up to 5)

Magnetic Reconnection?
✚ Cerutti, Uzdensky, & Begelman (2012)
Magnetic reconnection:- electrons accelerated by reconnection
electric field- focused inside the current layer where
B field is small (E>B)- a beam of PeV electrons

Bz = B0(dashed)
Bz = 0(solid)Fermi Observations of SNRs

Scientific Objectives:− SNRs are the best laboratories to study Diffuse Shock Acceleration− SNRs are thought to be the prime sources of Galactic Cosmic Rays

18

Galactic Diffuse Background
LAT (>1 GeV) three year

Hadronic:
π0-decay γ-rays ( CR p + H ➞ π0 ➞ 2γ )Leptonic:
bremsstrahlung ( CR e + H ➞ γ )Inverse Compton ( CR e + γ ➞ γ )Galactic diffuse emission− the distribution of interstellar gas; and− the distribution of Galactic cosmic rays

19Modeling Diffuse Background
CO (molecular gas) distribution

0-1.5 kpc

9.5-11.5 kpc

3.5-5.5 kpc

11.5-15.5 kpc✻These are not currently-used rings.

20

Diffusive Shock Acceleration
Shock wave (V~3000 km/s)
thermalE = 1 GeV

Problem 1: “injection”
How thermal (Maxwellian)
E = 1.03 GeV
particles can be injected into
Fermi acceleration?
➔ Energy transferred to CRs

Escaping

Shock crossing ➜ energy gain
Energy gain per one round trip:
ΔE/E 〜 V/c ~ 1% for young
SNRs. After 1000 round trips:
e.g. 1 GeV ➜ 20 TeV
Energy distribution (test particle
approximation):N(E)dE ∝	  E-2 dE
NB:✔ Non-linear effects✔ Magnetic field amplification

Problem 2: “escape”
How highest energy particles
escape from a shock?
➔ Maximum attainable energy

p2 N(p) ∝ p4 f(p)Diffusive Shock Acceleration
Young SNRs: Vsh ~ 0.01 c
k~100 cycles
CR number decreases asexp(−4v2 k/c) ~ e-1

Maxwellian

k ~100 cycles

Non-relativistic (linear)pk ≃ k (4V/3c) mcInjection?

p=mc 2mc 3mc

Relativistic (exponential)
pk ≃ p0 exp(4kV/3c)

Escape?
Non-relativistic

RelativisticpN(p) ∝ p−s with s = 2 for strong shock(1) “Injection” ➔ Energy transferred to CRs
CR particles
ξ = nCR/nth =thermal particles

Edmon+11

ξ is assumed to be constantξ = nCR/nthTycho

WCR

From the gamma-ray data, theamount of CRs in Tycho’s SNR at its
age of t = 439 yr is:
WCR ~ 7% of ESN

WCR will reach 14% of ESN
Supporting SNR originof Galactic CRs

(2) “Escape” ➔ Maximum EnergyEmaxBohm (B=const)

collisional wave damping

π0-decay γ-rays: Direct Probe of Accelerated Protons
ε2 dNγ/dε:
hard spectrum below 300 MeVdNγ/dε:
symmetric
about 68 MeV

10 8

s=2.5
s=2.0
s=1.5

γ-ray spectrum: dN/dε
small γπlarge γπ

E2 dN/dE [erg cm 2 s 1 ]π0 production photon index Γ ≃ s
10 9 microphysics
10 1010 1110 12ε✻ = mlogε2π c /2width = 2βπγπε✻10 13 7101081091010

1011
E [eV]

10121013

1014

Very hard spectrum below
300 MeV for any reasonableproton index101

CA: Funk, Tanaka, Uchiyama

Fermi Telescope Revealed Cosmic-ray Protons
SNR IC443
as seen by WISE (Wide-field Infrared Survey Explorer)(Atomic) shockwith v~100 km/s

(Molecular) shockwith v~30 km/s

Fermi Telescope Revealed Cosmic-ray Protons
Gamma-ray map
measured with Fermi
Large Area Telescope

Gamma-ray spectrum
measured with Fermi➜ consistent with π0decay

PreliminarySignature of π0-decay Gamma-rays

Our previous papers reported spectra only >200 MeV.
Here we report spectra down to 60 MeV thanks to:Recent update (“Pass-7”) of event reconstruction, which largely
improved effective area at low energies.
Increased exposure time: 1 yr ➜ 4 yr
Sub-GeV spectra of IC443/W44 agree well with π0-decay spectra.
29

Signature of π0-decay Gamma-rays

Our previous papers reported spectra only >200 MeV.
Here we report spectra down to 60 MeV thanks to:
Recent update (“Pass-7”) of event reconstruction, which largely
improved effective area at low energies.
Increased exposure time: 1 yr ➜ 4 yr
Sub-GeV spectra of IC443/W44 agree well with π0-decay spectra.
30

SNRs Detections with Fermi
1038

GeV Luminosity [erg s 1 ]1037

MCYoungSNRMC

1036

1035

G349.7+0.2
W49B
CTB37A

W51C

Kes17
3C391 W44

G8.7-0.1

IC443

CasA

1034

Middle-aged

W28
Puppis A

Tycho

RXJ1713
VelaJr

S147

Fermi-LAT Coll(Uchiyama+)2011
CygnusLoop

1033
101

102

103
D2 [pc2 ]104Updated from Thompson,Baldini,Uchiyama(2012)

10531

Gamma-ray Spectra of Fermi-Detected SNRs

Luminosity [erg s 1 ]Fermi

Cherenkov

1037(Fermi)
W44(Fermi)
CasRXJ1713
A (Fermi)(HESS)
W44(AGILE)
CasRXJ1713
A (VERITAS)(Fermi)
CasW51A (MAGIC)
W51 (MAGIC)

1036

SNRs interacting with MCs

1035

Vela(VERITAS)
Jr (HESS)IC443(Fermi)
Tycho
Vela(Fermi)
Jr (Fermi)
IC443(VERITAS)
Tycho
W49B (Fermi)

Young SNRs (Cas A & Tycho)
Synchrotron-dominated(RXJ1713, Vela Jr)

10 34

1033

1032
10 8

Uchiyama+(in prep)

109

1010

1011

E [eV]

1012

1013

1014
32

Gamma-ray Spectra of Fermi-Detected SNRs

10Cas A (Fermi)
Cas A (VERITAS)
Cas A (MAGIC)
Tycho (VERITAS)
Tycho (Fermi)
RXJ1713 (Fermi)
RXJ1713 (HESS)
Vela Jr (HESS)

37

Luminosity [erg s 1 ]1036

Vela Jr (Fermi)
W44 (Fermi)
W44 (AGILE)
W51 (Fermi)
W51 (MAGIC)
IC443 (Fermi)
IC443 (VERITAS)
W49B (Fermi)

1035

1034

1033

1032
108

Uchiyama+(in prep)

109

1010

1011

E [eV]

1012

1013

1014
33

Young SNR: Tycho’s SNR
Fermi-LAT Detection (5σ)

Photon index = 2.3 ± 0.1(favors hadronic origin)
6-8% of ESN
transferred to CRs.
34

Synchrotron-dominated SNR: Vela Jr.
Tanaka+2011

Suzaku X-ray(Y. Uchiyama)

HadronicTeV(H.E.S.S.)

Leptonic

B2 = 0.01 mG
LAT count map >10 GeV
LAT Detection at ~15σ level
Vela pulsar

ΓLAT = 1.87 ± 0.08(sta)± 0.17(sys)B2 = 0.01 mG in leptonic model would be difficult to bereconciled with X-ray measurements.
Hadronic model would require a large CR content
35(5×1050 erg for n=0.1 cm-3)

Middle-Aged SNR: Puppis A
Hewitt+2012
Diameter: 30 pc
Age: ~40,000 yr (Sedov phase)
ISM Density: 1 cm-3

WISE (infrared)

LAT Detection at ~13σ levelLAT

ΓLAT = 2.10 ± 0.07(sta)± 0.10(sys)π0-decayMC

Non-radiative shock

The gamma-ray emission can be modeled either
by bremsstrahlung with We = 1×1049 erg or byhadronic (π0-decay) with Wp = 4×1049 erg

brems

36

Old SNR: S147
Hα image (radiative shock)Katsuta, Uchiyama+2012
Diameter: 76 pc (d=1.3 kpc)
Age: ~30,000 yr
ISM Density (2 phases): 4 cm-3 / 0.1 cm-3
➜ compression ➜ 400 cm-3 / 0.4 cm-3Fermi vs HαThe gamma-ray emission can be explained byre-acceleration of Galactic CRs in opticalfilaments (Uchiyama+2010).
37

Cygnus Loop: LAT Results
Katagiri+ (2011)

Correlation with X-ray and Hα emissions➔ Gamma-ray-emitting particles distribute near shock
waves

NOTE: southern radio emission would be
another SNR.
Spectral steepening above ~ 2 GeV.
(simple power-law disfavored at 3.5σ level)
Gamma-ray Luminosity ~ 1×1033 erg/s (< otherLAT SNRs)
38

SNRs Detections with Fermi
1038

GeV Luminosity [erg s 1 ]1037MCYoung

SNRMC

1036

1035

G349.7+0.2
W49B
CTB37A

W51C

Kes173C391 W44

G8.7-0.1

IC443

CasA

1034

Middle-aged

W28
Puppis A

Tycho

RXJ1713
VelaJr

S147

Fermi-LAT Coll(Uchiyama+)2011
CygnusLoop

1033
101

102

103
D2 [pc2 ]104Updated from Thompson,Baldini,Uchiyama(2012)

10539

Growing Examples: MC-SNRs
IC 443

W49B Abdo+2010

W30(G8.7-0.1) Hanabata+

W44
LAT

H.E.S.S.
(Brun+2011)40

SNR-MC System
Crushed Cloud model: π0-decay γ-rays come from shocked
molecular clouds Blandford & Cowie (1982), Uchiyama et al. (2010)
Radiative shock ➜ high compression ➜ high CR & gas density
Shock: slow (~100 km/s), partially ionized ➜ Maximum energy < TeVThin filaments or sheets ➜ Hard to confine CRs at high energies
Reacceleration of pre-existing CRs may be important

The Case of W44:
Shocked MC mass of 5000 M⊙◉☉
Synchrotron radiation correlatedwith shocked H2 gas (infrared lines)

W44(synch. radio)41

Fermi-LAT Detection of W44 Surroundings
The presence of large-scale GeV emission was found in the vicinity of SNR W44

Uchiyama et al. (2012)count map 2-100 GeV

residual map (W44 subtracted)

SRC-1/2: TS = 75/102

Subtraction of W44
Gamma-rays from W44 itself are subtracted,assuming “radio map = gamma-ray map”

42

Large-scale GeV γ-rays vs CO map
W44 is known to be surrounded by a complex of MCs.
Size ~100 pc, Mass ~106 Msun (Dame+1986)

SNR W44

CO: contours
MC complex is illuminatedby a “CR halo” around W44
43

Gamma-ray Evidence for Leaking CRs(c) M. Malkov (2012)

CR self-confinement(Exciting Alfven waves)

After leaving SNR W44, CRs diffuse along theexternal B-field direction ➜ bipolar morphology

π0-decay γ-ray(and bremsstrahlung)Amount of CRs Escaped from W44
Molecular clouds illuminated by escaping
CRs (assumed to be uniform within r<L)

L ~100 pc, Mass = 0.5×105 M⊙◉☉

Diffusion coefficient of the ISM (isotropic)

D(p) = D28 (cp/10 GeV)0.6 1028 cm2 /s

W44

Surroundings

Solving the diffusion equationin the vicinity of W44, we canestimate the energy spectrum ofescaping CRs.
Case 1:
Slow diffusion (D28 = 0.1)
Nesc (E) = k E-2.6Wesc = 0.3×1050 erg
Case 2:
D28 = 1
Nesc (E) = k E-2.0
Wesc = 1.1×1050 erg
Case 3:
Fast diffusion (D28 = 3)
Nesc (E) = k E-2.0
Wesc = 2.7×1050 erg

CR Content and Maximum Energy as
Function of SNR Age
fee expansion

Sedov

c: Ellison+

WCR

Ep,maxNon-linear DSA

Tycho

W44

Summary✚ Fermi-LAT observations of the Crab Nebula:◆ Flares of synchrotron radiation challenge PWN models◆ Very efficient e-e+ acceleration (close to theoretical limit)◆ Change of a beaming factor seems to play a key role✚ Fermi-LAT observations of SNRs:••••

Historical SNRs Tycho & Cassiopeia A– Hadronic origin, Magnetic field amplification, CR energy content
Young TeV-bright SNRs RX J1713.7-3946 & Vela Jr.
– Leptonic origin? (B-field too low?)
SNRs interacting with molecular clouds– W51C, W44, IC443, W28, W49B, W30, CTB37A, ...
– Direct evidence for hadronic origin (IC443, W44)
– Evidence for Runaway CRs
Evolved SNRs without molecular cloud interactions– Cygnus Loop, Pup A, S147
47
– Hadronic origin? Perhaps from radiative shock (Hα)