Fermi LAT Observationsof GeV Gamma Rays
from Supernova Remnants

Hide Katagiri(Hiroshima Univ)
on behalf of Fermi LATcollaboration
Nov 16, 2010
Gamma-ray astronomy
workshop@ICRR
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Observations of GeV gamma raysfrom SNRs
Aharonian et al. (2006)

SNR RX J1713.7-3946

Leptonic model

Hadronic modelICfrom e-π0from nuclei

Observations of GeV gamma rays are crucial
for study of cosmic-ray acceleration in SNRs
Parent particles, momentum spectrum, diffusion…

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~0.8o

1AGL J1801-2317

GRO J18012320(EGRETreanalysis)Before theRadio boundary Fermi-LAT~ SNR W28
case3EGJ1800-2338

 Green contours:
4, 5, 6 sigmas of theHESS TeV emissions

 The uncertainty of position determination accuracy ~ 1deg. due todegree-scale PSF and low statistics
 Point-source or extended?
 Spectral studies at > a few GeV are limited due to the small
effective areas.
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Fermi Gamma-ray Space Telescope
International collaboration between US, Europe, and Japan
Launched on June 11 2008 (its science data is public now)
Consists of LAT and GBM

Large Area Telescope (LAT)
Tracker (16 towers):- Pair conversion
telescope→ Tungsten
conversion foils- Measures e-/e+track with Si-strip
detectors

Better PSF(~0.6o@1GeV)
Large Eff. Area & 2.4 sr
FoV~20 MeV < E < ~300 GeV

Calorimeter:-1536 CsI crystals-Measures photon
energyAnti-coincidence
detector:- Segmented- Vetos CR
background Source ID & extension
 Emission mechanism
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GeV-emitting SNRs
Fermi LAT 1yr Observations



10 SNRs so far (W28, W49B, RX J1713-3946, G8.7-0.1, …）
Two categories


Young (<~2000yr) SNRs (Cas A, RX J1713-3946)
Relatively old SNRs with molecular clouds (W28, W44, W49B,W51C, IC443, etc.)

Young SNR ~ Cas A

Historical supernova: age=330yr
The brightest radio source in our Galaxy-> efficient particle acceleration
TeV gamma rays detected by HEGRA, MAGIC, VERITAS
 Cannot be distinguishedon positional grounds
 No gamma-ray pulsation,
no time variability
 No pulsar-like cutoff onthe spectrum
GeV gamma rays~ from the SNR shells.

5’Abdo et al. (2009)Corresponding Authors (CAs): Funk, Uchiyama

Spectrum of Cas A

Leptonic scenario (Brems + IC):

B = 0.12 mG, We=1×1049 erg

Hadronic scenario (π0 decay):







B > 0.12 mG, Wp=3×1049 erg
Good fit with proton spectral index ~2.3(red) or ~2.1 (blue) with cut-off at 10 GeV

The hadronic scenario can better fit the data (but not conclusive
due to the systematics)

WCR(e+p)=1-4×1049 erg regardless of the parent particles.
B > 0.12 mG -> B amplification at the shockW44Castelletti+07~0.5o

Lines from H2 gas (Spitzer)
Radio synchrotron (VLA)★ OH masar spots

Mixed morphology
SNR Age ~2 x 104yr
 Distance ~3kpc
 Interactions withmolecular cloudsLAT viewCount Map(2–10 GeV)

Abdo+10(CAs: Tanaka, Uchiyama, Tajima)
Deconvolved Image(2–10 GeV)Black cross: location of PSR B1853+01, Green contours: Spitzer IRAC 4.5 μm
Results of maximum likelihood analysis also prefer ring-likemorphology rather than centrally filled morphology (> 8 σ)

LAT spectrum & modeling

π0-decay model can explain the data well

Leptonic scenarios have difficulties


Brems: difficult to reconcile with the radio data
IC: large amount of electrons (~1051ergs)

Protons need to have a spectral break at ~ 10 GeV/c

Possible explanation:
Fast escape of high energy particles with damping of magnetic
turbulence due to the dense environment (e.g. Ptuskin &Zirakashvili 2003)

Comparison of energy spectra
FunkHEAD meeging

W44, W51C,
IC443

RX J1713.7-3946

Cas A

Different evolutionary stages allows us to probe howparticles are accelerated and later released into the Galaxy11

W28 (G6.4-1.0)
ROSAT X-ray images
with HESS TeV contors

Mixed-morphology SNR
 Old age ~35000150000yr
 Distance 1.8-3.3kpc
 TeV detected by HESS South TeV sources are
spatially coincident withstar forming regions.
Radioboundary

0.8 deg.

Aharonian et al. (2008)

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Association with molecular clouds
NANTEN 12CO(J=0-1) 0-10km(0 – 2.5 kpc)

NANTEN 12CO(J=0-1) 10-20km(2.5 – 4 kpc)

Aharonian et al. (2008)
Most outstanding in the gamma-ray (HESS) – CO correlation
among SNRs.
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LAT view

2-10GeV with NANTEN CO(J=1-0) contours

Abdo+10 (CAs: Katagiri,Tajima, Tanaka, Uchiyama)






The GeV emission is extended,overlapping with molecularclouds
Spectral steepening with a break~1GeV.
π0-decay in dense molecular gas
provides a reasonable
explanation for the broadband
gamma-ray spectrum (similar toW44, etc.).
The compact HII region W28A
can be a possible energy sourcewith extremely high density(~107/cc).

•Mosaic: FERMI(1pixel=0.025deg. Smoothing Gaussian kernel=0.2deg.)•Contours: NANTEN (CO J=1-0) v=0-20km/s (25, 50,75%)•Diamonds : HII regions

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Comparison
with TeV

2-10GeV with HESS contoursUpper limits in theGeV band on HESS
J1800-240A, C•Mosaic: FERMI(1pixel=0.025deg. Smoothing Gaussian kernel=0.2deg.)•Contours: HESS excess map (20, 40, 60, 80%) with smoothing (Gaussian kernel=0.1deg.)• A,B,C=HESS J1800-240A,B,C ,Diamonds : HII regions
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Runaway CRs?
Gabici+07
Surrounding material (R=0pc)

Nearby clouds(surroundingmaterial)
GeV > TeV

8000yrDistant clouds

Distant cloud (R=100pc)

GeV < TeV

GeV-TeV spectrum
allow us to study
CR diffusion.

SNR with TeV & MCs~G8.7-0.1
VLA 90cm contours with 5,15 and 25% levels

PreliminaryYellow cross:
PSR J1803-2137
Green cross:
Suzaku J1804-2140

Abdo et al. (CAs: Hanabata, Katagiri),
to be submitted

NANTEN CO (J=1-0) 20-30
km/s with 25, 50 and 75%levels

Preliminary

HESS contours with 25, 50
and 75% levels

Preliminary

Extended GeV emission
Spatially associated with somemolecular clouds
With relatively bright TeV emission(~25% Crab)Theoretical activities


Runaway CRs (e.g., Fujita+10,Ohira+10) See Ohira-san’s talk“Crushed cloud” scenario(Uchiyama+10)






CRs escaping from SNR and colliding
with nearby MCs

Gamma rays coming from “cloudshock” (CRs and MC simultaneouslycompressed)
Spectral break:Ion-neutral collision → Damping ofAlfvén wave
Radio-GeV correlation naturallyexplainedEtc.

Blastwave

Summary of the LAT SNRs
Compiled by Uchiyama

References: Abdo+2009, 2010a, 2010b, 2010c, Castro & Slane 2010

Some of their emissions are well-explained by pi0 decay by making
detailed discussion (spectral modeling , association, etc.).
Observationally biased to SNRs with clouds so far.
Deeper observation would provide less-biased samples.

Fainter SNRs with deeper
observations ~ Cygnus LoopROSAT
H alpha
X-ray(Levenson+98)(Aschenbach+99)Middle-aged shell-type SNR
Large offset from the Galactic plane-> shock regions well-studied by
IR/optical/UV
Large size ~ 3oCan be easily compared with otherwaveband data to study particleacceleration.

Nearby SNR ~540pc; welldetermined by optical filaments(Blair+05)
Could leave a unique feature in theCR electron spectrum of 1-2 TeV inthe vicinity of the Earth(Koboyashi’03).

LAT view

0.5-10GeV with ROSAT X-ray contours

Abdo et al.
(CAs: Katagiri, Tibaldo),
to be submitted
The largest gamma-rayemitting SNRDetailed morphologicalstudy

Without clear
correlation with dense
molecular cloudsA valuable sample for aparticle acceleration

Fermi to CTA
Better source identifications
by extension with betterPSF (2’ at 1TeV)Extension is a useful toconstrain a pulsar/PWN
hypothesis.
Associations with molecularclouds, shells, etc.

Sensitivities of current
TeV experiments are notsufficient to search for all
SNRs in our Galaxy.
Most of SNRs would have asteep spectrum, except foryoung ones.

See Bamba-san’s talk (including
some quantitative estimations)100GeV

1TeV

10TeV

* C, B, E are configurations of telescope arrays.

Ｎｕｍｂｅｒ ｏｆ ＳＮＲｓ

PSFs of CTA

Apparent radius of SNRs
CTA (1TeV)
Fermi (10GeV)

SummaryThe Fermi-LAT detected GeV gamma rays from ~10SNRs
with 2yr data.
Two types:






(1)Young SNRs, (2) relatively old SNRs with MCs.

Basically gamma rays are naturally explained by π0
decay with surrounding material or molecular clouds.
CR diffusion & environment in acceleration region canbe also investigated.
Deeper observations would provide less-biased samples
for study of CRs.
The CTA will extend detailed studies of CR acceleration
to much more SNRs in our Galaxy with better angularresolution & sensitivity.
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