Transcript 銀河団からのガンマ線放射 - 宇宙線研究室

銀河団からのガンマ線放射
戸谷 友則 （京大理）
CANGAROO 望遠鏡によるガンマ線天文学の新展開
京都大学、平成１５年１２月１２日
Plan of the talk
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銀河団からのガンマ線： 理論レビュー
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EGRET での検出可能性 --- 未同定天体の中
にガンマ線銀河団はあるか？
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TeV 観測の可能性
銀河団からのガンマ線
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ショックと高エネルギー粒子生成
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銀河からの宇宙線の漏れ出し (<1043 erg/s)
AGN (~1044 erg/s)
銀河団形成、合体などの構造形成時のショック (~1044 erg/s)
放射機構
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ハドロン起源
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陽子衝突、π生成
π0 ガンマ、二次電子
e.g. Colafrancesco & Blasi 1998
（１次）電子起源
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逆コンプトン
e.g. Totani & Kitayama 2000
ハドロン v.s. 電子
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銀河団ガスの典型的密度 ~10-3 cm-3 << ISM in MW
pp reaction time scale:
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CMB photon density --- universal!
IC energy loss time scale:
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tcool = 2×106 (εγ/GeV)-1/2 yr << cluster age
Electron Lorentz factor
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(n σpp c)-1 ~ 3.3 x 1010 (n/10-3cm-3)-1 yr
IC(CMB), GeV: γe =1.1x106 (εγ/GeV)1/2
Synch: γe =1.9x104 (ν/GHz)1/2(B/μG)-1/2
同じエネルギーが注入されれば、
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LIC >> Lpp
Active time scale: IC << pp
Model prediction for pp-gamma
Typical EGRET limit
Colafrancesco & Blasi 1998
Prediction for IC gamma-ray clusters
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Standard ΛCDM universe
Dark halo formation rate dn(M,
z)/dt
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Time-differenciation of the
Press Schechter, Sasaki ’94,
Kitayama & Suto ’96
5% injection of the total
gravitational energy of bayron
gas into electrons
Electron energy spectrum:
dN/dε ∝ε-2
εγ,max ～6 (B/uG) V10002 TeV
tcool = 2×106 (εγ/GeV)-1/2 yr
Lγ = Eγ / tdyn
Totani & Kitayama 2000
ApJ, 545, 572
Properties of IC gamma-ray clusters
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M～1015 Msun, z ～ 0.05-0.1
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Very short electron cooling time
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100 Myr for GeV gamma-ray emitting electrons
Much shorter than dynamical time ( ～Gyr)
c.f. gamma-rays from hadronic processes
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Gamma-ray emission only from clusters with active shocks soon after
dynamical formation
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An interesting probe of dynamical processes of structure formation
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In sharp contrast to longer time-scale emissions:
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Thermal x-ray emission
Gamma-rays from hadronic processes
5% energy injection: reasonable?
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Generally it is believed that supernova remnants produce
cosmic-ray hadrons with efficiency of ～10%
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Energy flux of cosmic-ray electrons to the earth is only a
few percent of protons.
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Propagation effect?
Some indications for ～5% injection to electrons as well
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Radio flux from supernova remnants (e.g., Blandford & Eichler
1987)
EUV/hard X-ray emission from clusters of galaxies (e.g.,
Sarazin 2001)
Some recent numerical studies
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Keshet et al. 2003
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~10% contribution to extragalactic gamma-ray background
Cannot explain all isotropic unidentified EG sources (~60)
5% efficiency assumed.
Berrington & Dermer 2003
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A few unidentifed EGRET sources possible
EGRET all sky survey (>100MeV)
EGRET source catalog
Unidentified sources:
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Low galactic latitude (|b|<10°)
Supernova remnants
 pulsars
 Early stars and winds
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mid galactic latitude (10°<| b| < 45°)
Associated with the Gould belt
 Soft spectrum, steady source
 Pulsars?
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High galactic latitude (45°< |b|)
Variable sources  probably AGNs
 ~7 steady sources
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Can we find gamma-ray clusters in other
wavelength?
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No significant correlation between un-ID EGRET sources
and Abell or ROSAT clusters
 Not all un-ID EGRET sources should contain
detectable clusters in other wavelength
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z ～0.05: comparable with the depth of Abell/ROSAT allsky clusters
Contamination of AGNs in high-latitude EGRET sources
Forming/merging clusters may be more extended or not
concentrated to the center  more difficult to detect
Gamma-rays expected only from dynamically
forming/merging clusters.
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Not all clusters should be visible in gamma-rays
Merging signatures in radio or X-ray bands have longer time
scale than in gamma-rays
Search for gamma-ray clusters in the EGERET error
circles
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Search for forming/merging clusters by optical galaxy
catalog (Kawasaki & Totani 2002, ApJ in press)
Automated matched-filter search of galaxy clustering
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Search performed on high-latitude, steady un-ID EGRET
sources
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More systematic than the Abell catalog
7 sources exist in the steady source catalog of Gehrels et al.
We expect multiple groups or clusters of galaxies closely
interacting, from hierarchical structure formation, rather
than a single well-stabilized big cluster
Search for merging clusters in EGRET circles
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Seven steady, |b|>45°unidentified sources
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Correlation with Abell clusters:
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5 out of 7 associated with Abell clusters (1.7 sigma)
Expected number by chance: 2.4 +/- 1.5
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4076 clusters / 8.25 str
< theta95> = 0.85 degree for EGRET sources
0.34 cluster per 1 EGRET circle
Matched filter search of galaxy clusters
α(deg)
Matched filter search of galaxy clusters (2)
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Number of (single) clusters:
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21 clusters found in 20.07□2 field around EGRET
sources
Control field: 133 clusters in 162.86□2 field
Excess: (21-16.4)/16.41/2 = 1.6σ
Cluster pairs/groups (CPGs)
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Expected from hierarchical structure formation
Angular diameter < 2 Mpc/h
Same z within uncertainty (typically 20%)
z < 0.1
Matched-filter search for galaxy clusters (3)
6 cluster pairs/groups within 1 deg from centers of
EGRET sources
Statistics of cluster pairs/groups
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6 CPGs associated with 7 un-ID sources within 1
deg from the centers of EGRET sources
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Control field:
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6 per 20.07 □2
12 per 162.86□2 field
Excess:
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(6 – 1.5) / 1.51/2 = 3.7 σ
Chance probability from Poisson statistics: 0.4%
Properties of cluster pairs/groups
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Half of the 6 CPGs do not include any Abell clusters, but
complexes of relatively small clusters
 ‘forming cluster’?
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Relatively large total richness/mass
 C= 79, 109, 128, 165, 206, 217 ～ 1015 Msun
 The mass and z consistent with Totani-Kitayama
calculation
 Considerably larger than those in the control field:
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62, 67, 90, 91, 92, 99, 102, 110, 111, 114, 119, 154
Chance probability of the same distribution: 8.0%
Detectability of TeV gamma-rays
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Maximum gamma-ray energy:
 B   Vshock 
  3
 12 
 TeV
 G   10 km/s 
2
  ,max
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B～ 0.1 μG typically observed in clusters
V～1000 km/s for typical clusters
Spectrum extends to this energy with dN/dE ∝E-2
Motivation of CANGAROO observation:
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Detect extended TeV gamma-rays
Expected size of emission region <～ 1 degree
Image comparison with optical galaxy catalog
TeV Flux estimate for 3EG 1234-1318
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3EG 1234-1318:
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Hard spectral index of 2.09 +/0.24
Rich structure from optical
galaxy catalog
EGRET: 7.3×10-8 cm-2s-1
VHE flux 3.2×10-12 cm-2s-1
(>TeV, α=2.09)
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= 3.5×10-13 cm-2s-1
(α=2.09+0.24)
= 2.9×10-11 cm-2s-1 (α=2.090.24)
Suggested TeV Observation Targets
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Observability from CANGAROO
Spectral index should be hard
No variability evidence
Rich structure in the optical richness map
Matched filter search of galaxy clusters
α(deg)
Additional objects at 30<|b|<45 deg
他の最近の観測的研究へのコメント
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Colafrancesco 2002
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Scharf & Mukherjee 2002
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UID EGRET sources と Abell cluster に相関
ガンマ、X、電波強度に相関
読んではいけない
EGRET data と Abell clusters を直接比較、相関
Reimer et al. とは矛盾
Reimer et al. 2003
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X-ray selected clusters と EGRET ソースに相関なし
全ての銀河団を一緒にして上限値
 <6 x 109 cm-2 s-1 for E>100 MeV
Effect of preheating of intergalactic medium
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Cluster L-T relation suggests
preheating of intergalactic
gas by external entropy
sources
Self-similar
Effect of preheating of intergalactic medium(2)
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Preheating effects on high
energy gamma-rays:
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Reduced gravitational
energy
Softening of spectrum
by weakened shock
Gamma-ray background is
suppressed by a factor of 30
5-10 Gamma-ray clusters
still detectable by EGRET
Totani & Inoue (2001)