1. 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
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

2. Plan of the talk 銀河団からのガンマ線： 理論レビュー
銀河団からのガンマ線：　理論レビュー
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？
TeV 観測の可能性

3. 銀河団からのガンマ線 ショックと高エネルギー粒子生成 放射機構 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
ハドロン起源
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
逆コンプトン
e.g. Totani & Kitayama 2000

4. ハドロン v.s. 電子 銀河団ガスの典型的密度 ~10-3 cm-3 << ISM in MW
pp reaction time scale:
(n σpp c)-1 ~ 3.3 x 1010 (n/10-3cm-3)-1 yr
CMB photon density universal!
IC energy loss time scale:
tcool = 2×106 (εγ/GeV)-1/2 yr << cluster age
Electron Lorentz factor
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、
LIC >> Lpp
Active time scale: IC << pp

5. Model prediction for pp-gamma
Typical EGRET limit
Colafrancesco & Blasi 1998

6. Prediction for IC gamma-ray clusters
Standard ΛCDM universe
Dark halo formation rate dn(M, z)/dt
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

7. Properties of IC gamma-ray clusters
M～1015 Msun, z ～
Very short electron cooling time
100 Myr for GeV gamma-ray emitting electrons
Much shorter than dynamical time ( ～Gyr)
c.f. gamma-rays from hadronic processes
Gamma-ray emission only from clusters with active shocks soon after dynamical formation
An interesting probe of dynamical processes of structure formation
In sharp contrast to longer time-scale emissions:
Thermal x-ray emission
Gamma-rays from hadronic processes

8. 5% energy injection: reasonable?
Generally it is believed that supernova remnants produce cosmic-ray hadrons with efficiency of ～10%
Energy flux of cosmic-ray electrons to the earth is only a few percent of protons.
Propagation effect?
Some indications for ～5% injection to electrons as well
Radio flux from supernova remnants (e.g., Blandford & Eichler 1987)
EUV/hard X-ray emission from clusters of galaxies (e.g., Sarazin 2001)

9. Some recent numerical studies
Keshet et al. 2003
~10% contribution to extragalactic gamma-ray background
Cannot explain all isotropic unidentified EG sources (~60)
5% efficiency assumed.
Berrington & Dermer 2003
A few unidentifed EGRET sources possible

10. EGRET all sky survey (>100MeV)

11. EGRET source catalog

12. Unidentified sources:
Low galactic latitude (|b|<10°)
Supernova remnants
pulsars
Early stars and winds
mid galactic latitude (10°<| b| < 45°)
Associated with the Gould belt
Soft spectrum, steady source
Pulsars?
High galactic latitude (45°< |b|)
Variable sources  probably AGNs
~7 steady sources

13. Can we find gamma-ray clusters in other wavelength?
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
z ～0.05: comparable with the depth of Abell/ROSAT all-sky 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.
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

14. Search for gamma-ray clusters in the EGERET error circles
Search for forming/merging clusters by optical galaxy catalog (Kawasaki & Totani 2002, ApJ in press)
Automated matched-filter search of galaxy clustering
More systematic than the Abell catalog
Search performed on high-latitude, steady un-ID EGRET sources
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

15. Search for merging clusters in EGRET circles
Seven steady, |b|>45°unidentified sources
Correlation with Abell clusters:
5 out of 7 associated with Abell clusters (1.7 sigma)
Expected number by chance: 2.4 +/- 1.5
4076 clusters / 8.25 str
< theta95> = 0.85 degree for EGRET sources
0.34 cluster per 1 EGRET circle

16. Matched filter search of galaxy clusters
α(deg)

17. Matched filter search of galaxy clusters (2)
Number of (single) clusters:
21 clusters found in 20.07□2 field around EGRET sources
Control field: 133 clusters in □2 field
Excess: ( )/16.41/2 = 1.6σ
Cluster pairs/groups (CPGs)
Expected from hierarchical structure formation
Angular diameter < 2 Mpc/h
Same z within uncertainty (typically 20%)
z < 0.1

18. Matched-filter search for galaxy clusters (3)
6 cluster pairs/groups within 1 deg from centers of
EGRET sources

19. Statistics of cluster pairs/groups
6 CPGs associated with 7 un-ID sources within 1 deg from the centers of EGRET sources
6 per □2
Control field:
12 per □2 field
Excess:
(6 – 1.5) / 1.51/2 = 3.7 σ
Chance probability from Poisson statistics: 0.4%

20. Properties of cluster pairs/groups
Half of the 6 CPGs do not include any Abell clusters, but complexes of relatively small clusters
‘forming cluster’?
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:
62, 67, 90, 91, 92, 99, 102, 110, 111, 114, 119, 154
Chance probability of the same distribution: 8.0%

22. Detectability of TeV gamma-rays
Maximum gamma-ray energy:
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:
Detect extended TeV gamma-rays
Expected size of emission region <～ 1 degree
Image comparison with optical galaxy catalog

23. TeV Flux estimate for 3EG 1234-1318
Hard spectral index of /- 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)
= 3.5×10-13 cm-2s-1 (α= )
= 2.9×10-11 cm-2s-1 (α= )

24. Suggested TeV Observation Targets
Observability from CANGAROO
Spectral index should be hard
No variability evidence
Rich structure in the optical richness map

25. Matched filter search of galaxy clusters
α(deg)

26. Additional objects at 30<|b|<45 deg

27. 他の最近の観測的研究へのコメント Colafrancesco 2002 Scharf & Mukherjee 2002
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
Scharf & Mukherjee 2002
EGRET data と Abell clusters を直接比較、相関
Reimer et al. とは矛盾
Reimer et al. 2003
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

28. Effect of preheating of intergalactic medium
Cluster L-T relation suggests preheating of intergalactic gas by external entropy sources
Self-similar

29. Effect of preheating of intergalactic medium(2)
Preheating effects on high energy gamma-rays:
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)