銀河団からのガンマ線放射

1. 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 • 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 • 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 Totani & Kitayama 2000 ApJ, 545, 572 • 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

7. Properties of IC gamma-ray clusters • M～1015 Msun, z ～ 0.05-0.1 • 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 162.86□2 field • Excess: (21-16.4)/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 20.07 □2 • Control field: • 12 per 162.86□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 • 3EG 1234-1318: • 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) • = 3.5×10-13 cm-2s-1 (α=2.09+0.24) • = 2.9×10-11 cm-2s-1 (α=2.09-0.24)

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 • 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)