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

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

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

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

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

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

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

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)

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

EGRET all sky survey (>100MeV)

EGRET source catalog

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

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

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

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

Matched filter search of galaxy clusters α(deg)

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

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

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%

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%

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

TeV Flux estimate for 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)

Suggested TeV Observation Targets 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 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

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

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)