Gamma-ray absorption by cosmic radiation fields

1. blazar GRB Gamma-ray absorption by cosmic radiation fields Susumu Inoue (Kyoto University) with (more than) a little help from my friends Fermi z~60-6 CTA

2. outline 1. probe of high-z UV background SI, R. Salvaterra, T. R. Choudhury, A. Ferrara, B. Ciardi, R. Schneider, MNRAS in press (arXiv:0906.2495) Y. Inoue, M. Kobayashi, T. Totani, SI et al., work in prep. 2. probe of Lorentz invariance violation effect in Galactic sources?

3. diffuse extragalactic background radiation at z=0 EBL= extragalactic background light

4. 1. gamma-ray absorption:probe of diffuse radiation fields g + g→ e+ + e- e E threshold condition: E e (1-cos q )>2 me2c4 s peak ,, =4 me2c4 Costamante et al 03 e.g. TeV + 1eV (IR) 100 GeV + 10 eV (UV) probe of local IRB through gamma absorption in TeV blazars pair production cross section CM energy

5. HESS observations of TeV blazars @z=0.165, 0.186 probing local IR background with gamma-ray absorption Aharonian+ 06 Nat. no strong Pop III • strongly disfavors NIR peak • close to lower limits from galaxy counts

6. EBL constraints at z=0.536 Albert+ 08 Sci. MAGIC observation 80-500 GeV 3C279 @z=0.536 E(=1) close to lower limits from galaxy counts (little missing light) if “normal” blazar spectra>1.5

7. Abdo+ 09, Sci. 323, 1688 Fermi/LAT detection of GRB 080916C up to 13 GeV without cutoff EBL constraints at z=4.35

8. slides from T. R. Choudhury

9. z>6 current observational frontier cosmic reionization epoch When? early? late? two-epoch? How? topology? What? Pop III? Pop II? mini-QSOs? dark matter decay? So what? suppression of dwarf galaxy formation Madau 07

10. slides from T. R. Choudhury

16. crucial for understanding early universe high-z UV radiation field (“background”) • reionizes the IGM (feedback on later galaxy formation) • dissociates H2 molecules (feedback on later star formation) • Ly a pumping (determines HI hyperfine 21cm level population) but direct detection impossible!

17. Choudhury & Ferrara 05, 06, Choudhury 09 parameters e*II, e*III, hesc, l0IGM model of cosmic reionization semi-analytical model with Pop III+II stars+QSOs, radiative+chemical feedback consistent with large set of high-z observations: WMAP, xHI, HUDF NIR counts, etc. xHI WMAP3 LLS reionization begins z~15 90% z~8 100% z~6 (WMAP5 OK) Gphotoion SFR Lya TIGM Lyb HUDF

18. caveat: model only for 4<z<22, no Pop I/dust SI+ arXiv:0906.2945 intergalactic radiation field (volume average) spectra “local’’ optical depth • realistic UV IRF -> opaque for Erest~10’s-100’s GeV at z<~10 • sharp cutoff at Erest~18 GeV from HI absorption above Ly edge • (reionizing radiation cannot be probed directly)

19. SI+ arXiv:0906.2495  absorption optical depth low z model Kneiske+ 04 (high stellar UV) (high UV) see also e.g. Stecker+ 06 Razzaque+ 09 Gilmore+ 09 ~0.4 @z=4.35, E=13.2GeV (GRB 080916C) significant optical depth >12 GeV at z~5, down to 6-8 GeV at z~8-10 but not much effect z>~8 due to declining star formation, path length

20.  absorption attenuation factor low z model Kneiske+ 04 (high stellar UV) Fermi CTA, AGIS 5@5 (high UV) appreciable differences in attenuation between z~5-8 at several GeV → unique, important info on evolution of UV IRF below Ly edge during cosmic reionization/first star formation

21. e.g. late reionization model with no Pop III stars alternative models differences not large, attenuation not sensitive to reionization history → predictions reasonably robust?

22. GRBs detectability most luminous GRBs (LGeV~1054 erg/s, e.g. GRB 080916C) detectable @10GeV by Fermi out to z~7 out to higher z by future large arrays! CTA, AGIS, 5@5 blazars most luminous blazars (LGeV~1049 erg/s, e.g. 3C454.3) detectable @10GeV by Fermi out to z~7 few such objects plausible Y. Inoue, SI, Totani+, in prep. distinction between intrinsic cutoffs spectral variability: IRF cutoff t-indep. <-> intrinsic cutoff t-dep. statistical: similar cutoffs for similar z, decreasing trend with z

23. Y. Inoue, Totani, SI et al. in prep. cosmic star formation rate CAUTION: data at z>6 mostly lower limits to cosmic SFR large uncertainties in SFR at z>6 models with low Pop II but high Pop III also possible?

24. estimated from GRB rate Kistler+ 09 cosmic star formation rate

25. Y. Inoue et al., in prep.  optical depth: comparison large differences in absorption! -> distinguishable through CTA obs.

26. 2. Kifune 99, Aloisio+ 00, Protheroe & Meyer 00…  absorption: probe of Lorentz invariance violation? following Jacob & Piran 08, PRD 78, 124010 n=1  E*(TeV) 0.1 11 23 50 100 110 c.f. n=2 E*(=1)=0.9x1017 TeV

27. modified threshold Jacob & Piran 08

28. modified absorption feature Jacob & Piran 08 caveat: assume LIV affects only th

29. effect on blazar spectra CTA 100 TeV observations! Jacob & Piran 08 • BUT • depends on EBL uncertainties • blazar spectra to >10 TeV unlikely?

30. effect on Galactic sources? 20,0,90 0,0,0 R,z, 20,0,180 Moskalenko+ 06 • Galactic 100 TeV sources likely (e.g. SNRs) • appreciable >10 TeV absorption in Galactic Center region • probe of >20 TeV LIV recovery

31. -ray absorption by cosmic radiation fields probe of UV intergalactic radiation fields at high z based on a model of cosmic reionization significant >12 GeV at z~5, down to 6-8 GeV at z>~8 -> valuable info on evolution of UV IRF below Ly edge (constraints on Ly/H2 dissociating radiation) but not much effect above z~8 summary detectable in high-L GRBs/blazars to z~<7 by Fermi possibly to higher z by CTA, AGIS, 5@5 BUT alternative models with different Pop II/III SFRs possible? -> distinguishable through GeV observations? crucial contribution to understanding early star/galaxy formation probe of Lorentz invariance violation? recovery of absorption feature above ~20 TeV effect in Galactic center sources promising?